defined in io.h:

/*
 * Functions for searching for files. _findfirst returns -1 if no match
 * is found. Otherwise it returns a handle to be used in _findnext and
 * _findclose calls. _findnext also returns -1 if no match could be found,
 * and 0 if a match was found. Call _findclose when you are finished.
 */
int _findfirst (const char* szFilespec, struct _finddata_t* find);
int _findnext (int nHandle, struct _finddata_t* find);
int _findclose (int nHandle);

usage:

struct _finddata_t c_file;
long hFile;


if ( (hFile = _findfirst("*.c", &c_file)) == -1L ) {
    printf("No *.c files in current directory");
}
else {
    do {
        printf("%s\n", c_file.name);
    } while ( _findnext(hFile, &c_file) == 0 );
    _findclose(hFile);
}

the _finddata_t struct:

/*
 * The following structure is filled in by _findfirst or _findnext when
 * they succeed in finding a match.
 */
struct _finddata_t
{
        unsigned    attrib;             /* Attributes, see constants above. */
        time_t      time_create;
        time_t      time_access;        /* always midnight local time */
        time_t      time_write;
        _fsize_t    size;
        char       name[FILENAME_MAX];  /* may include spaces. */
};