recvfrommsg

recvfrommsg is a system call introduced in Linux kernels as part of the recvmsg interface to receive messages from datagram sockets. The call extends the older recvfrom and recvmsg by providing a single entry point that supports receiving both data and ancillary (control) data, along with the sender’s address, in one atomic operation. It is commonly used in high‑performance network stacks, such as the Linux kernel’s networking stack for raw sockets, UDP, or DCCP, where efficient receipt of large payloads with optional control information is required.

The system call signature is typically recovered from the kernel source as follows: int recvfrommsg(int sockfd, struct msghdr *msg, int flags). The sockfd specifies the socket descriptor, msg is a pointer to an msghdr structure that holds buffers for payload and ancillary data, and flags can set behavior such as MSG_DONTWAIT or MSG_CMSG_CLOEXEC. The msghdr structure contains pointers to the iovec array for the payload, a buffer for ancillary data, and fields for the address of the message sender. On return, the call fills the iovec buffers, the sender address, and the ancillary data header, and returns the number of bytes received or –1 on error, with errno set accordingly.

recvfrommsg is particularly useful for applications that need to inspect ancillary data such as QoS information or IP timestamps, or that require zero‑copy mechanisms to avoid data duplication between kernel and user space. It also supports receiving multiple message parts with a single call through the use of MSG_WAITFORONE. Because it is not part of the POSIX standard, it is limited to Linux environments, but many networking libraries have wrappers to expose its functionality to application developers.