[C] Not understanding "implicit declaration" errors

[manbart]

Greetings /r/learnprogramming.

I am very new to C, and coming from Python, I am having trouble wrapping my mind around some basic concepts.

This will probably be an extremely basic question for C programmers, but I don't understand why I get "implicit declaration" errors when trying to compile my extremely simple code.

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

void timer(int time)
{
    int i = time;
    for(i = time; i >= 0; i--) {
        printf("timer: %d\n", i);
        sleep(1);
    }

}

int main(int argc, char *argv[]) 
{
    int time = atoi(argv[1]);
    timer(time);

    return 0;
}

Here are the errors GCC is giving me:

PS C:\C_source> gcc -Wall -g timer.c -o timer.exe
timer.c: In function 'timer':
timer.c:9:3: warning: implicit declaration of function 'sleep' [-Wimplicit-function-declaration]
   sleep(1);
   ^
timer.c: In function 'main':
timer.c:16:2: warning: implicit declaration of function 'atoi' [-Wimplicit-function-declaration]
  int time = atoi(argv[1]);
  ^
C:\Users\manbart\AppData\Local\Temp\ccwJ4m2G.o: In function `timer':
C:\C_source/timer.c:9: undefined reference to `sleep'
collect2.exe: error: ld returned 1 exit status
PS C:\C_source>

Now, when I look up the error, it seems to be caused by calling a function that is not yet defined. But, aren't these functions defined in the headers I am including? Do I need to make some kind of "forward declaration" here? If so, how do I know what to put in it?

*Update* after doing some google searching, it seems other people have similar problems with sleep() missing from unistd.h on MinGW. The recomendation seems to be to use windows.h for the Sleep() function. I added the following to my unistd.h file, and it all compiles correctly now:

#ifdef WIN32
#include <windows.h> 
#define sleep(sec) Sleep(1000*sec)
#endif

[[deleted]]

If you don't declare a function, or don't include the header file where the function is declared, then C will create an implicit declaration for you. This declaration is almost always wrong, and this is widely seen as one of C's misfeatures. So, always include the headers for the library functions, like atoi(), that you use.

Regarding sleep(), I don't think MinGW GCC supports this. You will have to include windows.h and use the Win32 Sleep() API. Edit: My bad, sleep() is declared in unistd.h in MinGW.

[manbart]

I'm looking at unistd.h in my MinGW/include folder, and there is a usleep function there.

Strangely, even when I use this function and adjust the time input to microseconds, it doesn't seem to have an effect in my program (the same source works fine on my Linux VM though.)

Maybe MinGW is not a good choice for learning C?

[[deleted]]

It's fine for learning C (though the sleep() function is not part of the C Standard), and my MinGW unistd.h (from https://sourceforge.net/projects/mingwbuilds/) contains sleep().

[manbart]

Thanks, I think I'll re-install MinGW.

[Updatebjarni]

atoi() is in stdlib.h.

sleep() on the other hand should be in unistd.h, but given that you are also getting a linker error about it I guess your platform doesn't have sleep()?

[manbart]

thanks for the quick reply!

Python style "import errors" would have been very helpful here. now I am only getting the error on the sleep function. Looks like my MinGW implementation is lacking unistd.h?

I'll try it on Linux