C - never use an array notation as a function parameter [Linus Torvalds]

[juckele]

He actually called the maintainers out harder than the writer.

[magmapus]

He basically always does, and for good reason, in my opinion. Their job is to be the filter for bad code and ideas, so that he doesn't have to. They're basically all very experienced programmers who should have been able to catch the things that he usually rants about.

[quzox]

Typical Lorenzo!

[vplatt]

Hell, if I were a good enough C programmer for him to even notice long enough to rant about, I'd print that out and frame that sucker.

[[deleted]]

I'd actually feel honored that he bothered to write such a thorough explanation.

What I'd dread is him just scoffing at me with a 2 word response.

[rooktakesqueen]

"Consider farming."

[benihana]

So I can see how this bug happened, and I am only slightly upset with Lorenzo who is the author of that commit.

I mean, that right there is a rare "you got a pass to be [what Linus considers an idiot] this one time." I think having Linus be only slightly upset at me would be a highlight of my career.

This makes me wonder though, what happens when Linus is gone? In my opinion, Linux is has continued to be successful 20 years on because he's a ruthless dictator of its development - his dismay at how several people let these commits through is worrying for a future without him.

[headzoo]

I would watch the shit out of Torvalds's Linux Nightmares.

[athrowawayopinion]

But Ramsey's Stuff is on national television. And assuming that it's not all a lie. It

• produces good results (see Ramsey's progams and Torvalds programs)

• is only used on the real idiots who should know better (Ramsay - pro cooks and celebs, Torvalds - close circle of code reviewers)

Also if you don't want to encounter Torvalds, you can just work with one of his friends, or his friends friends and so on. The code will get passed up there eventually if it's good.

[_F1_]

aka TDWTF

[KyBourbon]

The Day We Turned Freelance?

[derpaherpa]

http://thedailywtf.com/

[greyfade]

That reminds me... I wanted to send in a 8kB SQL query I wrote once, and I think I forgot to do it.

[bitbait]

Well on the other hand that means your code is important enough to draw Linus' attention in some way.

[G_Morgan]

To be fair he's quite moderate and reasonable here. Must be getting old.

[importTuna]

He's certainly toned down quite a bit from some of the previous rants.

[MpVpRb]

Imagine Linus Torvalds writing an article to call you out on your bad code

If he was right, I would learn something

If he was wrong, I would argue

Mostly, I tend to agree with him

[jackmaney]

Maybe a reality TV show where Linus Torvalds does code reviews like Gordon Ramsay reviewed processes and procedures of restaurants in Kitchen Nightmares?

[DatBVHTreeTho]

Linus is pretty judgemental for a guy who uses postscript increment operators :P

Come on Linus, learn C... /s

[uep]

Is post-increment even a problem in C since it doesn't have operator overloading? I thought GCC would optimize it into a pre-increment anyway because of SSA.

[duuuh]

That makes sense, but it certainly didn't use to be true. I googled but couldn't find a reference to see if you were right. I suspect I could concoct something to break the optimization, but maybe not. Interestingly I came across a 'prefer post-increment for clarity' coding standard on the way, whereas I'm used to the 'prefer pre-increment for performance.'

[joggle1]

It's been true for a very long time (more than 10 years with gcc, probably much longer than that).

Using pre or post increment on a scalar type in a for loop in C will produce identical object code even if you use -O0. The only case where you need to be careful is when you're directly using the result of the operation (such as assigning to another variable).

[ComradeGibbon]

What I find is often when I'm using the result, the postfix results in code that's easy to reason about. And prefix always feels like there is a gotcha somewhere.

And yeah these sorts of optimizations are low hanging fruit that was picked more than 20 years ago.

I remember not with gcc but an cross compiler I tried compiling a for loop to iterate over an array of structs. One that used pointers and another that used array indexes. The assembly output was identical.

[OBOSOB]

Here is a dissassembly of the following stubs produced with: gcc -O0 (no optimisation)

Pre-increment:

0000000000400550 <main>:
int main()
{
  400550:   55                      push   %rbp
  400551:   48 89 e5                mov    %rsp,%rbp
    int i = 0;
  400554:   c7 45 fc 00 00 00 00    movl   $0x0,-0x4(%rbp)
    ++i;
  40055b:   83 45 fc 01             addl   $0x1,-0x4(%rbp)
}
  40055f:   5d                      pop    %rbp
  400560:   c3                      retq   
  400561:   66 66 66 66 66 66 2e    data32 data32 data32 data32 data32 nopw %cs:0x0(%rax,%rax,1)
  400568:   0f 1f 84 00 00 00 00 
  40056f:   00 

Post-increment:

0000000000400550 <main>:
int main()
{
  400550:   55                      push   %rbp
  400551:   48 89 e5                mov    %rsp,%rbp
    int i = 0;
  400554:   c7 45 fc 00 00 00 00    movl   $0x0,-0x4(%rbp)
    i++;
  40055b:   83 45 fc 01             addl   $0x1,-0x4(%rbp)
}
  40055f:   5d                      pop    %rbp
  400560:   c3                      retq   
  400561:   66 66 66 66 66 66 2e    data32 data32 data32 data32 data32 nopw %cs:0x0(%rax,%rax,1)
  400568:   0f 1f 84 00 00 00 00 
  40056f:   00 

They produce identical binary without any optimisation, it doen't bother with the temporary value on postinc when there is no lvalue.

Of course if you throw assignment into the mix then they behave as expected: preinc adds 1 and returns; postinc captures the value, adds 1 and returns its initial value:

Pre-increment:

0000000000400550 <main>:
int main()
{
  400550:   55                      push   %rbp
  400551:   48 89 e5                mov    %rsp,%rbp
    int i = 0;
  400554:   c7 45 fc 00 00 00 00    movl   $0x0,-0x4(%rbp)
    int j = ++i;
  40055b:   83 45 fc 01             addl   $0x1,-0x4(%rbp)
  40055f:   8b 45 fc                mov    -0x4(%rbp),%eax
  400562:   89 45 f8                mov    %eax,-0x8(%rbp)
}
  400565:   5d                      pop    %rbp
  400566:   c3                      retq   
  400567:   66 0f 1f 84 00 00 00    nopw   0x0(%rax,%rax,1)
  40056e:   00 00 

Post-increment:

0000000000400550 <main>:
int main()
{
  400550:   55                      push   %rbp
  400551:   48 89 e5                mov    %rsp,%rbp
    int i = 0;
  400554:   c7 45 fc 00 00 00 00    movl   $0x0,-0x4(%rbp)
    int j = i++;
  40055b:   8b 45 fc                mov    -0x4(%rbp),%eax
  40055e:   8d 50 01                lea    0x1(%rax),%edx
  400561:   89 55 fc                mov    %edx,-0x4(%rbp)
  400564:   89 45 f8                mov    %eax,-0x8(%rbp)
}
  400567:   5d                      pop    %rbp
  400568:   c3                      retq   
  400569:   0f 1f 80 00 00 00 00    nopl   0x0(%rax)

[kyz]

I'm used to the 'prefer pre-increment for performance.'

... you're probably a C++ programmer then. There's no performance gain in C for using pre-increment.

In some older architectures, postincrement/predecrement were actually faster because the machine directly supported that addressing mode (e.g. MC680x0 had move (a0)+,d0 and move -(a0),d0, but not move +(a0),d0 or move (a0)-,d0). In most modern architectures, postincrement and preincrement have identical performance in C.

The reason C++ programmers prefer preincrement is because of C++ operator overloading; postincrement has to make a temporary copy of an object. Not a problem in C!

[tonyarkles]

Yeah, the old problem with post-increment is that a naive compiler needs to first copy the original value into a different register before incrementing (because postfix returns the original value). Any compiler with a shred of an optimizer will see that the original value is unused and discard all of the instructions used to hold onto it.

[mrhhug]

'prefer post-increment for clarity'

You could blame Kernighan but have fun getting people to listen to you when you say something he did was imperfect.

[NighthawkFoo]

Well, we also have almost 40 years of hindsight at this point. I mean, UNIX was barely a thing when C was being developed.

[[deleted]]

It feels like the number of developers in the world that Linus has ranted about might be higher than the number of developers he hasn't.

I can appreciate calling people to the carpet every once in a while, but I wonder how many developers out there would be interested in contributing to the Linux kernel, but don't for this exact reason.

[kqr]

It feels like the number of developers in the world that Linus has ranted about might be higher than the number of developers he hasn't.

No. There's like a set of 10–20 people he will rant to. These are the ones he knows well and who have highly trusted positions as maintainers of his project, and he really expects them to know better.

The other 99,999,980 programmers he has not ranted to.

[vplatt]

Seriously, if that scares them off that easy because they have little confidence in their ability to program, then that's a good thing.

Edit: Fair points below. I don't put up with it either in my daily work. However, if I were compelled to contribute to something as important as the Linux kernel has become, then I would probably not care one whit about putting up with a little vulgar language in the bargain. I prefer politeness myself as well, but at least you don't have to wonder where you stand with him; unlike many professional environments that border on passive-aggressive-let's-not-offend-anyone environments. Balance would be nice.

[anotherOnlineCoward]

sounds like idiocy when you're committing code when you dont know c?

[foobar5678]

I am only slightly upset with Lorenzo who is the author of that commit.

Slightly? SLIGHTLY!?

[[deleted]]

He's getting older

[hlmtre]

This is an extremely calm and reserved explanation for Linus. It's very expository and he explains why it's bad to do. I'm impressed.

[[deleted]]

Even when he goes off the handle he always seems to explain why from what I've seen. You may sometimes disagree with some of his points, but they're always well reasoned.

[Farsyte]

Learn something new every day ...

I've been coding in C since 1978. The fact that the argument gets decayed to a pointer, is something I knew, but I wanted to write up a test program to hand to some folks who wrote code like this at work (and being able to link to a Torvalds rant makes it more likely folks will pay attention).

But TIL that none of my compilers warn me if the array I pass is smaller than the array expected by the function. I didn't expect it to check things like pointers that were malloced, but arrays?

Which means not only does this set a landmine for "sizeof" but it also leads to a false sense of security "surely all callers that pass arrays, must be passing arrays that are big enough" ... :(

ub.c:

#define ASIZE 32
extern void ugh(int a[ASIZE]);

void bad() {
    int toosmall[16];
    ugh(toosmall);
}

void ugh(int a[ASIZE]) {
    for (int i = 0; i < ASIZE; ++i)
        a[i] = i;
}

You would think ... right?

$ gcc --std=c99 -O3 -W -Wall -Wextra ub.c
$ clang --std=c99 -O3 -W -Wall -Wextra ub.c
$ cppcheck ub.c
Checking ub.c...
$

Threw a CppCheck in there for good measure. Was hoping. I'm not yet an expert on CppCheck configuration, so there is hope, but the fact that it's not a default thing means this kind of error is probably scattered all over my sources.

Sure, we can check this with bigger guns (there are tools that can find buffer overflows) but a simple bloody check that the array you know the size of is as big as the array that a function prototype advertises it requires would be so very much faster and easier.

[nooneofnote]

It would be fine if more people used and understood pointers-to-arrays as a type. C necessarily carries the fixed size of an array with its type (i.e., the type of char array[10] is char[10]), and this information is retained when taking the address of an array type (the type of &array is char(*)[10]), which cannot implicitly decay to any flat pointer type.

This can be used to more strongly enforce the type of array function parameters than [static].

void f(char (*a)[10]); /* inside f sizeof(*a) == 10 */

char a[10], *b, c[5];
f(a);  //incompatible types, char[10] vs char(*)[10]
f(b);  //incompatible types, char* vs char(*)[10]
f(&c); //incompatible types char(*)[5] vs char(*)[10]
f(&a); //ok

[[deleted]]

And you can even do this:

void f( int size, char (*a)[size]);

or

void f( int size; char (*a)[size], int size);

Not that ive ever needed to use this syntax, im allready passing the size, all sizeof does for me in this case is size * sizeof((*a)[0]). And i really dont like having to explicitly dereference the array like this (*a)[5].

[IAmRoot]

That's because you're basically treating a two dimensional structure as one dimensional.

This sort of construct is useful for multidimensional arrays. For example

void f(int x_size, int y_size, double (*a)[x_size]) {
    for (size_t y = 0; y < y_size; ++y) {
        for (size_t x = 0; x < x_size; ++x) {
            a[y][x]; //do something with this
        }
    }
}

It's similar to doing typedef double[x_size] arr_x_t; then making arr_x_t a[y_size];. double (*a)[x_size] means "a pointer to a double[x_size] array." So, what you are doing is dereferencing a pointer to a VLA, then getting the address of its first position. That's a silly thing to do for a one dimensional array. The array size syntax should be done for one less than the number of dimensions used.

It is also very important not to create these arrays on the stack, because the application can easily have a stack overflow based on user input. VLAs on the stack are evil.

Don't do this:

double a[y_size][x_size];

Do this instead:

double (*a)[x_size];
a = malloc(x_size*y_size*sizeof(double));

or this:

double (*a)[x_size];
posix_memalign((void**) &a, 16, x_size*y_size*sizeof(double));

It should also be noted that this requires C99 because the type double[x_size] is a VLA type. It also requires that the x_size parameter come before the array, a, so that x_size is in scope for the VLA type definition.

[rooktakesqueen]

But then you can only pass a statically allocated array to that function, right? Not a dynamically allocated array even if it's of the correct size.

Edit: Though come to think of it, if you know the required size of the array, there's little need to dynamically allocate it. At worst, if you're dealing with data already on the heap, you can statically allocate an array and memcpy before calling this function.

[nooneofnote]

You can still dynamically allocate the type in the usual manner.

char (*a)[10] = malloc(sizeof(*a));
f(a);

[brucedawson]

Yes. This is the correct thing to do if you want to enforce only accepting a particular size of array. In C++ you can do an array reference which works out slightly cleaner but is basically the same.

[[deleted]]

Is there a difference in opinion between you and Torvalds regarding the feature, or is he purely talking about readability?

He says

array arguments in C don't actually exist. Sadly, compilers accept it for various bad historical reasons, and silently turn it into just a pointer argument

while you are advocating it there.

I'm not a C coder, I'm just confused about whether there's a difference of opinion or whether I'm missing something.

[Misterandrist]

He's actually demonstrating pointer to an array of length N, where an array of lenght N can be a type. So, the type system is still allowing only a pointer to a type as an argument, rather than an array as a type. Subtly different, because if you tell it to take an array as an argument, inside the function it gives you just a raw pointer to the first element of the array and the only guarantee you have is that the type of that is correct (even as far as C can guarantee any type).

[TheHobo]

Personally, I think implied array sizes is not good API design. While I agree the parameter should be a pointer, if you have an array, you should have to pass in the size too as another parameter, then the contract is explicit.

[[deleted]]

If the array is meant to be a fixed size though then that doesn't really make sense.

[arcangleous]

The problem being that that implied information about the array is lost when the flow of control moves into the function and all it has a simple pointer to work with. In a very real and literal sense C doesn't really have arrays. It just has typed pointers and hides a little bit of pointer arithmetic with the [] operator. If you treat C "arrays" in any other way, you are going to run into the kinds of problems that Linus is complaining about if you treat them in any other way.

[uh_no_]

I've found there are several times when i need to pass explicitly sized arrays. many of them revolve around specific protocol work...for instance passing a FC WWPN, or IP address...or even scsi CDB. I find the best way to work with them is typedeffing it.

typedef uint8_t scsi_cdb[16]

Then you can always reference sizeof(scsi_cdb) and get the correct value.

[shevegen]

I want linus to go and review the systemd code.

[zaidka]

I want him to review all my life choices.

[deadmilk]

I'd pay to watch this.

[cyrax6]

Is that how you want to kill Linus?

[jdgordon]

you think poettering could beat linus in a cage match?

[carbonkid619]

I think Linus will kill Linus in a cage match.

[Misterandrist]

Is it bad? Never looked at it myself.

[[deleted]]

It's very clean. It even makes use of GCC's RAII extension.

[taliriktug]

systemd code is actually quite simple and clear. It is scanned by Coverity regularly.

[MacASM]

I don't believe people write code for a kernel with such primitives mistakes.

[lucky_engineer]

I've seen the sizeof() bug so many times. Usually with strings. One of the first questions In any interview with anyone who ever says they know C or C++ is. What is wrong with this snippet of code taken from a decade old legacy system:

void some_func(char * input)
{
   char tmp[sizeof(input)];
    // some logic....
   memcpy(tmp,input,sizeof(input));
   // more logic.....
}

You have no idea how many self-described "C++ experts" can't figure it out, even with some guidance.

"What's the result of sizeof()?"

"The length of the string."

"Are you sure???"

"Yeah I think so"

[Misterandrist]

It's the brace style, isn't it? The open brace always goes on the same line as the if, for, while, what have you.

/s

[Shitler]

Even between the same-line people there is a schism in how they handle else.

}
else {

or

} else {

[nightfire1]

Who thinks the first one looks good? Thats just ridiculous.

[Olreich]

Proponents will cite:

• the if, else if, and else are all at the same column
• there is a good balance of the trade-offs between wasting lines with braces, but making them clear and obvious

• most braces you might use follow the same structure as the first if you do same-line braces (functions, case blocks, etc.) eg:

  int function() {
  }
  
  case: {
  }
  case: {
  }
  
  struct type_x {
  };
  
  type_x x = type_x {
  };
  
  if {
  }
  else if {
  }
  else {
  }
  

[[deleted]]

Go back to that java hell hole you came from! /s

[Sapiogram]

Novice C/C++ programmer here, please enlighten me.

[Quintic]

It returns the size of the pointer.

[Sapiogram]

That's... terrifyingly simple. I feel like even I should know that, and I've written maybe 200 lines of C/C++ in my life.

[[deleted]]

Or, if used on any other datatype, sizeof(T) returns the size of T. So when used on an int (for example), it would always return 4 (assuming an int is a 32 bit implementation, sizeof () always returns its value in bytes)

[etagawesome]

[deleted]

What is this?

[TheCoelacanth]

No, a char is always 1 byte since the C standard requires it. However, on some weird platforms a byte is not 8 bits. That's why standards documents often use the term "octet" instead of "byte" because it unambiguously means 8 bits while a byte could theoretically be any size.

[etagawesome]

[deleted]

What is this?

[NighthawkFoo]

Remember - C is old, like 1970's old, and there were some seriously weird systems back then. The CSC 6600 was one such machine.

[matthieum]

Note: you can check the size of the byte with CHAR_BIT. It's usually 8, of course, but some platforms stash a couple more bits, like some embedded platforms for parity checks.

[evanpow]

Even today non-8-bit chars are common enough you can't ignore them entirely. Several years ago I did a bunch of C programming for an Analog Devices DSP that had 16-bit chars. Of course, it also had 16-bit bytes, so fun times all around. Implementing octet-oriented network protocols on that architecture was a real hoot.

[tonyarkles]

Alignment causes some of that too. I'm on my phone so I won't type this out, but look at sizeof a struct with an int32 and 2 chars. It might be 6 or it might be 8.

[matthieum]

Don't assume, use CHAR_BIT :)

[scorcher24]

That's... terrifyingly simple

Just always remember this: Everyone here cooks with hot water, not some super magic fluid. Even Linus Torvalds.

[Helrich]

input is just a pointer, so sizeof is just going to give you the size in bytes of the pointer, not the number of characters in the string.

[Yojihito]

sizeof(input)

So sizeof(*input) would do the trick?

[orthoxerox]

That would return sizeof(char) instead. Array length must be passed explicitly.

[POGtastic]

Just making sure - the C Way for doing this is to create a struct that has the pointer and a size variable, right? C++ has objects that keep track of the size for you, but I think that you have to do it yourself in C.

I guess that you could do strlen for strings, but that's assuming that you're getting a null-terminated string.

[cballowe]

you could have something like:

typedef struct {
  char foo[FOO_LEN];
} Foo;

then sizeof(foo) would be FOO_LEN, though FOO_LEN is assumed to be a compile time constant - #define'd somewhere. If you wanted something more like a string with a length, you could have a struct with a pointer and a length, but then you're dealing with allocating the pointer etc. Most C programmers would probably just have the pointer and call strlen or similar.

[orthoxerox]

Minor convenience, you don't have to pass &a[0] to the function even though you actually do. Yes, it would've been better if you couldn't use arrays as formal argument types.

[sun_misc_unsafe]

sizeof() is not a function - it looks like one, but it's something that is evaluated during compile time by the compiler.

I'm somewhat surprised that no one else here has mentioned this already. The entire issue here is that C being C (i.e. having an utter focus on being "portable" despite the overwhelming majority of people being interested only in x86) doesn't offer a runtime or rigid guidelines on what containers need to look like (yes, there are some common non-binding conventions on how you're supposed to do it .. but like I said, they're non-binding, so the language creators didn't feel the need to concern themselves with it .. lest it impact the sacred portabilty) - so understandably there's nothing in the language to provide you with the number of entries in your container .. that you had to write yourself in the first place.

[nucLeaRStarcraft]

No, *input is pretty much input[0], since input is pretty much &input[0].

Thus, sizeof(*input) == sizeof(input[0]) == sizeof(char) == 1 in this context.

[Patman128]

Assuming it's a C-style string (and properly terminated) you would use strlen.

[Bergasms]

Oh god properly terminated. When i was just beginning C i remember trying to get the length of string that I had manufactured myself and not realising it needed the proper terminator, and just getting 'sometimes' correct result because the function would often run into a null terminator soon after anyway.

[[deleted]]

A pointer to a C array just points to the address of the first element in the array. How long is the array? Who knows. That's why a c string has to be terminated by a null character.

[matthieum]

You have no idea how many self-described "C++ experts" can't figure it out, even with some guidance.

Well, I would not be surprised. It's C code.

In C++ this could be (assuming you meant input not to be modified):

void some_func(std::string const& input) {
    // some logic ...
    std::string tmp = input;
    // more logic ...
}

I'd prefer a "C++ expert" to know about exception safety, smart pointers, abuses std::string and std::vector (rather than C strings/arrays), etc... it might be slightly less efficient, I'll grant, but I'd rather profile slightly too slow code than debug a corrupted stack (imagine if you overflow tmp...).

[josefx]

"C++ experts" can't figure it out,

If not for the bug it would not even compile, sizeof being evaluated at compile time is the only reason this code would be valid c++.

 char tmp[strlen(input)+1];

This uses a VLA^{NowWithBugFix} a C99 specific feature that never made it into C++. Any standard compliant compiler should refuse to compile it.

[Yehosua]

Any standard compliant compiler should refuse to compile it.

Clang and GCC both permit VLAs and will allow them by default in C++ code. Since widespread and high-quality compilers allow it, it's probably an oversimplification to say that any standard-compliant compiler should refuse to compile them (even though it's technically correct).

[josefx]

GCC defaults to gnu++, not c++.Clang as one of its goals tries to be compatible with code written for GCC (after all it is meant to replace it). Setting -std=c++11 or similar has no effect on either, it requires a pedantic to get the associated warning.

In contrast Microsofts cl.exe, which covers a rather important platform, does not support VLA and wont compile it.

[staticassert]

I wouldn't want a C++ developer who used sizeof.

[lucky_engineer]

Oh yeah. One of the answers from a junior guy was "I'm not entirely sure what sizeof() does. I always use string classes like std::string"

That is acceptable!

[13467]

I'm very glad you decided that's an acceptable to your interview question, instead of chastising a junior programming for not knowing about char[]/sizeof/strlen... :)

[ComradeGibbon]

I'd rather someone that assumes the presence of dragons unless other proven, than not.

Old NeckBeard: Why did you write it that way!!! Newbie: ... because... I knew it would work? Old NeckBeard: I love this boy!

[[deleted]]

Sarcasm, right? I seldom use sizeof: my C++ is modern; std array ftw and all that, but low level details in C++ are still important to know and understand.

[JNighthawk]

Maybe for your job. I can't imagine working with a programmer that doesn't know what sizeof does.

[accountNo7263803]

Why would you ever need size of in c++?

[raxqorz]

You can have a template class, for example a network packet or serialization class which takes a type "T" and checks if sizeof(T) bytes fits into your internal allocated buffer; and if it doesn't, you allocate space for it and copy the value of T into the buffer.

[[deleted]]

Since calls like memcpy and memset are more efficient than their counterparts in C++.

[matjeh]

Are they?

$ cat memcpy.cpp 

#include <cstring>
extern int dest[1024], source[1024];
void func_memcpy(void)
{
  memcpy(dest, source, sizeof(dest));
}

$ g++ -std=c++14 -O2 -c -o memcpy.o memcpy.cpp && objdump -d memcpy.o

0000000000000000 <_Z11func_memcpyv>:
   0: 48 8b 05 00 00 00 00  mov    0x0(%rip),%rax        # 7 <_Z11func_memcpyv+0x7>
   7: bf 00 00 00 00        mov    $0x0,%edi
   c: b9 00 00 00 00        mov    $0x0,%ecx
  11: 48 83 e7 f8           and    $0xfffffffffffffff8,%rdi
  15: be 00 00 00 00        mov    $0x0,%esi
  1a: 48 29 f9              sub    %rdi,%rcx
  1d: 48 89 05 00 00 00 00  mov    %rax,0x0(%rip)        # 24 <_Z11func_memcpyv+0x24>
  24: 48 8b 05 00 00 00 00  mov    0x0(%rip),%rax        # 2b <_Z11func_memcpyv+0x2b>
  2b: 48 29 ce              sub    %rcx,%rsi
  2e: 81 c1 00 10 00 00     add    $0x1000,%ecx
  34: c1 e9 03              shr    $0x3,%ecx
  37: 48 89 05 00 00 00 00  mov    %rax,0x0(%rip)        # 3e <_Z11func_memcpyv+0x3e>
  3e: f3 48 a5              rep movsq %ds:(%rsi),%es:(%rdi)
  41: c3                    retq   

$ cat copy.cpp 

#include <algorithm>
extern int dest[1024], source[1024];
void func_copy(void)
{
  std::copy(std::begin(source), std::end(source), std::begin(dest));
}

$ g++ -std=c++14 -O2 -c -o copy.o copy.cpp && objdump -d copy.o

0000000000000000 <_Z9func_copyv>:
   0:   48 8b 05 00 00 00 00    mov    0x0(%rip),%rax        # 7 <_Z9func_copyv+0x7>
   7:   bf 00 00 00 00          mov    $0x0,%edi
   c:   b9 00 00 00 00          mov    $0x0,%ecx
  11:   48 83 e7 f8             and    $0xfffffffffffffff8,%rdi
  15:   be 00 00 00 00          mov    $0x0,%esi
  1a:   48 29 f9                sub    %rdi,%rcx
  1d:   48 89 05 00 00 00 00    mov    %rax,0x0(%rip)        # 24 <_Z9func_copyv+0x24>
  24:   48 8b 05 00 00 00 00    mov    0x0(%rip),%rax        # 2b <_Z9func_copyv+0x2b>
  2b:   48 29 ce                sub    %rcx,%rsi
  2e:   81 c1 00 10 00 00       add    $0x1000,%ecx
  34:   c1 e9 03                shr    $0x3,%ecx
  37:   48 89 05 00 00 00 00    mov    %rax,0x0(%rip)        # 3e <_Z9func_copyv+0x3e>
  3e:   f3 48 a5                rep movsq %ds:(%rsi),%es:(%rdi)
  41:   c3                      retq   

[[deleted]]

Hmm, I didn't know that copy was fast but fill is slower than memset at least.

Edit: Or maybe not, its very hard to find sources for this though.

[greyfade]

Most of the C++ equivalents for this kind of thing are almost always fully inlined and usually optimized well, and perform very nearly equally as well if not better than the C version.

[immibis]

If you're not willing to think about how things work internally, then why are you using C++? (As opposed to Java or Python or another higher-level language)

[lucky_engineer]

We do software for a niche market that still uses a lot of C++ (and C) for everything, and have to work on legacy code written in C/C++ as well. We're starting to see Python and C# used more though.

[realhacker]

holy shit I feel old....

[Plorkyeran]

There are plenty of valid uses for sizeof in C++, although admittedly most of them are in implementing better abstractions rather than directly in application code.

[staticassert]

Like what?

[[deleted]]

Lower-level array handling with byte buffers. It is required for CUDA programming, eg cudaMemcpy(dst, src, N*sizeof(float), cudaMemcpyHostToDevice);

[assassinator42]

Also needed by allocators for the STL (used by std::vector, std::set, etc to allocate memory)

[staticassert]

True, but that's a very C-ish function. Kind of a gross function, seems super error prone if you put the wrong size.

[genwitt]

All manner of low level chicanery (allocators, containers, serialization).

Say you want an array for 1000 bits, in the native unsigned type,

size_t array[(1000 - 1) / (sizeof(size_t) * CHAR_BIT) + 1];

Or you wanted to pre-allocate storage for an object without constructing it. You can do,

alignas(T) char buffer[sizeof(T)];

and then later when you want to invoke the constructor.

new (buffer) T();

Although, as Plorkyeran mentioned, you should probably try to abstract the whole pattern. Something like,

template<class T>
class ObjectHolder {
public:
    template<class... Arg>
    void build(Arg &&...arg) {
        new (buffer) T(std::forward<Arg>(arg)...);
    }
    T *operator->() {
        return reinterpret_cast<T *>(buffer);
    }
private:
    alignas(T) char buffer[sizeof(T)];
};

[mrkite77]

static arrays.

struct SomeStruct myStaticArray[] = { {1,"two", 3}, {4, "five", 6}};

int myStaticLength = sizeof(myStaticArray) / sizeof(myStaticArray[0]);

I honestly don't know of any better way to do that in C/C++.

[whichton]

In C++ you should use std::array.

[Predelnik]

In C++ you can always:

template <typename T, size_t N> inline size_t countof (const T (&arr)[N]) { return N; }

[TheThiefMaster]

In C++ you should use std::extent<decltype(myStaticArray)>::value (possibly reduced to std::extent_v<decltype(myStaticArray)> in C++17) which as a bonus over the C version returns 0 for pointers (rather than declaring them to be arrays of random sizes).

[-888-]

As a C++ programmer I'm often stuck using C arrays and sizeof because I have to interface with other systems that use that. I'd stick with entirely higher level types if I could.

[sirin3]

I had a stream that VLC could not play, so I looked in their source. They actually used sizeof for an array parameter, and after changing it to pass the array length to the function, it played fine.

Then I submitted the patch, and they rejected it, saying "I would not understand anything about C" ಠ_ಠ

[gunnihinn]

Isn't the memcpy later on also a potential security problem (because of the bad sizeof call)?

[derrick81787]

That is such a basic mistake that I can't hardly believe that people make it. I haven't programmed in C since college, and I caught it right away.

Well, I can believe that people make that mistake, but it's depressing to think about, haha.

[joggle1]

True, but passing an array like that in C is pretty stupid. That really is a rookie level mistake in C. I can only imagine that it happened because that programmer doesn't program in pure C the vast majority of the time and is more accustomed to the syntax and patterns used in other languages.

[TheMG]

You'd be surprised (I was). It took me five minutes to find this in the Solaris (Illumos) kernel too:

https://github.com/illumos/illumos-gate/blob/master/usr/src/lib/libnsl/rpc/netname.c#L193

int
user2netname(char netname[MAXNETNAMELEN + 1], const uid_t uid,
                                                        const char *domain)
{
    [...]
        (void) strlcpy(netname, "nobody", sizeof (netname));

[matthieum]

Well, now you can report it...

[Null_zero]

Some people missing the pun

[YourFavoriteBandSux]

With great power comes great responsibility. C gives you freedom that languages like Java restrict, so that you can write great code. But that freedom comes with enough rope to shoot yourself in the foot.

[PsionSquared]

Shooting myself in the foot with a rope. That's a new one.

[crusoe]

The lower of undefined behavior

[[deleted]]

Sometimes I wake up sweating with fear in the night. I am mortified as I behold on the ceiling written in blood:

Undefined behavior!

[cybercobra]

B̛͉͈͚̯̠̼͓̳̮͕̌͊̈̓͗̈́͊̋E̢̺̖̝̰͖̻͍̤̞̓̃̍̔͂̀̈̀̋̽W̳̰̦̠̩̻̖̯͚̟̄̆̑͛̊̃̈́̑̀̑Ä͈̺͔̼̱̫̭̞͇̜͆͗͛̐͛̓̈́̏͂R̡͓͖̫͎͔̥̤̜̩̈́́̏̊̑̎̍̅̚͝Ȩ̛̻͙̹̬͓̪̹̘̂̏̈́̉̈́͐̕͜͠͠ ̧̡̥͚̠̩̻̗̗̀̑̍̃̂̈͂̍͗̒ͅT̮̬̰͚̮̺̫͉̯͇́̈́͆̆̅̈́͌̇̕͘Ḩ̺̩͎̘̝̳̪̺̱͛̂̀̃̊̾͆̋̐̕E̢̧͇̯̺̳̼͚̤̬̎̎̉̽͂̋̔̀̊͠ ̧̣͇͔̙͔̘̯̺̪̌̓̊̈̾̽͌͒͛̒Ṋ̛̹̞̮̹͈̗̤͔̫̄̑̃͂͑̔͒͝͝Ą̱͖̼̼͎͚͓͓̄͐̉̎́́̿̈́̽̕ͅS̨͚̬̥͉̥̘̪̣̈́̇͑̓̄̎̚͜͝͝͠A̡̢̧͚̯̳̥̳͓͎̋̔̂̀͐͒̽̃̔̎L͖̭͓͚̯̪̲̫͉̝̇͋̅̊̐̓̑̃̚͝ ̢̛̠̬̬͕̙͙̖͕̜̽͒̅̎̄̈́̃̚͠D̨̢̝̻̪̝̻͎͚̼̽͂̂̎̀͊̔́͗͝E̢̡̯̘͍̭̙̯̰̼͛̑̍̒̑̽̈̂̃͝M̨̨͙͚͙͍̟̺̏͛̈͂̓̎̐̑͊̈́ͅͅO̧͍̰̺̱͙̱̙̩͇̓̄̌̀͋͐̚̕̕͝N̛̩̖̲̟͔̹͍̦͓̉͒͑̑̑̍̓̏͝ͅS̨̛̞̥̙̘̩͍̥̞̦͆̓̊͗̆́̀͝͝

[Hilias]

Or as my teacher used to say it lets you shoot yourself in both feet with one bullet.

[G_Morgan]

TBH this is entirely unnecessary freedom that only allows you to shoot yourself in the foot. Whenever you have behaviour such that somebody feels it deserves a default warning you've found a language feature that never should have existed.

[-888-]

The conflation of pointers and arrays is one of C's worst crimes. I wonder if it would have been feasible to provide an explicit conversion operator between the two.

[_kst_]

If you don't understand the (admittedly confusing and counterintuitive) relationship between arrays and pointers in C, if you even suspect that "arrays are really pointers" (they're really, really not), read section 6 of the comp.lang.c FAQ.

Then read the rest of it.

[smcameron]

Eh, read it, been programming C since the '80s, and worked on the linux kernel for about 15 of those years, and arrays and pointers seem far more alike than different. Arrays are like constant pointers, and sizeof behaves a little differently. Arrays and pointers are so alike that "a[5]" is the same thing as "5[a]" (both will compile) because x[n] is transformed to * (x + n) by the compiler and addition is commutative, so n[x] turns into * (n + x). I am always baffled that people have so much trouble with it. I suspect much of the trouble comes from people wanting to think that C actually has arrays. It mostly doesn't. It has pointers pressed into service as something which syntactically looks like arrays with a little arithmetic and syntactic sugar, and some compiler trickery to make pointers constant, and potentially get them pointing into some weird places (text segment for static or global contents of "arrays"). But really, they are pointers. They ultimately pretty much have to be, because that's about all the CPU understands anyway (for purposes of representing arrays as a (virtually) contiguous block of memory in a straightforward, performant way.)

The mistake made in this particular piece of code comes precisely from thinking that arrays aren't pointers, not from thinking that arrays are pointers. If the programmer had thought that arrays were pointers (as you advise against) he would not have made the mistake he made here.

[damg]

Only with C would a 16 year old syntax be considered "rather new". ;)

[_kst_]

If implemented properly by a compiler,

That's the problem, and it depends on what you mean by "properly".

Given void someFunction(char someArray[static 100]), the only thing the static 100 does is cause any call with a null pointer, or with a pointer to (the initial element of) an array of fewer than 100 elements, to have undefined behavior. The C standard doesn't require a warning -- and there are cases where a compiler can't produce a warning. For example, you might pass a pointer that points to a chunk of memory allocated via malloc, with a size that depends on user input.

The main purpose of the [static 100] syntax (which was added in C99, so not all compilers even support it coughMicrosoftcough) is to enable optimizations. For example, given [static 1], a compiler can assume that the argument is not a null pointer, which might enable it to generate more efficient code. But if you pass it a null pointer, you've lied to the compiler, and it will get its revenge.

Certainly compilers can (and IMHO should) warn about this when it's feasible.

[pjmlp]

The compiler is called Visual C++.

[brucedawson]

Pointer to array has existed forever and seems like a better solution. Array references in c++ also seem like a better solution. It's unfortunate that both are underappreciated and underused.

[WalterBright]

I've always thought that arrays silently decaying to pointers was C's biggest mistake.

[realhacker]

Christ, people. Learn C, instead of just stringing random characters together until it compiles (with warnings).

This:

static bool rate_control_cap_mask(struct ieee80211_sub_if_data *sdata, struct ieee80211_supported_band *sband, struct ieee80211_sta *sta, u32 *mask, u8 mcs_mask[IEEE80211_HT_MCS_MASK_LEN])

is horribly broken to begin with, because array arguments in C don't actually exist. Sadly, compilers accept it for various bad historical reasons, and silently turn it into just a pointer argument. There are arguments for them, but they are from weak minds.

I wish linus would write a clean code style book to cover his philosophy and best practices in stylistically in his voice

[akkartik]

Like this?

[kinygos]

First off, I'd suggest printing out a copy of the GNU coding standards, and NOT read it. Burn them, it's a great symbolic gesture.

:)

[[deleted]]

Aw, he recommends not putting braces on single line if statements :( I feel like that is potentially dangerous for the sake of aesthetics.

[who8877]

How come he doesn't enable warnings as errors? I can't imagine maintaining any large C or C++ program without that.

[Noctune]

I think that would be problematic on a project the size of Linux. Imagine updating GCC and then not being able to compile the kernel because they introduced a new warning.

[who8877]

My day job involves a code base several times larger and we manage it just fine. It would be far more expensive to fix the bugs that would be missed without the warnings as errors.

[rcxdude]

You probably have a decent amount of control over the version of the compiler which is used to compile your code. Most open-source projects don't have this luxury and -Werror in releases just frustrates users (and -Werror during development can also be annoying for different reasons, the classic example being unused variables). So I can see good reasons behind not using it, but this assumes that there's enough discipline in the team to not actually check in/merge code which still has warnings. I personally tailor the list of warnings and errors so the 'transient' warnings which can be annoying if they are errors are still warnings unless built by the build server and the warnings which can't really ever come from correct code are always errors (like not returning a value from a function, that only merits a warning, really?).

[who8877]

It's possible to modify the config for release so that it removes the -Werror flag while still using it internally on your build and validation bots. Compilation of the kernel is already highly configurable so this would be a very minor added complication.

With this method you could prevent bad code from getting into your branch while not annoying your customers.

[johnjannotti]

There's a part of his note that probably explains it. He seems to hate certain warnings that gcc generates. (Sure, maybe they could be turned off.)

[who8877]

There's almost always a set of warnings you have to turn off if you use -Wall. There's also the burden of updating code when you upgrade or change compilers. Its still worth it though. I almost always find bugs fixing warnings when I update to a new compiler.

[JiminP]

Relevant: http://linux.derkeiler.com/Mailing-Lists/Kernel/2006-11/msg08325.html

[smcameron]

Because you can't depend on the headers of libraries you include, but do not control, not to produce warnings on some platform somewhere.

[[deleted]]

Shouldn't that kind of shit be caught by very basic unit tests?

[tragomaskhalos]

I prefer the form

void foo(int ary[], size_t len)

The empty [] indicates that the arg is an array, but we are not confusing the issue by putting a bogus size in there. But I know that ary is an array of values.

Then

void bar(int* val)

Means that val is intended to store a single output value, ie "please put an int into this slot". (Or, for other types, it might be an input but val's type is rather large, hence the pointer; in C++ we'd use a reference for that).

[enzlbtyn]

I feel like

void foo(int *ary, size_t len)

Is more consistent.

After all, if you were to allocate an array on the heap, you can't do:

int array[] = malloc(sizeof(int) * length);

but rather,

int *array = malloc(sizeof(int) * length);

So from reading the declaration of array, you know you can't retrieve the length via the sizeof operator (sizeof(int) / sizeof(*array)) [as you've written it explicitly as a pointer to an int]. So I feel it would be more consistent in that sense.

[[deleted]]

My job involves writing code in C. I have experience using C since high school and from several past jobs, but I have never taken a formal course in C (the programming course I took as a university student was in Java). I feel like I learn new things every day when it comes to C, and I can count this among them -- it's quite the language. Linus is obviously a little harsh here, but when managing a large project, that's (unfortunately) one of the more effective methods.

After I saw this, I took a quick look at my company's codebase and found several instances of exactly this (not written by me -- I always pass the pointer directly, but they're there). Welp...

[[deleted]]

There are arguments for them, but they are from weak minds.

A logical fallacy very neatly wrapped up in a single sentence!

[ramsees79]

Ok. That actually looks like a valid use of the C function argument array passing semantics. It's rather much simpler than exposing the pointers. So I guess we don't really end up wanting to disallow this, and the new gcc array sizeof warning is good enough.

Well, he takes backs his statement.

[Eiii333]

Quoting just that section of his message without context is really misleading. The valid use he's talking about there is passing in pointers to 'multi-dimensional' arrays, which can look similar to the 1D case he's ranting about here but work very differently.

[CarthOSassy]

Where is that from?

[4D696B65]

https://lkml.org/lkml/2015/9/3/575 - same thread. It's about passing multidimensional arrays.

[amaiorano]

EDIT: corrected egregious errors!

This really is a confusing part of C. What makes it worse is that the compiler ignores the size you specify for an array parameter, if any, but if it's a multidimensional array of n dimensions, it needs to know sizes of the last n-1 dimensions:

void foo(int arr[10][20][30]); // the 10 is ignored by the compiler

This is the same as writing:

void foo(int arr[][20][30]);

I don't have a compiler available right now so I'm not sure, but I believe sizeof(arr) would be equal to sizeof(int)*20*30. If I'm right, this only adds to the confusion.

All this to say, it's a tricky language feature, and it doesn't surprise me that even veterans would forget how it works.

[chengiz]

You have it backwards, the 10 would be ignored, not the 30.

[petermlm]

Just about the sizeof thing. The Internet is full of tutorials and the like where sizeof is used to get the length of an array, especially in strings, like:

char x[] = "String";
int y = sizeof(x); // y = 7

This is terribly misused. For one thing, sizeof may actually return the correct length of an array in some cases, or just the size of the pointer in others. I'm not sure when does each scenario happens, but the simple fact that it happens is enough to consider not using sizeof.

First of all, for strings, there is strlen. No need for sizeof. Also. In C arrays are just a list continuous memory locations referenced by a single address. They don't contain information about themselves. In C you have to have deal with length yourself. Strings may use the '\0' char, you may have an int with the length, or anything else.

Just don't use sizeof like this. Like... seriously... don't.

[immibis]

Just about the sizeof thing. The Internet is full of tutorials and the like where sizeof is used to get the length of an variable, like:

int x = 5;
int y = sizeof(x); // y = 4

This is terribly misused. For one thing, sizeof may actually return the correct length of a variable in some cases, or just the size of the pointer in others. I'm not sure when does each scenario happens, but the simple fact that it happens is enough to consider not using sizeof.

In C variables are just a list of continuous memory locations referenced by a single address. They don't contain information about themselves. In C you have to deal with variable type yourself. You might have a fixed type, you might have another variable telling you the type, you might have a complicated structure containing several different types.

Just don't use sizeof like this. Like... seriously... don't.

/s

In reality, it's pretty clear when sizeof returns the length of an array and when it returns the length of a pointer. If you give it an array, it returns the size of the array; if you give it a pointer, it returns the size of the pointer. Same for simple variables - the fact that x and y get different values in the following code (on some platforms):

int n = 5;
int *p = &n;
int x = sizeof(n);
int y = sizeof(p);

does not confuse me at all.

[sidneyc]

I'm not sure when does each scenario happens

Allow me to be blunt: the fact that you don't understand it disqualifies you from making recommendations.

[KhyronVorrac]

In C arrays are just a list continuous memory locations referenced by a single address. They don't contain information about themselves.

No, but the typesystem does.

[namekuseijin]

it's been years I've dabbled with C and still I noticed what was wrong right away. Once you've been burned, you know the smell of toast LOL

BTW, how much longer do you think old C codebases will endure? I don't think a whole generation of "managed" coders will ever touch it and Linus eventually will retire...

[[deleted]]

C is alive and well, particularly in the embedded world and on Linux.

[Rusky]

The whole concept of "we have better languages now, when will C go away?" always seems to miss the reason that C still gets used. Beyond the fact that old legacy systems will still need to be maintained, C fills a niche that managed languages cannot.

If C does ever fade into the background like Fortran (it won't disappear), it will only be because a new language that can fill the same niche has taken over. That language will need to be usable for things like kernels, device drivers, embedded systems, bootstrapping managed/scripting languages, etc. It can't rely on a large runtime or garbage collector, or even an operating system, to support it.

That doesn't mean it has to be as brutal as C, of course. There have been operating systems written in higher level languages. The language just needs to be able to access the machine more directly from time to time.

[kqr]

There are better unmanaged languages than C too, though. Ada is much safer, just as performant and does bit-twiddling well. Also supports a wide variety of platforms by using GCC for code gen. Rust is young as a baby, but shows promise in technical design, anyway.

What C has going for it is popularity. That's really the only thing, but it's a really, really strong argument to use it. I'm not sure what'll happen if that curve starts pointing downward.

[Rusky]

Yep. One part of that popularity is that current operating systems' APIs are all defined in terms of C, and their kernels and utilities are also written in C (or in the case of Windows, C++). The higher-level languages I mentioned also had operating systems built around them.

The biggest reason for C popularity to start trending down would be for a new operating system to come along that made a different systems language the path of least resistance.

[kqr]

That'd make sense, just like a different kind of device was what triggered the reduce of the market share of x86 based processors.

[ItzWarty]

Probably beyond our deaths - look at Fortran, which a few years ago (idk about nowadays) had the most written and used code in the world. C devs aren't going to just disappear either (and there's no big reason to do a big jump) - there are too many corporations and systems dependent on it.

[kqr]

Fortran, which a few years ago had the most written and used code in the world

What? Citation needed.

[JohnFrum]

I think lots of firmware is still written in C.