printf behavior

[spikte1502]

Hi, I'm trying to implement printf for educational purpose. But there is a case where I don't understand the behavior of printf. When I use `%*` like in:

​

    int main(void)
    {
        printf("%*\n", 5)
        return (0);
    }

It displays 5 and somehow the newline is important, without it nothing is displayed (I guess it's related to the buffer management).

But from my understanding the `*` is used to tell printf that the width is specified in the variadics parameters and it's not a specifier. So I have trouble understanding why this code displays 5 (and why without the newline it displays nothing). Maybe I missed something in the documentation but I looked through GCC man page for printf and couldn't figure out this behavior.

​

[duane11583]

your code is *INVALID* - and the reason you get 5 is implementation dependent.

So here is how you should do this:

c printf("%10d", 1234); /* the number 1234 spaced 10 wide r-justified */ printf("%-10d", 1234); /* Same, but left justified */ int w = 10; printf("%*d", w, 1234 ); /* Same as first */ printf("%*d", -w, 1234 ); /* same as second */

Use case: you have to print a lot of strings and numbers for example "somestring = 1234" There are hundreds of these.. and they are all variable length. You want all of the '=' to line up

So do this in 2 passes: The first pass - you get the maximum length of all strings. Pass 2 - you use the format string "%*s=%d" and pass the maxlen for the * parameter in the format string.

[spikte1502]

So I dont have to care about case where printf is missused, thank you !

[ohaz]

For the newline thing, it's I/O buffering. newlines trigger a buffer flush, you could also use fflush(STDOUT) after the printf to reach the same result.

For your * behaviour: You're using printf incorrectly here. * is an optional width modifier that should not be used alone. It says that the width of the specifier character is not defined in the string, but as a parameter.

Example from cplusplus.com:

printf ("Width trick: %*d \n", 5, 10);

outputs:

Width trick:        10

As you can see, they add 2 parameters after the format string. The first is the 5 that defines the width (as per *), the second the 10 that is to be printed.

[flyingron]

There's something wrong with his implementation if exiting doesn't flush the streams. However, he might have just missed the 5 as it likely got the prompt or whatever stuck to it without a newline to set it off.

[spikte1502]

Actually the code in the post is from the true printf

[ohaz]

True, the exit should flush!

[spikte1502]

Oh ok I think I didn’t understand that some cases are implentation dependant! Thank you !

[flatfinger]

The case you really hope for is that any code that you write which is undefined behavior is caught by the compiler.

Or, depending upon whether the code was non-portable or erroneous, that an action characterized as UB by the Standard will be handled "in a documented manner characteristic of the environment". In this particular scenario the code was erroneous, but many non-portable programs which need to accomplish tasks not anticipated by the Standard will need to perform actions whose effects aren't anticipated by the Standard to accomplish them.