readn,writen,readline函數

字節流套接字上的read和write函數所表現的行爲不同於通常的文件I/O。字節流套接字上調用的read和write輸入或輸出的字節數可能比請求的數量少，然而這不是出錯的狀態，這個現象的原因在於內核中用於套接字的緩衝區可能已達到了極限， 此時所需要的是調用者再次調用read和write函數，以輸入或輸出剩餘的字節。爲防止萬一，不讓實現返回一個不足的字節計數值，我們總是改爲調用readn和writen函數來實現：如下：

#include "unp.h"
ssize_t readn(int filedes, void *buff, size_t nbytes);
ssize_t writen(int filedes, const void *buf, size_t nbytes);
ssize_t readline(int filedes, void *buff, size_t maxlen);

函數實現如下：

#include "unp.h"
ssize_t 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;
            else 
                return -1;
        }else if(nread == 0)
            break;
        nleft -= nread;
        ptr += nread;
    }
    return (n-nleft);
}

#include "unp.h"
ssize_t writen(int fd, const void *vptr, size_t n)
{
    size_t nleft;
    ssize_t nwriten;
    const char *ptr;
    ptr = vptr;
    nleft = n;
    while(nleft > 0){
        if((nwrite = write(fd, ptr,nleft)) <= 0){
            if(nwriten < 0 && errno == EINTR)
                nwriten = 0;
            else
                return -1;
        }
        nleft -= nwriten;
        ptr += nwriten;
    }
    return n;
}

#include "unp.h"
ssize_t readline(int fd, void *vptr, size_t maxlen)
{
    ssize_t n, rc;
    char c, *ptr;
    ptr = vptr;
    for(n = 1; n < maxlen; n++){
        again:
            if((rc = read(fd, &c, 1)) == 1){
                *ptr++ = c;
                if(c == '\n')
                    break;
            }else if(rc == 0){
                *ptr = 0;
                return (n-1);
            }else{
                if(errno == EINTR)
                    goto again;
                return -1;
            }
    }
    *ptr = 0;
    return n;
}