UNIX / Linux IPC：从管道读取.如何在运行时知道数据长度？

从管道(7)手册页：

If all file descriptors referring to
the write end of a pipe have been
closed,then an attempt to read(2)
from the pipe will see end-of-file
(read(2) will return 0).

以下示例取自pipe(2)联机帮助页并进行了反转,以便子进行写入,父进行读取(只是为了确定).我还添加了一个可变大小的缓冲区.孩子会睡5秒钟.延迟将确保子项的exit()与pipeio无关(父项将在子项退出之前打印完整的行).

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

char *
slurpfd(int fd)
{
    const int bytes_at_a_time = 2;
    char *read_buffer = NULL;
    int buffer_size = 0;
    int buffer_offset = 0;
    int chars_io;
    while (1) {
      if (buffer_offset + bytes_at_a_time > buffer_size) {
        buffer_size = bytes_at_a_time + buffer_size * 2;
        read_buffer = realloc(read_buffer,buffer_size);
        if (!read_buffer) {
          perror("memory");
          exit(EXIT_FAILURE);
        }
      }

      chars_io = read(fd,read_buffer + buffer_offset,bytes_at_a_time);
      if (chars_io <= 0) break;
      buffer_offset += chars_io;
    }

    if (chars_io < 0) {
      perror("read");
      exit(EXIT_FAILURE);
    }

    return read_buffer; /* caller gets to free it */
}

int
main(int argc,char *argv[])
{
  int pipefd[2];
  pid_t cpid;

  assert(argc == 2);

  if (pipe(pipefd) == -1) {
    perror("pipe");
    exit(EXIT_FAILURE);
  }

  cpid = fork();
  if (cpid == -1) {
    perror("fork");
    exit(EXIT_FAILURE);
  }

  if (cpid == 0) {     /* Child writes argv[1] to pipe */
    close(pipefd[0]);  /* Close unused read end */

    write(pipefd[1],argv[1],strlen(argv[1]) + 1);

    close(pipefd[1]);  /* Reader will see EOF */
    /* sleep before exit to make sure that there
       will be a delay after the parent prints it's
       output */
    sleep(5);
    exit(EXIT_SUCCESS);
  } else {             /* Parent reads from pipe */
    close(pipefd[1]);  /* Close unused write end */

    puts(slurpfd(pipefd[0]));

    close(pipefd[0]);
    wait(NULL);        /* Wait for child */
    _exit(EXIT_SUCCESS);
  }
}

从您的评论中我现在看到,您可能希望在数据可用时读取数据,更新UI或其他任何内容,以反映系统的状态.为此,在非阻塞(O_NONBLOCK)模式下打开管道.重复读取任何可用的东西,直到-1返回并且errno == EAGAIN并进行解析.重复unil读取返回0,表示子项已关闭管道.

要为File *函数使用内存缓冲区,可以在GNU C库中使用fmemopen().