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RECV(2)			   Linux Programmer's Manual		       RECV(2)

       recv, recvfrom, recvmsg - receive a message from a socket

       #include <sys/types.h>
       #include <sys/socket.h>

       ssize_t recv(int sockfd, void *buf, size_t len, int flags);

       ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
			struct sockaddr *src_addr, socklen_t *addrlen);

       ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);

       The  recv(),  recvfrom(),  and recvmsg() calls are used to receive mes‐
       sages from a socket.  They may be used to receive data on both  connec‐
       tionless	 and  connection-oriented  sockets.  This page first describes
       common features of all three system calls, and then describes the  dif‐
       ferences between the calls.

       All  three calls return the length of the message on successful comple‐
       tion.  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.

       If no messages are available at the socket, the receive calls wait  for
       a  message  to arrive, unless the socket is nonblocking (see fcntl(2)),
       in which case the value -1 is returned and the external variable	 errno
       is set to EAGAIN or EWOULDBLOCK.	 The receive calls normally return any
       data available, up to the requested amount,  rather  than  waiting  for
       receipt of the full amount requested.

       An  application	can  used select(2), poll(2), or epoll(7) to determine
       when more data arrives on a socket.

   The flags argument
       The flags argument is formed by ORing one or more of the following val‐

       MSG_CMSG_CLOEXEC (recvmsg() only; since Linux 2.6.23)
	      Set  the close-on-exec flag for the file descriptor received via
	      a UNIX domain file descriptor  using  the	 SCM_RIGHTS  operation
	      (described  in  unix(7)).	 This flag is useful for the same rea‐
	      sons as the O_CLOEXEC flag of open(2).

       MSG_DONTWAIT (since Linux 2.2)
	      Enables nonblocking operation; if the operation would block, the
	      call  fails  with the error EAGAIN or EWOULDBLOCK (this can also
	      be enabled using the O_NONBLOCK flag with the F_SETFL fcntl(2)).

       MSG_ERRQUEUE (since Linux 2.2)
	      This flag specifies that queued errors should be	received  from
	      the  socket  error  queue.   The error is passed in an ancillary
	      message  with  a	type  dependent	 on  the  protocol  (for  IPv4
	      IP_RECVERR).   The  user	should	supply	a buffer of sufficient
	      size.  See cmsg(3) and ip(7) for more information.  The  payload
	      of the original packet that caused the error is passed as normal
	      data via msg_iovec.  The original	 destination  address  of  the
	      datagram that caused the error is supplied via msg_name.

	      For local errors, no address is passed (this can be checked with
	      the cmsg_len member of the cmsghdr).  For	 error	receives,  the
	      MSG_ERRQUEUE  is	set  in	 the  msghdr.  After an error has been
	      passed, the pending socket error is  regenerated	based  on  the
	      next  queued  error and will be passed on the next socket opera‐

	      The error is supplied in a sock_extended_err structure:

		  #define SO_EE_ORIGIN_NONE    0
		  #define SO_EE_ORIGIN_LOCAL   1
		  #define SO_EE_ORIGIN_ICMP    2
		  #define SO_EE_ORIGIN_ICMP6   3

		  struct sock_extended_err
		      uint32_t ee_errno;   /* error number */
		      uint8_t  ee_origin;  /* where the error originated */
		      uint8_t  ee_type;	   /* type */
		      uint8_t  ee_code;	   /* code */
		      uint8_t  ee_pad;	   /* padding */
		      uint32_t ee_info;	   /* additional information */
		      uint32_t ee_data;	   /* other data */
		      /* More data may follow */

		  struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);

	      ee_errno contains the errno number of the queued error.  ee_ori‐
	      gin is the origin code of where the error originated.  The other
	      fields  are  protocol-specific.	The   macro   SOCK_EE_OFFENDER
	      returns a pointer to the address of the network object where the
	      error originated from given a pointer to the ancillary  message.
	      If  this address is not known, the sa_family member of the sock‐
	      addr contains AF_UNSPEC and the other fields of the sockaddr are
	      undefined.   The	payload of the packet that caused the error is
	      passed as normal data.

	      For local errors, no address is passed (this can be checked with
	      the  cmsg_len  member  of the cmsghdr).  For error receives, the
	      MSG_ERRQUEUE is set in the msghdr.   After  an  error  has  been
	      passed,  the  pending  socket  error is regenerated based on the
	      next queued error and will be passed on the next	socket	opera‐

	      This flag requests receipt of out-of-band data that would not be
	      received in the normal data stream.  Some protocols place	 expe‐
	      dited  data  at the head of the normal data queue, and thus this
	      flag cannot be used with such protocols.

	      This 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

       MSG_TRUNC (since Linux 2.2)
	      For    raw   (AF_PACKET),	  Internet   datagram	(since	 Linux
	      2.4.27/2.6.8), netlink (since Linux 2.6.22), and	UNIX  datagram
	      (since  Linux 3.4) sockets: return the real length of the packet
	      or datagram, even when it was longer than the passed buffer.

	      For use with Internet stream sockets, see tcp(7).

       MSG_WAITALL (since Linux 2.2)
	      This 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  discon‐
	      nect  occurs,  or the next data to be received is of a different
	      type than that returned.

       recvfrom() places the received message into the buffer buf.  The caller
       must specify the size of the buffer in len.

       If  src_addr  is	 not  NULL,  and  the underlying protocol provides the
       source address of the message, that source address  is  placed  in  the
       buffer pointed to by src_addr.  In this case, addrlen is a value-result
       argument.  Before the call, it should be initialized to the size of the
       buffer  associated  with	 src_addr.  Upon return, addrlen is updated to
       contain the actual size of the source address.  The returned address is
       truncated  if  the  buffer provided is too small; in this case, addrlen
       will return a value greater than was supplied to the call.

       If the caller is not interested in the source address, src_addr	should
       be specified as NULL and addrlen should be specified as 0.

       The  recv()  call is normally used only on a connected socket (see con‐
       nect(2)).  It is equivalent to the call:

	   recvfrom(fd, buf, len, flags, NULL, 0));

       The recvmsg() call uses a msghdr structure to minimize  the  number  of
       directly	 supplied  arguments.  This structure is defined as follows in

	   struct iovec {		     /* Scatter/gather array items */
	       void  *iov_base;		     /* Starting address */
	       size_t iov_len;		     /* Number of bytes to transfer */

	   struct msghdr {
	       void	    *msg_name;	     /* optional address */
	       socklen_t     msg_namelen;    /* size of address */
	       struct iovec *msg_iov;	     /* scatter/gather array */
	       size_t	     msg_iovlen;     /* # elements in msg_iov */
	       void	    *msg_control;    /* ancillary data, see below */
	       size_t	     msg_controllen; /* ancillary data buffer len */
	       int	     msg_flags;	     /* flags on received message */

       Here msg_name and msg_namelen specify the source address if the	socket
       is unconnected; msg_name may be given as a null pointer if no names are
       desired or required.  The fields msg_iov and msg_iovlen describe	 scat‐
       ter-gather locations, as discussed in readv(2).	The field msg_control,
       which has length msg_controllen, points to a buffer for other  protocol
       control-related	 messages   or	miscellaneous  ancillary  data.	  When
       recvmsg() is called, msg_controllen should contain the  length  of  the
       available  buffer in msg_control; upon return from a successful call it
       will contain the length of the control message sequence.

       The messages are of the form:

	   struct cmsghdr {
	       socklen_t     cmsg_len;	   /* data byte count, including hdr */
	       int	     cmsg_level;   /* originating protocol */
	       int	     cmsg_type;	   /* protocol-specific type */
	   /* followed by
	       unsigned char cmsg_data[]; */

       Ancillary data should  be  accessed  only  by  the  macros  defined  in

       As  an  example,	 Linux	uses  this  ancillary  data  mechanism to pass
       extended errors, IP options, or file descriptors over UNIX domain sock‐

       The  msg_flags  field  in the msghdr is set on return of recvmsg().  It
       can contain several flags:

	      indicates end-of-record; the data returned  completed  a	record
	      (generally used with sockets of type SOCK_SEQPACKET).

	      indicates	 that the trailing portion of a datagram was discarded
	      because the datagram was larger than the buffer supplied.

	      indicates that some control data were discarded due to  lack  of
	      space in the buffer for ancillary data.

	      is  returned to indicate that expedited or out-of-band data were

	      indicates that no data was received but an extended  error  from
	      the socket error queue.

       These  calls  return  the  number  of bytes received, or -1 if an error
       occurred.  In the event of an error,  errno  is	set  to	 indicate  the

       When a stream socket peer has performed an orderly shutdown, the return
       value will be 0 (the traditional "end-of-file" return).

       Datagram sockets in  various  domains  (e.g.,  the  UNIX	 and  Internet
       domains)	 permit	 zero-length  datagrams.   When	 such  a  datagram  is
       received, the return value is 0.

       The value 0 may also be returned if the requested number	 of  bytes  to
       receive from a stream socket was 0.

       These  are  some	 standard errors generated by the socket layer.	 Addi‐
       tional errors may be generated and returned from the underlying	proto‐
       col modules; see their manual pages.

	      The socket is marked nonblocking and the receive operation would
	      block, or a receive timeout had been set and the timeout expired
	      before  data  was received.  POSIX.1-2001 allows either error to
	      be returned for this case, and does not require these  constants
	      to  have	the same value, so a portable application should check
	      for both possibilities.

       EBADF  The argument sockfd is an invalid descriptor.

	      A remote host refused to allow the network connection (typically
	      because it is not running the requested service).

       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; see signal(7).

       EINVAL Invalid argument passed.

       ENOMEM Could not allocate memory for recvmsg().

	      The socket is associated with a connection-oriented protocol and
	      has not been connected (see connect(2) and accept(2)).

	      The argument sockfd does not refer to a socket.

       4.4BSD (these function calls first appeared in 4.2BSD), POSIX.1-2001.

       POSIX.1-2001 describes only  the	 MSG_OOB,  MSG_PEEK,  and  MSG_WAITALL

       The  prototypes	given above follow glibc2.  The Single UNIX Specifica‐
       tion agrees, except that it has return values of	 type  ssize_t	(while
       4.x  BSD	 and libc4 and libc5 all have int).  The flags argument is int
       in 4.x BSD, but unsigned int in libc4 and libc5.	 The len  argument  is
       int in 4.x BSD, but size_t in libc4 and libc5.  The addrlen argument is
       int * in 4.x BSD,  libc4	 and  libc5.   The  present   socklen_t *  was
       invented by POSIX.  See also accept(2).

       According  to  POSIX.1-2001,  the  msg_controllen  field	 of the msghdr
       structure should be typed as socklen_t, but glibc currently types it as

       See recvmmsg(2) for information about a Linux-specific system call that
       can be used to receive multiple datagrams in a single call.

       An example of the use of recvfrom() is shown in getaddrinfo(3).

       fcntl(2), getsockopt(2), read(2), recvmmsg(2), select(2),  shutdown(2),
       socket(2), cmsg(3), sockatmark(3), socket(7)

       This  page  is  part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux				  2014-02-11			       RECV(2)

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