--- name: "recv" section: "2" platform: "macos" package: "xnu" version: "12377.121.6" release: "macOS 26.5" platforms: ["linux", "freebsd", "macos"] url: "https://new.carta.tech/recv" --- # recv(2) receive a message from a socket ## NAME **recv**, **recvfrom**, **recvmsg** – receive a message from a socket ## LIBRARY Standard C Library (libc, -lc) ## SYNOPSIS **#include \** *ssize\_t*\ **recv**(*int socket*, *void \*buffer*, *size\_t length*, *int flags*); *ssize\_t*\ **recvfrom**(*int socket*, *void \*restrict buffer*, *size\_t length*, *int flags*, *struct sockaddr \*restrict address*, *socklen\_t \*restrict address\_len*); *ssize\_t*\ **recvmsg**(*int socket*, *struct msghdr \*message*, *int flags*); ## DESCRIPTION The **recvfrom**() and **recvmsg**() system calls are used to receive messages from a socket, and may be used to receive data on a socket whether or not it is connection-oriented. If *address* is not a null pointer and the socket is not connection-oriented, the source address of the message is filled in. The *address\_len* argument is a value-result argument, initialized to the size of the buffer associated with *address*, and modified on return to indicate the actual size of the address stored there. The **recv**() function is normally used only on a *connected* socket (see [connect(2)](/connect/2.md) or [connectx(2)](/connectx/2.md)) and is identical to **recvfrom**() with a null pointer passed as its *address* argument. As it is redundant, it may not be supported in future releases. All three routines return the length of the message on successful completion. If a message is too long to fit in the supplied buffer, excess bytes may be discarded depending on the type of socket the message is received from (see [socket(2)](/socket/2.md)). If no messages are available at the socket, the receive call waits for a message to arrive, unless the socket is nonblocking (see [fcntl(2)](/fcntl/2.md)) in which case the value -1 is returned and the external variable *errno* set to EAGAIN. The receive calls normally return any data available, up to the requested amount, rather than waiting for receipt of the full amount requested; this behavior is affected by the socket-level options SO\_RCVLOWAT and SO\_RCVTIMEO described in [getsockopt(2)](/getsockopt/2.md). The [select(2)](/select/2.md) system call may be used to determine when more data arrive. If no messages are available to be received and the peer has performed an orderly shutdown, the value 0 is returned. The *flags* argument to a **recv**() function is formed by *or*'ing one or more of the values: | | | | --- | --- | | MSG\_OOB | process out-of-band data | | MSG\_PEEK | peek at incoming message | | MSG\_WAITALL | wait for full request or error | The MSG\_OOB flag requests receipt of out-of-band data that would not be received in the normal data stream. Some protocols place expedited data at the head of the normal data queue, and thus this flag cannot be used with such protocols. The MSG\_PEEK flag causes the receive operation to return data from the beginning of the receive queue without removing that data from the queue. Thus, a subsequent receive call will return the same data. The MSG\_WAITALL flag requests that the operation block until the full request is satisfied. However, the call may still return less data than requested if a signal is caught, an error or disconnect occurs, or the next data to be received is of a different type than that returned. The **recvmsg**() system call uses a *msghdr* structure to minimize the number of directly supplied arguments. This structure has the following form, as defined in ⟨sys/socket.h⟩: ``` struct msghdr { void *msg_name; /* optional address */ socklen_t msg_namelen; /* size of address */ struct iovec *msg_iov; /* scatter/gather array */ int msg_iovlen; /* # elements in msg_iov */ void *msg_control; /* ancillary data, see below */ socklen_t msg_controllen; /* ancillary data buffer len */ int msg_flags; /* flags on received message */ }; ``` Here *msg\_name* and *msg\_namelen* specify the destination address if the socket is unconnected; *msg\_name* may be given as a null pointer if no names are desired or required. The *msg\_iov* and *msg\_iovlen* arguments describe scatter gather locations, as discussed in [read(2)](/read/2.md). *msg\_iovlen* 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. The *msg\_control* argument, which has length *msg\_controllen*, points to a buffer for other protocol control related messages or other miscellaneous ancillary data. The messages are of the form: ``` struct cmsghdr { u_int cmsg_len; /* data byte count, including hdr */ int cmsg_level; /* originating protocol */ int cmsg_type; /* protocol-specific type */ /* followed by u_char cmsg_data[]; */ }; ``` As an example, one could use this to learn of changes in the data-stream in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting a **recvmsg**() with no data buffer provided immediately after an **accept**() system call. Open file descriptors are now passed as ancillary data for AF\_UNIX domain sockets, with *cmsg\_level* set to SOL\_SOCKET and *cmsg\_type* set to SCM\_RIGHTS. The *msg\_flags* field is set on return according to the message received. MSG\_EOR indicates end-of-record; the data returned completed a record. MSG\_TRUNC indicates that the trailing portion of a datagram was discarded because the datagram was larger than the buffer supplied. MSG\_CTRUNC indicates that some control data were discarded due to lack of space in the buffer for ancillary data. MSG\_OOB is returned to indicate that expedited or out-of-band data were received. ## RETURN VALUES These calls return the number of bytes received, or -1 if an error occurred. For TCP sockets, the return value 0 means the peer has closed its half side of the connection. ## ERRORS The calls fail if: - \[EAGAIN\] The socket is marked non-blocking, and the receive operation would block, or a receive timeout had been set, and the timeout expired before data were received. - \[EBADF\] The argument *socket* is an invalid descriptor. - \[ECONNRESET\] The connection is closed by the peer during a receive attempt on a socket. - \[EFAULT\] The receive buffer pointer(s) point outside the process's address space. - \[EINTR\] The receive was interrupted by delivery of a signal before any data were available. - \[EINVAL\] MSG\_OOB is set, but no out-of-band data is available. - \[ENOBUFS\] An attempt to allocate a memory buffer fails. - \[ENOTCONN\] The socket is associated with a connection-oriented protocol and has not been connected (see connect(2,) connectx(2,) and [accept(2)](/accept/2.md)). - \[ENOTSOCK\] The argument *socket* does not refer to a socket. - \[EOPNOTSUPP\] The type and/or protocol of *socket* do not support the option(s) specified in *flags*. - \[ETIMEDOUT\] The connection timed out. The **recvfrom**() call may also fail if: - \[EINVAL\] The total of the iov\_len values overflows a ssize\_t. The **recvmsg**() call may also fail if: - \[EMSGSIZE\] 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. - \[ENOMEM\] Insufficient memory is available. ## SEE ALSO [fcntl(2)](/fcntl/2.md), [getsockopt(2)](/getsockopt/2.md), [read(2)](/read/2.md), [select(2)](/select/2.md), [socket(2)](/socket/2.md) ## HISTORY The **recv**() function appeared in 4.2BSD.