#include 
     
      #include 
      
       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; /* call read() again */            } else {                return (-1);            }        } else if (nread == 0) {            break;         /* EOF */        }        nleft -= nread;        ptr += nread;    }    return (n - nleft);} 函数功能：从一个描述符读n字节