compiling with gcc with system()

Question

My code does not compile right when I use:

system("gcc -o filename temp.c");

I am getting:

implicit declaration of function system

I'm not sure what is missing, because it only throws the system error on the gcc call.

Here is my code:

#include <stdio.h>

int main() {
        ...
        system("gcc -o filename temp.c");
        return 0;
}

Solution 1

Add #include <stdlib.h> at the top of main().

Tip: When you see implicit declaration of a built-in function, you have to search for the function (with google, for example now you should have searched with "system() C"), in order to find the corresponding header, i.e. where the function is declared. Then one of the results should be the ref of the function.

In our case this link. There you can see:

SYNOPSIS

#include <stdlib.h>
int system(const char *command);

which tells you that you have to include stdlib header to use system().

As Mr. Bright noticed, if you're on on a inux-like OS, man 3 system should do the trick too.

Example:

samaras@samaras-A15:~$ man 3 system
SYSTEM(3)                  Linux Programmer's Manual                 SYSTEM(3)

NAME
       system - execute a shell command

SYNOPSIS
       #include <stdlib.h>  <-- this is what we are looking for!

       int system(const char *command);
...

Solution 2

Since it appears you are using a Posix system, you should know about the man command which shows documentation for most library calls. On my system, when I type:

$ man system

I get:

SYSTEM(3)                  Linux Programmer's Manual                 

NAME
       system - execute a shell command

SYNOPSIS
       #include <stdlib.h>

       int system(const char *command);

Notice that in the synopsis it tells you the include file you need to use. The man page also includes a lot of other documentation such as the return value.

Comments

[Sean Bright]

If you're on on a unixy OS, man 3 system should do the trick too.

[Josh Buchanan]

Ok, so if something throws a implicit declaration of function, it means that there is header file missing?

[gsamaras]

Yes @JoshBuchanan, not any header though, the header that needs to be included by that particular function at the time. Now system() required stdlib.h to be included. Nice question though, you got my +1. Hope you keep my tip, it will help I believe!

[Josh Buchanan]

@G.Samaras Thank you, I will keep that in mind, this is like my third C program. Still trying to figure out what to look and when.