Can argc be zero on a POSIX system?

Given the standard definition for the main program:

int main(int argc, char *argv[]) {
   ...
}

Under which circumstances can argc be zero on a POSIX system?

Yes, it is possible. If you call your program as follows:

execl("./myprog", NULL, (char *)NULL);

Or alternately:

char *args[] = { NULL };
execv("./myprog", args);

Then in "myprog", argc will be 0.

The standard also specifically allows for a 0 argc as noted in section 5.1.2.2.1 regarding program startup in a hosted environment:

1 The function called at program startup is named main. The implementation declares no prototype for this function. It shall be defined with a return type of int and with no parameters:

int main(void) { /* ... */ } 

or with two parameters (referred to here as argc and argv, though any names may be used, as they are local to the function in which they are declared):

int main(int argc, char *argv[]) { /* ... */ }

or equivalent; or in some other implementation-defined manner.

2 If they are declared, the parameters to the main function shall obey the following constraints:

• The value of argc shall be nonnegative.
• argv[argc] shall be a null pointer.

...

Note also that this means that if argc is 0 then argv[0] is guaranteed to be NULL. How printf treats a NULL pointer when used as the argument to a %s specifier is not spelled out in the standard however. Many implementations will output "(null)" in this case but it's not guaranteed.

To add to the other answers, there is nothing in C (POSIX or not) preventing main() from being called as a function within the program.

int main(int argc, int argv[]) {
    if (argc == 0) printf("Hey!\n");
    else main(0,NULL);

    return 0;
}