test2.c

这个是学习嵌入式开发的重要例子

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <errno.h>

#define BUFFER_SIZE	1024

void child_process_main(int fd);
void parent_process_main(int fd);

int main(int argc, char **argv)
{
  int fds[2];

  // int pipe(int fildes[2]);
  // Upon successful completion, 0 shall be returned; otherwise, -1 shall be returned and errno set to  indi-
  // cate the error.
  if (pipe(fds) < 0)
  {
    fprintf(stderr, "Create pipe failed: %s\n", strerror(errno));
    exit(1);
  }

  fprintf(stdout, "Created a new pipe, fds[0] = %d, fds[1] = %d\n", fds[0], fds[1]);

  pid_t pid;

  if ((pid = fork()) < 0)
  {
    fprintf(stdout, "fork() failed: %s\n", strerror(errno));
    exit(1);
  }
  else if (pid == 0)
  {
    // XXX: child process close write endpoint, open read endpoint for reading
    close(fds[1]);
    child_process_main(fds[0]);
    exit(0);
  }

  // XXX: parent process close read endpoint, open write endpoint for writting
  close(fds[0]);
  parent_process_main(fds[1]);

  close(fds[0]);
  close(fds[1]);

  return 0;
}

void child_process_main(int fd)
{
  fprintf(stdout, "[%d]Read data from pipe, fd = %d\n", getpid(), fd);

  char buf[BUFFER_SIZE];
  ssize_t n;

  memset(buf, 0, BUFFER_SIZE);
  // ssize_t read(int fildes, void *buf, size_t nbyte);
  n = read(fd, buf, BUFFER_SIZE);

  if (n < 0)
  {
    fprintf(stdout, "[%d]Read from pipe failed: %s\n", getpid(), strerror(errno));
    // FIXME: handle read error
  }
  else if (n == 0)
  {
    fprintf(stdout, "[%d]Read from pipe return 0.\n", getpid());
  }
  else
  {
    fprintf(stdout, "[%d]Read %d bytes from pipe.\n", getpid(), n);
    fprintf(stdout, "[%d]Message: %s\n", getpid(), buf);

    if (strlen(buf) < 8)
    {
      fprintf(stdout, "[%d]Can not get data length.\n", getpid());
      // FIXME: read rest data.
    }
    else
    {
      int length;
      char str[BUFFER_SIZE];

      memset(str, 0, BUFFER_SIZE);
      memcpy(str, buf, 8);

      length = atoi(str);

      if ((strlen(buf)) < 8 + length)
      {
	fprintf(stdout, "[%d]There's no enough data for parsing", getpid());
	// FIXME: read rest data
      }
      else
      {
	fprintf(stdout, "[%d]We have read all required data.\n", getpid());
      }
    }
  }
}

void parent_process_main(int fd)
{
  pid_t pid;

  pid = getpid();

  fprintf(stdout, "[%d]Write data to pipe, fd = %d\n", getpid(), fd);

  char buf[BUFFER_SIZE];
  char wbuf[BUFFER_SIZE];

  memset(buf, 0, BUFFER_SIZE);
  // char *fgets(char *restrict s, int n, FILE *restrict stream);
  if (fgets(buf, BUFFER_SIZE, stdin) == NULL)
  {
    fprintf(stdout, "[%d]Read failed from stdin: %s\n", getpid(), strerror(errno));
    // FIXME: handle read from stdin failed
  }
  else
  {
    // ssize_t write(int fildes, const void *buf, size_t nbyte);
    ssize_t n;
    int length;

    length = strlen(buf);
    memset(wbuf, 0, BUFFER_SIZE);

    // int snprintf(char *str, size_t size, const char *format, ...);
    snprintf(wbuf, BUFFER_SIZE, "%08d%s", length, buf);

    fprintf(stdout, "[%d]Composed to-be-sent buffer: %s\n", pid, wbuf);

    n = write(fd, wbuf, strlen(wbuf));

    fprintf(stdout, "[%d]Write %d bytes to pipe.\n", pid, n);
  }
}