Question

I have such a funny problem I thought I’d share with you.

I cornered it down to the most little program I could :

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

int cmd_left(char *name)
{
  pid_t pid;
  int   f_d;

  if ((pid = fork()) == -1)
    {
      perror("");
      exit(1);
    }
  f_d = open(name);
  printf("%d\n", f_d);
  close(f_d);
}

int main(int ac, char **av, char **env)
{
  char **dummy_env;

  if (ac < 2)
    return (0);
  dummy_env = malloc(10);
  cmd_left(av[1]);
}

Basically, if I remove the malloc, opening works just fine.
You just have to compile and give the program a (valid) file to see the magic.

Answer 1

You need #include <fcntl.h> to get a declaration for open() in scope, which would then tell you that you are not calling it with enough arguments:

int open(const char *filename, int flags, ...);

(The optional argument – singular – is the permissions for the file (mode_t perms) if you have O_CREAT amongst the options in the flags argument.)

The call to malloc() scribbles over enough stack to remove the zeroes on it initially, which leaves the ‘extra arguments’ to open() in a state where they are not zero and you run into problems.

Undefined behaviour – which you’re invoking – can lead to any weird result.

Make sure you compile with at least ‘gcc -Wall‘ and I recommend ‘gcc -Wmissing-prototypes -Wstrict-prototypes -Wall -Wextra‘.