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 I don't believe it's 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;}

Yes, it can be zero, meaning that argv[0] == NULL.

It's a convention that argv[0] is the name of the program.You can have argc == 0 if you launch yourself the binary, like with execve family and don't give any argument. You can even give a string that is nowhere near to be the program name. That's why using argv[0] to get the name of the program is not entirely reliable.

Usually, the shell where you type your command-line always add the program name as the first argument, but again, it's a convention. If argv[0] == "--help" and you use getopt to parse option, you will not detect it because optind is initialized to 1, but you can set optind to 0, use getopt and "help" long option will show up.

long story short : It's perfectly possible to have argc == 0 (argv[0] is not really special by itself). It happen when the launcher doesn't give argument at all.