recvfrom

recvfrom is a function in the Berkeley sockets API used to receive data from a socket, with the ability to obtain the address of the sender. It is especially common for datagram sockets (UDP) where knowing the source is essential for responding or logging, but it can be used with other socket types as well. The function fills a user-provided buffer with incoming data and, if requested, stores the sender's address in a separate buffer.

In POSIX systems, the typical prototype is ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags, struct sockaddr *src_addr, socklen_t *addrlen). Windows uses a similar form with different types and return conventions. The parameters are: sockfd, the socket descriptor; buf, the buffer to receive data; len, the maximum number of bytes to read; flags, controlling behavior (for example, MSG_PEEK to peek at data without removing it); src_addr, an optional buffer to receive the sender’s address; and addrlen, a pointer to a socklen_t that should initially hold the size of the buffer pointed to by src_addr and, on return, hold the actual length of the address.

Return value is the number of bytes received, or -1 (or SOCKET_ERROR on Windows) on error. In blocking mode, the call waits for data; in non-blocking mode, it may return -1 with EAGAIN or EWOULDBLOCK if no data is available. If src_addr is non-NULL, the function writes the sender’s address (up to the length specified by addrlen). If the socket is connected, the behavior of src_addr may reflect the connected peer, or be unused depending on implementation.

Compared with recv, recvfrom adds the ability to capture the sender’s address, which is particularly useful for UDP servers and similar applications.