head.c

   1 /* `head.c` - copy the first part of files
   2    Copyright (c) 2022, Alan Potteiger
   3    See `LICENSE` for copyright and license details */
   4
   5 #define _POSIX_C_SOURCE 200809L
   6
   7 #include <stdio.h>
   8 #include <stdlib.h>
   9 #include <string.h>
  10 #include <errno.h>
  11 #include <unistd.h>
  12
  13 static const char *usage = {
  14         "usage: head [-n number] [file...]\n"
  15 };
  16
  17 static char *strstdin = "stdin";
  18
  19 int
  20 main(int argc, char *argv[])
  21 {
  22         char ch;
  23         char *linep;
  24         int n, i, mult;
  25         ssize_t s;
  26         size_t linecapp;
  27         FILE *f;
  28
  29         linep = NULL;
  30         n = 10;
  31         s = 0;
  32         i = mult = 0;
  33         linecapp = 0;
  34
  35         while ((ch = getopt(argc, argv, "n:")) != -1) {
  36                 switch (ch) {
  37                 case 'n':
  38                         linep = optarg;
  39                         n = (int) strtol(optarg, &linep, 10);
  40                         if (optarg == linep) {
  41                                 fprintf(stderr, "head: invalid line number\n");
  42                                 return 1;
  43                         }
  44
  45                         if (n < 1) {
  46                                 fprintf(stderr, "head: number is required to "
  47                                         "be a positive decimal integer.\n");
  48                                 return 1;
  49                         }
  50                         linep = NULL;
  51                         break;
  52                 case '?':
  53                 default:
  54                         fputs(usage, stderr);
  55                         return 1;
  56                 }
  57         }
  58
  59         argc -= optind;
  60         argv += optind;
  61
  62         /* head of stdin */
  63         if (argc < 1) {
  64                 while (n > 0) {
  65                         s = getline(&linep, &linecapp, stdin);
  66                         if (s < 0)
  67                                 break;
  68                         fwrite(linep, s, 1, stdout);
  69                         n--;
  70                 }
  71                 free(linep);
  72                 return 0;
  73         }
  74
  75         if (argc > 1)
  76                 mult = 1;
  77
  78         /* head for listed input files */
  79         for (;argc > 0; argc-- && argv++) {
  80                 if (argv[0][0] == '-' && argv[0][1] == '\0') {
  81                         f = stdin;
  82                         argv[0] = strstdin;
  83                 } else {
  84                         f = fopen(argv[0], "r");
  85                 }
  86                 if (f == NULL) {
  87                         fprintf(stderr, "head: %s: %s\n", argv[0],
  88                                                 strerror(errno));
  89                         break;
  90                 }
  91
  92                 if (mult == 1)
  93                         printf("==> %s <==\n", argv[0]);
  94                 if (mult > 1)
  95                         printf("\n==> %s <==\n", argv[0]);
  96
  97                 i = n;
  98                 while (i > 0) {
  99                         s = getline(&linep, &linecapp, f);
 100                         if (s < 0)
 101                                 break;
 102                         fwrite(linep, s, 1, stdout);
 103                         i--;
 104                 }
 105
 106                 mult++;
 107                 fclose(f);
 108         }
 109         free(linep);
 110
 111         return 0;
 112 }
 113