RECVMSG(3P)

🌸 PROLOG 🌸

okay bestie, quick heads up before we dive in: this manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (so like, go consult the corresponding Linux manual page for the deets on Linux behavior, fr), or honestly the interface may not be implemented on Linux at all. just so you know. 💅

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✨ NAME ✨

recvmsg — receive a message from a socket

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🎀 SYNOPSIS 🎀

#include <sys/socket.h>

ssize_t recvmsg(int socket, struct msghdr *message, int flags);

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🦋 DESCRIPTION 🦋

So the recvmsg() function shall receive a message from a connection-mode or connectionless-mode socket. It's normally used with connectionless-mode sockets because it lets your app retrieve the source address of the received data — which is lowkey the whole point, iykyk. ✨

The recvmsg() function takes the following arguments:

• socket — Specifies the socket file descriptor. the vibe check for "which socket are we even talking about". 💅
• message — Points to a msghdr structure, containing both the buffer to store the source address and the buffers for the incoming message. The length and format of the address depend on the address family of the socket. The msg_flags member is ignored on input, but may contain meaningful values on output. 👀
• flags — Specifies the type of message reception. Values of this argument are formed by logically OR'ing zero or more of the following values:
  • MSG_OOB — Requests out-of-band data. The significance and semantics of out-of-band data are protocol-specific. (so like, ask your protocol, not me 🫶)
  • MSG_PEEK — Peeks at the incoming message. yes it's exactly as nosy as it sounds. 🙈
  • MSG_WAITALL — On SOCK_STREAM sockets this requests that the function block until the full amount of data can be returned. The function may return the smaller amount of data if the socket is a message-based socket, if a signal is caught, if the connection is terminated, if MSG_PEEK was specified, or if an error is pending for the socket. so like... it tries to wait for all of it. sometimes it simply cannot. 😭

The recvmsg() function shall receive messages from unconnected or connected sockets and shall return the length of the message.

The recvmsg() function shall return the total length of the message. For message-based sockets, such as SOCK_DGRAM and SOCK_SEQPACKET, the entire message shall be read in a single operation. If a message is too long to fit in the supplied buffers, and MSG_PEEK is not set in the flags argument, the excess bytes shall be discarded, and MSG_TRUNC shall be set in the msg_flags member of the msghdr structure. For stream-based sockets, such as SOCK_STREAM, message boundaries shall be ignored. In this case, data shall be returned to the user as soon as it becomes available, and no data shall be discarded. 💖

If the MSG_WAITALL flag is not set, data shall be returned only up to the end of the first message.

If no messages are available at the socket and O_NONBLOCK is not set on the socket's file descriptor, recvmsg() shall block until a message arrives. If no messages are available at the socket and O_NONBLOCK is set on the socket's file descriptor, the recvmsg() function shall fail and set errno to [EAGAIN] or [EWOULDBLOCK]. no waiting around, periodt.

In the msghdr structure, the msg_name member may be a null pointer if the source address is not required. Otherwise, if the socket is unconnected, the msg_name member points to a sockaddr structure in which the source address is to be stored, and the msg_namelen member on input specifies the length of the supplied sockaddr structure and on output specifies the length of the stored address. If the actual length of the address is greater than the length of the supplied sockaddr structure, the stored address shall be truncated. If the socket is connected, the msg_name and msg_namelen members shall be ignored. The msg_iov and msg_iovlen fields are used to specify where the received data shall be stored. The msg_iov member points to an array of iovec structures; the msg_iovlen member shall be set to the dimension of this array. In each iovec structure, the iov_base field specifies a storage area and the iov_len field gives its size in bytes. Each storage area indicated by msg_iov is filled with received data in turn until all of the received data is stored or all of the areas have been filled. it's giving "an assembly line of buffers". 🏭✨

Upon successful completion, the msg_flags member of the message header shall be the bitwise-inclusive OR of all of the following flags that indicate conditions detected for the received message:

• MSG_EOR — End-of-record was received (if supported by the protocol).
• MSG_OOB — Out-of-band data was received.
• MSG_TRUNC — Normal data was truncated.
• MSG_CTRUNC — Control data was truncated.

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🌈 RETURN VALUE 🌈

Upon successful completion, recvmsg() shall return the length of the message in bytes. If no messages are available to be received and the peer has performed an orderly shutdown, recvmsg() shall return 0. Otherwise, -1 shall be returned and errno set to indicate the error. sad girl hours fr. 😭

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💀 ERRORS 💀

The recvmsg() function shall fail if:

• EAGAIN or EWOULDBLOCK — The socket's file descriptor is marked O_NONBLOCK and no data is waiting to be received; or MSG_OOB is set and no out-of-band data is available and either the socket's file descriptor is marked O_NONBLOCK or the socket does not support blocking to await out-of-band data.
• EBADF — The socket argument is not a valid open file descriptor.
• ECONNRESET — A connection was forcibly closed by a peer.
• EINTR — This function was interrupted by a signal before any data was available.
• EINVAL — The sum of the iov_len values overflows a ssize_t, or the MSG_OOB flag is set and no out-of-band data is available.
• EMSGSIZE — The msg_iovlen member of the msghdr structure pointed to by message is less than or equal to 0, or is greater than {IOV_MAX}.
• ENOTCONN — A receive is attempted on a connection-mode socket that is not connected.
• ENOTSOCK — The socket argument does not refer to a socket.
• EOPNOTSUPP — The specified flags are not supported for this socket type.
• ETIMEDOUT — The connection timed out during connection establishment, or due to a transmission timeout on active connection.

The recvmsg() function may fail if:

• EIO — An I/O error occurred while reading from or writing to the file system.
• ENOBUFS — Insufficient resources were available in the system to perform the operation.
• ENOMEM — Insufficient memory was available to fulfill the request.

The following sections are informative. (translation: cute, but not the binding contract, bestie 🫶)

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💜 EXAMPLES 💜

None.

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🎀 APPLICATION USAGE 🎀

The select() and poll() functions can be used to determine when data is available to be received. like, "hey is my snack ready yet" but for sockets. 🍓

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🧚 RATIONALE 🧚

None.

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👑 FUTURE DIRECTIONS 👑

None.

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💖 SEE ALSO 💖

poll(), pselect(), recv(), recvfrom(), send(), sendmsg(), sendto(), shutdown(), socket()

The Base Definitions volume of POSIX.1‐2017, <sys_socket.h>

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📜 COPYRIGHT 📜

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

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okay that's the whole thing bestie, and this page came to you courtesy of IEEE/The Open Group, dated 2017, under the name RECVMSG(3P) — so like, shoutout to them, we love a documentation icon. go forth and recvmsg() responsibly. 💅✨🦋