pager.c

#include <unistd.h>
#include <stdio.h>
#include <sys/wait.h>
#include <errno.h>
#include <stdlib.h>

/* Usage:
$ clang -o pager main.c
$ ./pager 200
*/

#define READ_END 0
#define WRITE_END 1

void perror_die(char* msg) {
  perror(msg);
  exit(EXIT_FAILURE);
}

void print_n_lines(int n) {
  for (int i = 0; i < n; i++) {
    printf("%d\n", i);
  }
}

int main(int argc, char *argv[]) {
  int fds[2];
  if (pipe(fds) == -1) {
    perror_die("pipe");
  }

  int child_pid = fork();
  switch (child_pid) {
  case -1: {
    /* fork failed */
    perror_die("fork");
    break;
  }
  case 0: {    /* Child(pager) */
    /* Pager process doesn't write to pipe */
    close(fds[WRITE_END]);

    /* Make READ_END of pipe pager's STDIN */
    dup2(fds[READ_END], STDIN_FILENO);

    /* F -> quit-if-one-screen */
    /* R -> preserve color formatting */
    /* X -> don't send some special instructions eg. to clear terminal screen before starting */
    char *less_argv[] = {"less", "-FRX", NULL};
    int exec_status = execvp(less_argv[0], less_argv);

    fprintf(stderr,
            "execvp failed with status: %d and errno: %d\n", exec_status, errno);
    break;
  }
  default: { /* Parent */
    /* STDOUT is now fds[WRITE_END] */
    dup2(fds[WRITE_END], STDOUT_FILENO);
    /* parent doesn't read from pipe */
    close(fds[READ_END]);

    /* "Business" logic which determines what to actually print */
    int num_lines = 1024;
    if (argc > 1) {
      num_lines = atoi(argv[1]);
    }
    print_n_lines(num_lines);
    
    fflush(stdout);

    /* Signal EOF to the pager process */
    close(STDOUT_FILENO);

    int stat_loc;
    waitpid(child_pid, &stat_loc, 0);
    break;
  }
  }
}

This example didn't work for me until I flushed all output in the buffer before the call to close(2).
fflush(stdout) did the trick.

Thanks, fixed.

Actually, for this purpose it's better to call fdopen() for fds[1], use that for fprintf and call fclose at the end.

I experimented with this here.