EvenLines

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
static char **read_lines(FILE *in, int *n) {
  char **lines = NULL;
  char line[BUFSIZ];
  *n = 0;
  while (fgets(line, BUFSIZ, in)) {
    lines = (char **) realloc(lines, ++(*n) * sizeof(char *));
    lines[*n - 1] = (char *) malloc(strlen(line) + 1);
    strcpy(lines[*n - 1], line);
  }
  return lines;
}
static void free_lines(char **lines, int n) {
  int i;
  for (i = 0; i < n; i++)
    free(lines[i]);
  free(lines);
}
int main(const int argc, const char * const *argv) {
  FILE *in;
  char **lines;
  int i, n;
  if (argc != 1 + 1) {
    fprintf(stderr, "usage: %s DATA_FILE\n", argv[0]);
    exit(1);
  }
  in = fopen(argv[1], "r");
  lines = read_lines(in, &n);
  for (i = 0; i < n; i += 2)
    printf("%s", lines[i]);
  free_lines(lines, n);
  return 0;
}