Implicit declaration of scandir; alphasort is undeclared

问题

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.

回答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.

回答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.

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.

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.
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
```
save your file and make.

回答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)

来源：https://stackoverflow.com/questions/19439855/implicit-declaration-of-scandir-alphasort-is-undeclared

标签

c99

scandir