昨天用recvfrom去接收數據，好久沒有過這個函數，按照man的手冊，一個一個傳參，但是遇到一個問題，數據可以接收正常，但是總是errno一直爲22，並且sendto一直失敗返回-1。這裏做一下記錄，提醒自己細節的重視程度決定技術的高度。
函數原型：
ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
struct sockaddr *src_addr, socklen_t *addrlen);
前面幾個參數沒什麼好說，一搜一大把。遇到的問題就在於最後一個參數：socklen_t *addrlen。看到傳入指針，我的第一反應就是，這是一個結果參數，所以初始化完之後直接傳入。導致在之後調用sendto時，一直失敗。後來才發現recvfrom的函數說明中有下面一段話：
recvfrom()
recvfrom() places the received message into the buffer buf. The caller must specify the size of the buffer inlen.
If src_addr is not NULL, and the underlying protocol provides the source address of the message, that source address is placed in the buffer pointed to by src_addr. In this case, addrlen is a value-result argument. Before the call, it should be initialized to the size of the buffer associated with src_addr. Upon return, addrlen is updated to contain the actual size of the source address. The returned address is truncated if the buffer provided is too small; in this case, addrlen will return a value greater than was supplied to the call.
If the caller is not interested in the source address, src_addr and addrlen should be specified as NULL.
真神奇，粗心大意浪費了時間，打自己一下。。