How can I read a directory in a C program?

See if you can use the opendir and readdir functions, which are part of the POSIX standard and are available on most Unix variants. Implementations also exist for MS-DOS, VMS, and other systems. (MS-DOS also has FINDFIRST and FINDNEXT routines which do essentially the same thing, and MS Windows has FindFirstFile and FindNextFile.) readdir returns just the file names; if you need more information about the file, try calling stat. To match filenames to some wildcard pattern,
Here is a tiny example which lists the files in the current directory:
#include <stdio.h>
#include <sys/types.h>
#include <dirent.h>

main()
{
struct dirent *dp;
DIR *dfd = opendir(".");
if(dfd != NULL) {
while((dp = readdir(dfd)) != NULL)
printf("%s\n", dp->d_name);
closedir(dfd);
}
return 0;
}

(On older systems, the header file to #include may be <direct.h> or <dir.h>, and the pointer returned by readdir may be a struct direct *. This example assumes that "." is a synonym for the current directory.)
In a pinch, you could use popen to call an operating system list-directory program, and read its output. (If you only need the filenames displayed to the user, you could conceivably use system