The fastest way on Linux (the question is tagged as Linux), is to use a direct system call. Here's a little program that counts files (only, no directories) in a directory. You can count millions of files and it is around 2.5 times faster than "ls -f" and around 1.3-1.5 times faster than Christopher Schultz's answer.
#define _GNU_SOURCE
#include <dirent.h>
#include <stdio.h>
#include <fcntl.h>
#include <stdlib.h>
#include <sys/syscall.h>
#define BUF_SIZE 4096
struct linux_dirent {
long d_ino;
off_t d_off;
unsigned short d_reclen;
char d_name[];
};
int countDir(char *dir) {
int fd, nread, bpos, numFiles = 0;
char d_type, buf[BUF_SIZE];
struct linux_dirent *dirEntry;
fd = open(dir, O_RDONLY | O_DIRECTORY);
if (fd == -1) {
puts("open directory error");
exit(3);
}
while (1) {
nread = syscall(SYS_getdents, fd, buf, BUF_SIZE);
if (nread == -1) {
puts("getdents error");
exit(1);
}
if (nread == 0) {
break;
}
for (bpos = 0; bpos < nread;) {
dirEntry = (struct linux_dirent *) (buf + bpos);
d_type = *(buf + bpos + dirEntry->d_reclen - 1);
if (d_type == DT_REG) {
// Increase counter
numFiles++;
}
bpos += dirEntry->d_reclen;
}
}
close(fd);
return numFiles;
}
int main(int argc, char **argv) {
if (argc != 2) {
puts("Pass directory as parameter");
return 2;
}
printf("Number of files in %s: %d\n", argv[1], countDir(argv[1]));
return 0;
}
PS: It is not recursive, but you could modify it to achieve that.