Implicit declaration of scandir; alphasort is undeclared

Question

I am trying to use scandir to print a list of files in the current directory. When I try to compile, I am receiving the following errors and warnings:

warning: implicit declaration of function ‘scandir’ [-Wimplicit-function-declaration]
error: ‘alphasort’ undeclared (first use in this function)
note: each undeclared identifier is reported only once for each function it appears in

I am including <dirent.h>, which to my knowledge should define scandir() and all related functions. And I don't see any errors in my code:

#include <dirent.h>
...
int printFiles(){
    struct dirent **nameList;
    int numOfFiles = scandir(".", &nameList, 0, alphasort);

    //TODO print file names
    return numOfFiles;
}
....

I am running Ubuntu 12.04, and I'm compiling using gcc with the -c99 flag.

Am I simply overlooking something? I can't figure out why it's failing to compile.

Answer 1

If you use -std=c99, only functions that are strictly a part of the C99 standard are included by the header files. scandir() is not in the C99 standard. Therefore, you have to set a preprocessor variable to ensure that the function prototype is included. For example, the man page for scandir() indicates that setting the _BSD_SOURCE or _SVID_SOURCE preprocessor variables before you do the #include will fix the problem. Or, you can use #define _GNU_SOURCE which will in turn set quite a few different variables for you (including _BSD_SOURCE and _SVID_SOURCE).

Your code will still compile with the warning and work because C allows you to compile with implicitly defined functions, and the linker will correctly link the call to scandir() to the proper function.

Answer 2

what macro you use is decided by the macro in dirent.h on your own computer.usually it located in /usr/include/dirent.h.

1. find scandir in dirent.h ,you will find the macro which make scandir Invisible.for example in my conmputer,it is "__USE_BSD ,__USE_MISC".

   #if defined __USE_BSD || defined __USE_MISC
   /* Return the file descriptor used by DIRP.  */
   extern int dirfd (DIR *__dirp) __THROW __nonnull ((1));
   
   # if defined __OPTIMIZE__ && defined _DIR_dirfd
   #  define dirfd(dirp)   _DIR_dirfd (dirp)
   # endif
   
   # ifndef MAXNAMLEN
   /* Get the definitions of the POSIX.1 limits.  */
   #  include <bits/posix1_lim.h>
   
   /* `MAXNAMLEN' is the BSD name for what POSIX calls `NAME_MAX'.  */
   #  ifdef NAME_MAX
   #   define MAXNAMLEN    NAME_MAX
   #  else
   #   define MAXNAMLEN    255
   #  endif
   # endif
   
   # define __need_size_t
   # include <stddef.h>
   
   # ifndef __USE_FILE_OFFSET64
   extern int scandir (__const char *__restrict __dir,
       struct dirent ***__restrict __namelist,
       int (*__selector) (__const struct dirent *),
       int (*__cmp) (__const void *, __const void *))
   __nonnull ((1, 2));
   # else
   #  ifdef __REDIRECT
   extern int __REDIRECT (scandir,
       (__const char *__restrict __dir,
       struct dirent ***__restrict __namelist,
       int (*__selector) (__const struct dirent *),
       int (*__cmp) (__const void *, __const void *)),
       scandir64) __nonnull ((1, 2));
   #  else
   #  define scandir scandir64
   #  endif
   # endif
   

   you know what to do now? don't be worry!it not a nice way to #define this macro directly.

2. open file features.h(located /usr/include/features.h),search "__USE_MISC" inside,you can see codes below:

   #if defined _BSD_SOURCE || defined _SVID_SOURCE
   # define __USE_MISC 1
   #endif
   

   it tells that if"_BSD_SOURCE "or"_SVID_SOURCE"is defined,then __USE_MISC would be defined auto.

3. consider Compatibility of you own codes,add statement below(any one or both) at start(before any statement like #include <.h>||".h") of you file which will use scandir().

   #define _BSD_SOURCE  1
   #define _SVID_SOURCE 1
   

4. save your file and make.

Answer 3

try to #include <sys/dir.h> file to use scandir and define extern int alphasort(const void*,const void*); or extern int alphasort(); above your printFiles

also - you should link you program with standard library (hope it is already done)