system () in stdio or stdlib?

Question

I used the system("pause") with stdio.h and it worked without error. When I looked at the stdio functions, system() is in stdlib. How come it worked, and here is the code?

#include <stdio.h>

int main() {
    printf("Hello World\n" );
    system("pause");
    return 0;
}

Sounds like a dupe of: http://stackoverflow.com/questions/4800102/not-including-stdlib-h-does-not-produce-any-compiler-error — mwcz, Jan 31 '12 at 15:51
You have to understand the fundamental difference between "if" and "only if". Standard documents usually say, "If you do X, you get Y." That *does not say anything* about what happens if you *don't* do X. — Kerrek SB, Jan 31 '12 at 15:51
Generally it's better to avoid using system calls [link](http://stackoverflow.com/questions/900666/system-calls-in-c-and-their-roles-in-programming)
[link](http://www.gidnetwork.com/b-61.html)
Calling system in windows is basically the same as running cmd.exe and typing pause. — k3oy, Jan 31 '12 at 16:07
@KerrekSB, in C an header must provide only the declarations and definitions it has to (in C++ an header can provide more). — AProgrammer, Jan 31 '12 at 16:40
@AProgrammer: Very interesting, thanks! I guess that makes sense, since there are no namespaces and you don't want to permit silent pollution with names you didn't expect. — Kerrek SB, Jan 31 '12 at 16:47
@KerrekSB, I think it also has to do with template definitions dragging in implementation details. — AProgrammer, Jan 31 '12 at 17:01

score 11 · Accepted Answer · answered Jan 31 '12 at 15:51

11

The answer is that it's an implicit declaration. If the compiler doesn't see a prototype for a function, it assumes it was declared like:

int system();

If you turn up the warning level on your compiler, you'll likely see that this causes a warning. Implicit declarations are generally undesirable, but in this case it's why this works without causing any errors.

answered Jan 31 '12 at 15:51

FatalError

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The current C language does not have implicit declarations. The compiler should give an error. – R.. GitHub STOP HELPING ICE Jan 31 '12 at 18:30

score 0 · Answer 2 · answered Jan 31 '12 at 15:51

0

According to the manpages, it is in stdlib.h

answered Jan 31 '12 at 15:51

C--

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njsg · Answer 3 · 2012-01-31T16:34:38.413

0

#include only carries the function declaration (prototype), the functionality is provided by a library, that's included in the linking stage.

Even if you do not #include it, as far as the definition assumed by the compiler when compiling matches the one in the library carrying it, there is no error and it will work.

edited Jan 31 '12 at 16:34

answered Jan 31 '12 at 15:52

njsg

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the prototype is a *declaration*, not a *definition*. The definition is provided by the C library in this case. – FatalError Jan 31 '12 at 15:53
you're right, my bad. read that as `s/definition/declaration/`. – njsg Jan 31 '12 at 16:00
@njsg: please feel free to edit your answer and correct the error – Paul R Jan 31 '12 at 16:21

score 0 · Answer 4 · answered Jan 31 '12 at 15:52

0

From the Standard at 4.10.4.5 The system function is in stdlib.h:

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

answered Jan 31 '12 at 15:52

ipc

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system () in stdio or stdlib?

4 Answers4