--- name: "recv" section: "3p" platform: "linux" package: "man-pages" version: "6.19-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/recv/3p" --- # recv(3p) receive a message from a connected socket ## PROLOG This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux. ## NAME recv — receive a message from a connected socket ## SYNOPSIS ``` #include ``` ssize\_t recv(int *socket*, void \**buffer*, size\_t *length*, int *flags*); ## DESCRIPTION The *recv*() function shall receive a message from a connection-mode or connectionless-mode socket. It is normally used with connected sockets because it does not permit the application to retrieve the source address of received data. The *recv*() function takes the following arguments: - *socket* Specifies the socket file descriptor. - *buffer* Points to a buffer where the message should be stored. - *length* Specifies the length in bytes of the buffer pointed to by the *buffer* argument. - *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\_PEEK Peeks at an incoming message. The data is treated as unread and the next *recv*() or similar function shall still return this data. - MSG\_OOB Requests out-of-band data. The significance and semantics of out-of-band data are protocol-specific. - 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. The *recv*() function shall return the length of the message written to the buffer pointed to by the *buffer* argument. 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 buffer, and MSG\_PEEK is not set in the *flags* argument, the excess bytes shall be discarded. 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, *recv*() 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, *recv*() shall fail and set *errno* to **\[EAGAIN\]** or **\[EWOULDBLOCK\]**. ## RETURN VALUE Upon successful completion, *recv*() 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, *recv*() shall return 0. Otherwise, -1 shall be returned and *errno* set to indicate the error. ## ERRORS The *recv*() 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 file descriptor. - **ECONNRESET** A connection was forcibly closed by a peer. - **EINTR** The *recv*() function was interrupted by a signal that was caught, before any data was available. - **EINVAL** The MSG\_OOB flag is set and no out-of-band data is available. - **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 or protocol. - **ETIMEDOUT** The connection timed out during connection establishment, or due to a transmission timeout on active connection. The *recv*() 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.* ## EXAMPLES None. ## APPLICATION USAGE The *recv*() function is equivalent to *recvfrom*() with null pointer *address* and *address\_len* arguments, and to *read*() if the *socket* argument refers to a socket and the *flags* argument is 0. The *select*() and *poll*() functions can be used to determine when data is available to be received. ## RATIONALE None. ## FUTURE DIRECTIONS None. ## SEE ALSO *poll()*, *pselect()*, *read()*, *recvmsg()*, *recvfrom()*, *send()*, *sendmsg()*, *sendto()*, *shutdown()*, *socket()*, *write()* The Base Definitions volume of POSIX.1‐2017, ***\*** ## 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 . 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