"man pipe" example extended to N childs+pipes

pipeN.c

/* gcc -Wall -Wextra -pedantic pipeN.c -o pipeN
*/
#include <sys/types.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <assert.h>

int main(int argc, char *argv[])
{
  assert(argc >= 3   || !"Usage: ./pipeN N <string> d1 ... dN");
  int N = atoi(argv[1]);
  assert(argc == 3+N || !"Usage: ./pipeN N <string> d1 ... dN");
  int fd[N];
  pid_t pid[N];
  char buff[strlen(argv[2])+2];


  for(int i=0; i<N; ++i) /* setup N childs+pipes */
  {
    int pipefd[2];
    pid_t cpid;
    char buf;

    assert(pipe(pipefd) != -1);

    cpid = fork();
    assert(cpid != -1);

    if (cpid == 0) {     /* Child reads from pipe */
      close(pipefd[1]);  /* Close unused write end */

      while (read(pipefd[0], &buf, 1) > 0)
        write(STDOUT_FILENO, &buf, 1);

      close(pipefd[0]);
      _exit(EXIT_SUCCESS);
    } else {             /* Parent writes to pipe */
      close(pipefd[0]);  /* Close unused read end */

      fd[i] = pipefd[1]; /* store for ... */
      pid[i] = cpid;     /* ... later use */
    }
  }


  for(int i=0; i<N; ++i) /* send stuff to requested child */
  {
    int c = atoi(argv[3+i]);
    assert(0<=c && c<N); 

    strncpy(buff, argv[2]+c, strlen(argv[2])-N+1);
    buff[strlen(argv[2])-N+1]='\n';
    buff[strlen(argv[2])-N+2]='\0';

    write(fd[c], buff, strlen(argv[2])-N+3);
    close(fd[c]);        /* Reader will see EOF */

    usleep(10000);
  }


  for(int i=0; i<N; ++i) /* Wait for all childs */
  {
    waitpid(pid[i], NULL, 0);
  }


  exit(EXIT_SUCCESS);
}