readn.c

#include <errno.h>
#include <unistd.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; /* 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