--- name: "recvfrom" section: "3p" platform: "linux" package: "man-pages" version: "6.19-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/recvfrom/3p" --- # recvfrom(3p) receive a message from a 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 recvfrom — receive a message from a socket ## SYNOPSIS ``` #include ``` ssize\_t recvfrom(int *socket*, void \*restrict *buffer*, size\_t *length*,\ int *flags*, struct sockaddr \*restrict *address*,\ socklen\_t \*restrict *address\_len*); ## DESCRIPTION The *recvfrom*() function shall receive a message from a connection-mode or connectionless-mode socket. It is normally used with connectionless-mode sockets because it permits the application to retrieve the source address of received data. The *recvfrom*() function takes the following arguments: - *socket* Specifies the socket file descriptor. - *buffer* Points to the 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 *recvfrom*() 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. - *address* A null pointer, or points to a **sockaddr** structure in which the sending address is to be stored. The length and format of the address depend on the address family of the socket. - *address\_len* Either a null pointer, if *address* is a null pointer, or a pointer to a **socklen\_t** object which on input specifies the length of the supplied **sockaddr** structure, and on output specifies the length of the stored address. The *recvfrom*() 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\_RAW, 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. Not all protocols provide the source address for messages. If the *address* argument is not a null pointer and the protocol provides the source address of messages, the source address of the received message shall be stored in the **sockaddr** structure pointed to by the *address* argument, and the length of this address shall be stored in the object pointed to by the *address\_len* argument. 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 *address* argument is not a null pointer and the protocol does not provide the source address of messages, the value stored in the object pointed to by *address* is unspecified. If no messages are available at the socket and O\_NONBLOCK is not set on the socket's file descriptor, *recvfrom*() 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, *recvfrom*() shall fail and set *errno* to **\[EAGAIN\]** or **\[EWOULDBLOCK\]**. ## RETURN VALUE Upon successful completion, *recvfrom*() 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, *recvfrom*() shall return 0. Otherwise, the function shall return -1 and set *errno* to indicate the error. ## ERRORS The *recvfrom*() 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** A signal interrupted *recvfrom*() 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. - **ETIMEDOUT** The connection timed out during connection establishment, or due to a transmission timeout on active connection. The *recvfrom*() 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 *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()*, *recv()*, *recvmsg()*, *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 . 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 .