c - C 中 write(2) 的返回值 0 是错误吗？

Question

在系统调用的手册页中 write(2) -

ssize_t write(int fd, const void *buf, size_t count);

内容如下:

Return Value

On success, the number of bytes written are returned (zero indicates nothing was written). On error, -1 is returned, and errno is set appropriately. If count is zero and the file descriptor refers to a regular file, 0 may be returned, or an error could be detected. For a special file, the results are not portable.

我会将其解释为返回 0 只是意味着没有写入任何内容，无论出于何种原因。

然而，Stevens in UNP在处理作为 TCP 套接字的文件描述符时将返回值 0 视为 fatal error (这由另一个调用 exit(1) 的函数包装):

ssize_t /* Write "n" bytes to a descriptor. */
writen(int fd, const void *vptr, size_t n)
{
    size_t      nleft;
    ssize_t     nwritten;
    const char  *ptr;

    ptr = vptr;
    nleft = n;
    while (nleft > 0) {
        if ( (nwritten = write(fd, ptr, nleft)) <= 0) {
            if (nwritten < 0 && errno == EINTR)
                nwritten = 0;       /* and call write() again */
            else
                return(-1);         /* error */
        }

        nleft -= nwritten;
        ptr   += nwritten;
    }
    return(n);
}

只有当 errno 表明对 write 的调用被进程接收信号中断时，他才会将 0 视为合法的返回值。

为什么？

Answer 1

Stevens 这样做可能是为了捕捉旧的实现write() 表现不同。例如，Single Unix 规范说(http://www.opengroup.org/onlinepubs/000095399/functions/write.html)

Where this volume of IEEE Std 1003.1-2001 requires -1 to be returned and errno set to [EAGAIN], most historical implementations return zero