head

gistfile1.c

#include <stdio.h>
#include <stdlib.h>
#define _GNU_SOURCE
#include <getopt.h>


static void do_head(FILE *f, long nlines);

#define DEFAULT_N_LINES 10

static struct option longopts[] = { 
  { "lines", required_argument, NULL, 'n' },
  { "help", no_argument, NULL, 'h' },
  { 0, 0, 0, 0 }
};

int main(int argc, char const *argv[])
{

  int opt;
  int nlines = DEFAULT_N_LINES;

  while ((opt = getopt_long(argc, argv, "n:", longopts, NULL)) != -1) {
    switch (opt) {
      case 'n':
        nlines = atoi(optarg);
        break;
      case 'h':
        fprintf(stdout, "Usage: %s [-n LINES] [FILE ...]¥n", argv[0]);
        exit(0);
      case '?':
        fprintf(stderr, "Usage: %s [-n LINES] [FILE ...]¥n", argv[0]);
        exit(1);
    }
  }

  if (optind == argc) {
    do_head(stdin, nlines);
  } else {
    int i;
    for (i = optind; i < argc; i++) {
      FILE *f = fopen(argv[i], 'r');
      if (!f) {
        perror(argv[i]);
        exit(1);
      }
      do_head(f, nlines);
      fclose(f);
    }
  }

  exit(0);

}

static void do_head(FILE* f, long nlines)
{
  
  if (nlines <= 0) {
    return;
  }
  int c;
  while ((c = getc(f)) != EOF) {
    if (putchar(c) < 0) {
      exit(1);
    }
    if (c == '\n') {
      nlines--;
      if (nlines == 0) {
        return;
      }
    }
  }

}