为什么我得到：未解析的外部符号错误 - C.

I try to compile this code:

我尝试编译这段代码:

static uint64_t
push(int fd, SOCKET sock, SSL *ssl, const char *buf, uint64_t len)
{
    uint64_t    sent;
    int     n, k;

    sent = 0;
    while (sent < len) {

        /* How many bytes we send in this iteration */
        k = len - sent > INT_MAX ? INT_MAX : (int) (len - sent);

        if (ssl != NULL) {
            n = SSL_write(ssl, buf + sent, k);
        } else if (fd != -1) {
            n = write(fd, buf + sent, k);
        } else {
            n = send(sock, buf + sent, k, 0);
        }

        if (n < 0)
            break;

        sent += n;
    }

    return (sent);
}
stat