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.