Ошибка implicit declaration of function

“A young man is smoking one cigarette after each other without a pause. An elderly woman observes that and says: “Young man, you are smoking like crazy! Don’t you know that there is a warning on each cigarette package that this can kill you?” The young man finishes his cigarette, looks at the elderly person and says: “Yes, I know. But look, I’m a programmer, and it is only a warning.”

I don’t smoke, and I do pay attention to warnings :-). I always try to keep my source code free of compiler warnings. And I always pay special attention to the following on:

So what does the gcc warning ‘implicit declaration of function’ mean (other compiler report a ‘implicit parameter declaration’)? Basically it means that the compiler has found a call to function for which he does not have a prototype. For the case it missed a declaration like this:

 
errorCode_t calcMatrix(matrix_t *m, double weight);

Either because I did not provide a ‘forward declaration’ of it or I missed to include the header file with that declaration, or that declaration is missing in the header file.

Without knowing the interface, the compiler has to assume (implicitly) the interface from the actual parameters used, and that the function returns an int. So the compiler has to assume according to the rules:

int calcMatrix(int*, int); 

Which of course is wrong in many ways: not only the parameter 3 is not converted to a double, it means that the wrong size (int vs. double) is assumed. If that parameter is passed on the stack it will result in a stack space mismatch, so overall very bad. It might ‘work’ if the types by chance match the actual implementation or will be less of a runtime problem if parameters and return values are passed in registers. Still, it is clearly wrong and bad, bad, bad. It might be only a warning if the linker does not find that symbol (calcMatrix in this case) somewhere, so if you do not pay attention to warnings, your application might crash or behave in wrong way.

Such an ‘implicit declaration’ is really an oversight or error by the programmer, because the C compiler needs to know about the types of the parameters and return value to correctly allocate them on the stack. Unfortunately in C this is not an error but a warning (for legacy reasons, to be able to compile old non-compliant code). The good news is that compiling such a thing with in C++ will produce an error, another good reason to compile plain C code with C++ mode.

The other good news is that every C compiler usually has an option to turn this warning into an error (what I recommend if you are not paying attention to warnings). From the gcc help page:

-Werror-implicit-function-declarationGive a warning (or error) whenever a function is used before being declared. The form -Wno-error-implicit-function-declaration is not supported. This warning is enabled by -Wall (as a warning, not an error).

With this option added to the compiler settings it gets flagged as an error:

Problem solved.

Happy warning 🙂

Links

• GNU gcc warning options: Warning Options – Using the GNU Compiler Collection (GCC)

This tutorial discusses removing the warning of implicit function declaration by declaring the functions above the main function in C.

Implicit Declaration of Function in C

Sometimes, the compiler shows a warning of implicit declaration of the function in C language, which means that the function is not declared on top of the main() function or its header file is not included.

For example, the printf() function belongs to the stdio.h header file, and if we don’t include it before using it in a C source file, the compiler will show a warning that the function declaration is implicit.

In this case, we have to include the header file, which includes the function declaration or declare the function above the main() function. For example, let’s use the printf() function without including its header file.

See the code below.

int main()
{
    int a = 10;
    printf("a = %d",a);
    return 0;
}

Output:

main.c: In function 'main':
main.c:5:5: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
    5 |     printf("a = %d",a);
      |     ^~~~~~
main.c:5:5: warning: incompatible implicit declaration of built-in function 'printf'
main.c:1:1: note: include '<stdio.h>' or provide a declaration of 'printf'
  +++ |+#include <stdio.h>
    1 |
a = 10

In the above output, we can see that the compiler gave a warning saying that the declaration of the printf() function is implicit and we have to include the <stdio.h> file, or we must provide a declaration of the printf() function in the source file.

We can also see that the value of a is printed, which means the printf() function worked perfectly even though we have not included its header file.

Now, let’s include the header file and repeat the above example. See the code below.

#include <stdio.h>

int main()
{
    int a = 10;
    printf("a = %d",a);
    return 0;
}

Output:

We can see above the warning of the implicit function declaration is not shown this time because we have included the header file for the printf() function.

The implicit function declaration warning will also be shown if we have created a function in a source file but have not declared it above the main() function.

The compiler warns when we try to call the function that the function declaration is implicit. For example, let’s create a function and call it from the main() function without declaring it above the main() function.

#include <stdio.h>

int main()
{
    int a = fun(2, 3);
    printf("a = %d",a);
    return 0;
}

int fun(int x, int p)
{
    int a = x+p;
    return a;
}

Output:

main.c: In function 'main':
main.c:4:13: warning: implicit declaration of function 'fun' [-Wimplicit-function-declaration]
    4 |     int a = fun(2, 3);
      |             ^~~
a = 5

In the above output, we can see that the warning is shown for the function we created. We have to declare the function above the main() function to solve this problem.

For example, let’s declare the function above the main() function and repeat the above example. See the code below.

#include <stdio.h>

int fun(int x, int p);
int main()
{
    int a = fun(2, 3);
    printf("a = %d",a);
    return 0;
}

int fun(int x, int p)
{
    int a = x+p;
    return a;
}

Output:

We can see that the warning is now gone. We can also declare the function in a header file and then include the header file in the source file, which is useful in the case of many functions because it will simplify the code.

1. 01-26-2006

   
   

   #1

   

   
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   Implicit declaration of function …???
   

   Hi,
   I keep getting the warning of:

   Code:

    Implicit declaration of funciton getline()

   whenever i use getline() function

   What does this warning mean actually ?
   Is this a serious warning, or just ignorable.

   Cheer!!

   ---

2. 01-26-2006

   
   

   #2

   

   
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   getline() is c++ not C. There is no standard C getline() function.

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3. 01-26-2006

   
   

   #3

   

   
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   getline() is also a GNU provided extension available under #include <stdio.h>. It will not appear if you use the -ansi flag, because the -ansi flag excludes all GNU extensions. It’s not part of standard C, as Ancient Dragon said.

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4. 01-26-2006

   
   

   #4

   

   
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   replace getline() with fgets() and your program will (maybe) work.

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5. 01-27-2006

   
   

   #5

   

   
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   What’s happening is that you have tried to use a function called getline() that hasn’t been declared anywhere (eg in a header file) and the compiler takes your attempt to call it as an implicit declaration.

   As other have sort of said, your real problem is probably that you’re calling a function that doesn’t exist and you need to work out what function you really intend to call. If the function is one you actually know you’re trying to call, find the corresponding header file and #include it.

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6. 01-28-2006

   
   

   #6

   

   
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   Code:

   $ grep -n getline /usr/include/*.h

   

   If you’re trying to use the C++ getline, then you need <iosteam> etc.

   dwk

   Seek and ye shall find. quaere et invenies.

   «Simplicity does not precede complexity, but follows it.» — Alan Perlis
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7. 08-07-2006

   
   

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   Sorry if I am a topic digging here, but I came across this thread while searching for the answer for exactly this problem and I noticed noone here gave the right answer.

   The answer turned out pretty simple:
   getline() IS a valid C function which is defined in stdio.h. As some people rightfully suggested it is not a function that is defined in the ANSI standard, but it is a GNU extension. In order to correctly include these extensions you have to add a define:

   Code:

   #define _GNU_SOURCE
   #include <stdio.h>
   
   /* functions like getline() and getdelim() should be defined now */

   Ofcourse this only works if you are actually using glibc (e.g. on Linux or cygwin).
   Hope this clarifies the issue.
   — LT

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8. 08-07-2006

   
   

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   Originally Posted by Lord Thunder

   Sorry if I am a topic digging here, but I came across this thread while searching for the answer for exactly this problem and I noticed noone here gave the right answer.

   Depends on your perspective. I consider your post to be incorrect and misleading, as you make some statements early on which are compiler/library specific but you state them as generalities. Yes you correct them later, sort of, but the information you give is not generally true.

   Originally Posted by Lord Thunder

   The answer turned out pretty simple:
   getline() IS a valid C function which is defined in stdio.h.

   No. The functions that are declared in <stdio.h> are specified in the ISO C standard. They are not required to be defined in stdio.h.

   Reliance on stdio.h declaring any function that is not specified in the C standard is invalid, unless you are prepared to constrain yourself to only one compiler and it’s library (and possibly to a particular version of that compiler and library).

   Originally Posted by Lord Thunder

   As some people rightfully suggested it is not a function that is defined in the ANSI standard, but it is a GNU extension.

   Exactly my point. There are a few commonly used compilers that are produced by vendors not associated with GNU. Most of those compilers do not support GNU extensions and are not bunded with glibc.

   Originally Posted by Lord Thunder

   In order to correctly include these extensions you have to add a define:

   Code:

   #define _GNU_SOURCE
   #include <stdio.h>
   
   /* functions like getline() and getdelim() should be defined now */

   Ofcourse this only works if you are actually using glibc (e.g. on Linux or cygwin).

   Sort of. glibc, in practical terms, often relies on using a correct version of gcc (the GNU compiler collection). Several versions of glibc only work with particular versions of gcc. This is the reason that the gnu compiler and glibc are often bundled together, particularly with recent versions. Both the gnu compiler and library are supported on more systems than linux; cygwin is just one of several windows ports.

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9. 08-07-2006

   
   

   #9

   

   
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   Originally Posted by Lord Thunder

   Sorry if I am a topic digging here, but I came across this thread while searching for the answer for exactly this problem and I noticed noone here gave the right answer.

   The answer turned out pretty simple:
   getline() IS a valid C function which is defined in stdio.h. As some people rightfully suggested it is not a function that is defined in the ANSI standard, but it is a GNU extension. In order to correctly include these extensions you have to add a define:

   Code:

   #define _GNU_SOURCE
   #include <stdio.h>
   
   /* functions like getline() and getdelim() should be defined now */

   Ofcourse this only works if you are actually using glibc (e.g. on Linux or cygwin).
   Hope this clarifies the issue.
   — LT

   Why is it exactly that you are dredging up a 7 month old thread? Was the urge so strong to blabber that you could not resist? Give me a break.

   Forum Guidelines. Read before posting:

   5. Don’t bump threads. (Bumping: posting messages on threads to move them up the list or to post on a thread that has been inactive for two weeks or longer).

   Any questions?

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