确定准备好recv（）的字节数(Determing the number of bytes ready to be recv()'d)

确定准备好recv（）的字节数(Determing the number of bytes ready to be recv()'d)

我可以使用select（）来确定对recv（）的调用是否会阻塞，但是一旦我确定它们是要读取的字节，它是一种在我实际调用recv之前查询当前可用的字节数的方法（ ）？

I can use select() to determine if a call to recv() would block, but once I've determined that their are bytes to be read, is their a way to query how many bytes are currently available before I actually call recv()?

最满意答案

如果你的操作系统提供它（大多数都提供），你可以使用ioctl（..，FIONREAD，..）：

int get_n_readable_bytes(int fd) { int n = -1; if (ioctl(fd, FIONREAD, &n) < 0) { perror("ioctl failed"); return -1; } return n; }

Windows提供了一个类似的ioctlsocket（..，FIONREAD，..），它需要一个指向unsigned long的指针：

unsigned long get_n_readable_bytes(SOCKET sock) { unsigned long n = -1; if (ioctlsocket(sock, FIONREAD, &n) < 0) { /* look in WSAGetLastError() for the error code */ return 0; } return n; }

ioctl调用应该适用于套接字和其他一些fds，但不适用于所有fds。 我相信它可以在几乎任何你可能使用的免费类Unix操作系统上使用TCP套接字。 它的语义与UDP套接字略有不同：对于它们，它告诉你下一个数据报中的字节数。

Windows上的ioctlsocket调用（显然）只适用于套接字。

If your OS provides it (and most do), you can use ioctl(..,FIONREAD,..):

int get_n_readable_bytes(int fd) { int n = -1; if (ioctl(fd, FIONREAD, &n) < 0) { perror("ioctl failed"); return -1; } return n; }

Windows provides an analogous ioctlsocket(..,FIONREAD,..), which expects a pointer to unsigned long:

unsigned long get_n_readable_bytes(SOCKET sock) { unsigned long n = -1; if (ioctlsocket(sock, FIONREAD, &n) < 0) { /* look in WSAGetLastError() for the error code */ return 0; } return n; }

The ioctl call should work on sockets and some other fds, though not on all fds. I believe that it works fine with TCP sockets on nearly any free unix-like OS you are likely to use. Its semantics are a little different for UDP sockets: for them, it tells you the number of bytes in the next datagram.

The ioctlsocket call on Windows will (obviously) only work on sockets.