MSGWAITALL

MSGWAITALL is a flag used with the recv() system call on Linux that requests the kernel to block until the full amount of data requested by the caller has been received. When this flag is set, the function does not return until either exactly the specified number of bytes has been read, an error occurs, or a signal interrupts the call. It is primarily used with stream-oriented sockets (such as TCP) to avoid partial reads when the caller needs a complete payload.

Behavior and semantics: With MSG_WAITALL, recv() will attempt to fill the user-provided buffer up to the requested length. If the data cannot be delivered in one go due to the peer sending less data, or the connection closing, the return value may be less than requested and errno may reflect the condition, depending on the exact circumstances. If a signal interrupts the call or a non-blocking condition prevents progress, the call may fail or return with partial data according to standard error handling. The flag is designed to simplify protocols that require a fixed-size message, reducing the need for higher-layer buffering.

Portability and availability: MSG_WAITALL is a Linux-specific flag. It is not defined in the POSIX standard and may not be available on other operating systems or older kernels. Developers targeting portability should use explicit protocols with length prefixes and timeouts, or implement their own logic to read until the desired amount is obtained.

See also: MSG_PEEK, MSG_DONTWAIT, recv(2).