c - 使用 write() 系统调用写一个完整的缓冲区

Question

我想写一个充满 BUFF_SIZE 的缓冲区使用 write system call 在 TCP 套接字上传输字节:

#include <unistd.h>
ssize_t write(int fd, const void *buf, size_t count);

documentation指出

write() writes up to count bytes from the buffer pointed buf to the file referred to by the file descriptor fd.

当然，可以通过返回值检测实际写入的字节数。但是，如果我想确保我的整个字节缓冲区都通过连接发送，那么执行此操作的好方法是什么？到目前为止，我在想:

while ( (ch = fgetc(pipe) ) != EOF )
{
    buff[ 0 ] = ch;
    bytes_in_buff++;

    // fill a buffer's worth of data from the pipe
    for (int i = 1; i < BUFF_SIZE; ++i, ++bytes_in_buff)
    {
        if ( (ch = fgetc(pipe) ) == EOF)
            break;

        buff[ i ] = ch;
    }

    // write that buffer to the pipe
    int bytes_sent = 0;
    while (bytes_sent < BUFF_SIZE)
    {
        bytes_sent = write(fd, buff, bytes_in_buff);
    }
}

当然，如果我继续每次发送整个缓冲区，就会发送一些冗余字节bytes_sent < BUFF_SIZE .

Answer 1

看看下面的函数，它围绕着 write() 循环，直到 b 中的 s 字节被写入或者出现 fatal error 发生:

int writen(const int sd, const char * b, const size_t s, const int retry_on_interrupt)
{
  size_t n = s;
  while (0 < n)
  {
    ssize_t result = write(sd, b, n);
    if (-1 == result)
    {
      if ((retry_on_interrupt && (errno == EINTR)) || (errno == EWOULDBLOCK) || (errno == EAGAIN))
      {
        continue;
      }
      else
      {
        break;
      }
    }

    n -= result;
    b += result;
  }

  return (0 < n) ?-1 :0;
}

这样调用它:

int retry_on_interrupt = ... /* {0|1} depending on wether a signal reception shall abort the write operation or not. */
int result = writen(fd, buff, sizeof(buf), retry_on_interrupt)
if (-1 == result)
{
  perror("writen()");
}