socket上的read write 操作不同與一般的文件IO操作，socket上的用read write讀寫的字節數可能比要求的少,但這并不是錯誤，原因是socket的緩沖區可能已經達到了極限。此時所需要的就是再次調用read write 以寫入或輸出剩余的字符。這種情況在socket中很常見，但在寫字節流socket時只能在socket非堵塞的情況下才會出現，然而為預防實現萬一返回不足的字符數值，我們總是調用writen和readn函數，而不是read和write.
下面就是readn、writen函數源碼：
ssize_t??????/* Read "n" bytes from a descriptor. */
readn(int fd, void *vptr, size_t n)
{
?size_t?nleft;
?ssize_t?nread;
?char?*ptr;
?ptr = vptr;
?nleft = n;
?while (nleft > 0) {
??if ( (nread = read(fd, ptr, nleft)) < 0) {
???if (errno == EINTR)
????nread = 0;??/* and call read() again */
???else
????return(-1);
??} else if (nread == 0)
???break;????/* EOF */
??nleft -= nread;
??ptr?? += nread;
?}
?return(n - nleft);??/* return >= 0 */
}
?
?
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);
}
?
)
