Tầng thấp so với stdio

int main(int argc, char **argv) {
    int bufsz = (argc > 1) ? atoi(argv[1]) : 4096;

    int fd = open("big.dat", O_RDONLY);
    char *buf = malloc(bufsz);

    long tong = 0, lan = 0;
    ssize_t n;
    clock_t t0 = clock();
    while ((n = read(fd, buf, bufsz)) > 0) {
        tong += n;
        ++lan;
    }
    clock_t t1 = clock();

    printf("buffer %6d byte: %ld lan read, %ld byte, %.0f ms\n",
           bufsz, lan, tong, 1000.0 * (t1 - t0) / CLOCKS_PER_SEC);

    close(fd);
    free(buf);
    return 0;
}

#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>

int main(void) {
    int fd = open("fd.txt", O_WRONLY | O_CREAT | O_TRUNC, 0644);

    FILE *f = fdopen(fd, "w");       /* fd -> FILE*: boc de dung fprintf */
    fprintf(f, "ghi qua FILE* boc quanh fd\n");
    fclose(f);                        /* dong ca FILE va fd ben trong */

    FILE *r = fopen("fd.txt", "r");
    int rfd = fileno(r);             /* FILE* -> fd: lay so fd ben trong */
    printf("fileno cua FILE* = %d\n", rfd);

    char buf[64];
    fgets(buf, sizeof buf, r);
    printf("noi dung: %s", buf);
    fclose(r);
    return 0;
}