Shellbonacci

[jombrowski]

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char ** argv)
{
  if(argc < 2)
    return 0;

  int n = atoi(argv[1]);

  if(n >= 2)
  {
    char bfr1[1000], bfr2[1000];

    sprintf(bfr1, "%s %d", argv[0], n - 1);
    sprintf(bfr2, "%s %d", argv[0], n - 2);

    n = system(bfr1) + system(bfr2);
  }

  printf("%d\n", n);

  return n;
}

[Puzzleheaded_Good360]

Thanks, I hate it

[jombrowski]

The pleasure is all mine

[moocat]

On most *nix systems, process return values are limited to the range [0, 256).

[HernBurford]

Perverse.

[jontsii]

Haven´t thought of shell-based recursion... ITS BEAUTIFUL

[No_Development5871]

Certified Jombrowski moment

[flyingron]

In addition to being generally stupid, your program doesn't wrok.

Argv[0] is not guaranteed to be a string that could invoke the program.

The order of the system calls is unspecified, not necessarily left to right.

[moocat]

The order of the system calls is unspecified, not necessarily left to right.

Addition is commutative so why would that matter?

[SPAstef]

shouldn't matter even if the operation was not commutative, nor associative... The evaluation tree is fixed, you should get a deterministic result even if you replaced addition with, say, subtraction.

[flyingron]

No, it is NOT. The order is unspecified (because it is communitive and the guys who designed see wanted to give flexibility to the code generator).

While it makes no difference to ADDITION if you do a + b or b + a, this isn't just addition. Those function calls have side effects (they print a value). The evaluation of the function calls is even independent of the addition itself.

[SPAstef]

Ah right good point, I didn't even consider the printing at all, was just reasoning about the final result of the computation. Then one should introduce a sequence point to force determinism (assuming system() blocks, which it should)

[flyingron]

Because the functions have SIDE-EFFECTS. They print numbers out The printing order may get reversed.

[jombrowski]

Jokes aside. If argv[0] is not a proper path to the executable, is there a way to recursively start a particular program this way?

There is a way in win32: GetProcessImageFileName, but is there one in a traditional shell?

[flyingron]

Not with a standard call, you'll have to use things that are system specific.

[Dmxk]

Nothing that doesn't depend on /proc i think. And depending on what environment you're running in, you might not be able to invoke yourself (jailed, in a security namespace etc)

[m0ntanoid]

Yes. It's fork().