Begin

[tl-append] / tl-append.c

#include <stdio.h>
#include <stdlib.h>

const char *FILENAME = "tl.log";
const size_t BUF_SIZE = 1024;

static void die(const char *message) {
  fputs(message, stderr);
  fputc('\n', stderr);
  exit(1);
}

static void die_err(const char *message) {
  perror(message);
  exit(1);
}

static void read_line(char *buf) {
  if (fgets(buf, BUF_SIZE, stdin) == NULL) {
    if (ferror(stdin))
      die("I/O error reading line");
    if (feof(stdin)) {
      buf[0] = '\0'; /* Unclear if fgets does this already */
      return;
    }
    die("Unexpected error reading line");
  }
}

static void write_line(const char *line) {
  FILE *f = fopen(FILENAME, "a");
  if (f == NULL)
    die_err("Error opening output file");
  if (fputs(line, f) == EOF)
    die("Error writing to output file");
  if (fclose(f) != 0)
    die_err("Error closing output file");
}

int main() {
  char buf[BUF_SIZE];
  for (read_line(buf); buf[0]; read_line(buf)) {
    write_line(buf);
  }
  return 0;
}