readn/writen/readline

字节流套接字上调用read、write输入或输出的字节数可能比请求的数量少，然而这不是出错的状态。这个现象的原因在于内核中用于套接字的缓冲区可能已达到了极限。此时需要再次调用read或write函数，以输入或输出剩余的字节。
以下三个函数就是用于此目的：

#include <unistd.h>
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include "readn.h"

int readn(int fd, void* buffer, size_t n)
{
    int nread;
    int nleft;
    char *ptr;

    ptr = buffer;
    nleft = n;
    while(nleft >0) {
        if((nread=read(fd,ptr,nleft)) < 0){
            if(errno == EINTR) {
                nread = 0;
                continue;
            }else {
                return -1;
            }
        }else if (nread==0) {
            break;
        }
        ptr += nread;
        nleft -= nread;
    }
    return (n - nleft);
}

int writen(int fd, void* buffer, size_t n)
{
    int nwrite;
    int nleft;
    char* ptr;

    ptr = buffer;
    nleft = n;
    while(nleft > 0) {
        if((nwrite = write(fd,ptr,nleft)) < 0) {
            if(errno == EINTR) {
                nwrite = 0;
                continue;
            }else {
                return -1;
            }
        }else if(nwrite == 0){
            break;
        }
        ptr += nwrite;
        nleft -= nwrite;
    }
    return (n-nleft);
}

size_t readline(int fd, void * buffer, size_t maxlen)
{
    int i;
    int nread;
    char c;
    char *ptr;

    ptr=buffer;
    for(i = 0;i < maxlen;i++) {
again:
        if((nread = read(fd,&c,1)) == 1){
            *ptr++ = c;
            if(c == '\n')
                break;
        }else if(nread == 0) {
            *ptr = 0;
            return i-1;
        }else {
            if(errno == EINTR) {
                goto again;
            }else {
                return -1;
            }
        }
    }
    *ptr = 0;
    return i;
}
static char *ptr;
static int len = 0;
char read_buffer[MAXLINE];
ssize_t my_read(int fd, char* buffer)
{
    int nread;

    if(len <= 0) {
again:
        if((nread = read(fd,read_buffer,MAXLINE)) < 0) {
            if(errno == EINTR)
                goto again;
            return -1;
        }else if(nread == 0) {
            return 0;
        }
        ptr = read_buffer;
        len = nread;
    }
    *buffer = *ptr++;
    len--;
    return 1;
}