warning implicit declaration of function

Question

My compiler  GCC  is giving me the warning      warning  implicit declaration of function   Please help me understand why is it coming

User · Accepted Answer

You are using a function for which the compiler has not seen a declaration ("prototype") yet.

For example:

int main()
{
    fun(2, "21"); /* The compiler has not seen the declaration. */       
    return 0;
}

int fun(int x, char *p)
{
    /* ... */
}

You need to declare your function before main, like this, either directly or in a header:

int fun(int x, char *p);

User · Answer

Don t forget  if any functions that are called in your function and their prototypes must be situated above your function in the code otherwise the compiler might not find them before it attempts to compile your function  This will generate the error in question

User · Answer

You need to declare the desired function before your main function    include  lt stdio h gt  int yourfunc void    int main void        yourfunc

User · Answer

I think the question is not 100  answered  I was searching for issue with missing typeof    which is compile time directive   Following links will shine light on the situation   https   gcc gnu org onlinedocs gcc-5 3 0 gcc Typeof html  https   gcc gnu org onlinedocs gcc-5 3 0 gcc Alternate-Keywords html Alternate-Keywords  as of conculsion try to use   typeof     instead  Also gcc     -Dtypeof   typeof       can help

User · Answer

The right way is to declare function prototype in header    Example   main h   ifndef MAIN H  define MAIN H  int some main const char  name     endif   main c   include  main h   int main         some main  Hello  World n       int some main const char  name        printf   s   name       Alternative with one file  main c   static int some main const char  name    int some main const char  name           do something

User · Answer

When you get the error  implicit declaration of function it should also list the offending function  Often this error happens because of a forgotten or missing header file  so at the shell prompt you can type man 2 functionname and look at the SYNOPSIS section at the top  as this section will list any header files that need to be included  Or try http   linux die net man  This is the online man pages they are hyperlinked and easy to search  Functions are often defined in the header files  including any required header files is often the answer  Like cnicutar said       You are using a function for which the compiler has not seen a   declaration   prototype   yet

User · Answer

When you do your  includes in main c  put the  include reference to the file that contains the referenced function at the top of the include list  e g  Say this is main c and your referenced function is in  SSD1306 LCD h     include  SSD1306 LCD h       include  system h          include  lt stdio h gt   include  lt stdlib h gt   include  lt xc h gt   include  lt string h gt   include  lt math h gt   include  lt libpic30 h gt           http   microchip wikidot com faq 74  include  lt stdint h gt   include  lt stdbool h gt   include  GenericTypeDefs h      This has the  BYTE  type definition   The above will not generate the  implicit declaration of function  error  but below will-    include  system h           include  lt stdio h gt   include  lt stdlib h gt   include  lt xc h gt   include  lt string h gt   include  lt math h gt   include  lt libpic30 h gt           http   microchip wikidot com faq 74  include  lt stdint h gt   include  lt stdbool h gt   include  GenericTypeDefs h         This has the  BYTE  type definition  include  SSD1306 LCD h        Exactly the same  include list  just different order   Well  it did for me

User · Answer

If you have the correct headers defined  amp  are using a non GlibC library  such as Musl C  gcc will also throw error  implicit declaration of function when GNU extensions such as malloc trim are encountered   The solution is to wrap the extension  amp  the header    if defined    GLIBC      malloc trim 0    endif

[c] warning: implicit declaration of function

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