recv: Receive data on a connected socket

The recv socket function receives data on a socket with descriptor s and stores it in a buffer.

Last updated

  • Changed for PUT15 (information only; no code change).
  • Changed for PUT14.
  • Added for PUT00.

Format

#include  <sys/socket.h>
int       recv(int s,
               char *buf,
               int len,
               int flags);
s
The socket descriptor.
buf
A pointer to the buffer that receives the data.
len
The length, in bytes, of the buffer pointed to by the buf parameter. The maximum amount of data that can be received is 1048576 bytes.
flags
Must be set to 0 or one or more of the following flags. If you specify more than one flag, use the logical OR operator (|) to separate them:
MSG_OOB
Reads any out-of-band data on the socket.
MSG_PEEK
Peeks at the data present on the socket; the data is returned but not consumed, so a later receive operation sees the same data.

Normal return

If successful, the function returns the length, in bytes, of the message or datagram:
  • For UDP sockets, zero length messages can be received on the socket.
  • For TCP sockets, if the connection is closed, 0 is returned.

Error return

A return code equal to -1 indicates an error. You can get the specific error code by calling sock_errno. For more information about socket errors, see Socket error return codes.
E1052STATE
The socket was closed because the system was in or cycling down to 1052 state.
ESYSTEMERROR
A system error has occurred and closed the socket.
SOCFAULT
Using buf and len would result in an attempt to access a protected address space.
SOCINVAL
The MSG_OOB option was specified for a socket other than a stream socket or the MSG_OOB option was specified, but out-of-band data is queued inline for this socket.
SOCNETUNREACH
An ICMP message was received on this socket (in response to sent data) indicating that the destination IP address and/or destination port are not reachable.
SOCNOTCONN
A stream socket was used to issue the recv function, and the socket was not connected.
SOCNOTSOCK
The s parameter is not a valid socket descriptor.
SOCTIMEDOUT
The operation timed out. The socket is still available.
SOCWOULDBLOCK
The s parameter is in nonblocking mode and either no data is available to read or the traffic limit has been reached for the socket or application.

Programming considerations

  • This function applies only to connected sockets.
  • This function returns up to len bytes of data. If there are fewer than the requested number of bytes available, the function returns the number currently available.
  • If data is not available for socket s, and s is in blocking mode, the recv function blocks the caller until data arrives. If data is not available, and s is in nonblocking mode, recv returns a -1 and sets sock_errno to SOCWOULDBLOCK.
  • If a bind has not yet been issued to the socket, a bind is issued on behalf of the application for a non-stream socket.
  • The receive timeout value (the SO_RCVTIMEO setsockopt option) determines how long to wait for data to be received before the recv function times out.
  • The receive low-water mark (the SO_RCVLOWAT setsockopt option) determines the minimum amount of data that must be received before the recv function is completed. If the recv function times out, any data that was received is returned to the application even if the amount of data received is less than the receive low-water mark value.
  • The recv function cannot be issued if an activate_on_receipt, activate_on_receipt_with_length, activate_on_receipt_of_TCP_message, or tpf_read_TCP_message call is pending for the socket. These operations are mutually exclusive.

Examples

The following example reads in 1 byte of out-of-band data. 
#include <sys/socket.h>
⋮
int  rc;
int  server_sock;
char oob_data;
⋮
rc = recv(server_sock,oob_data,sizeof(oob_data),MSG_OOB);
if (rc > 0)
{
  /* Process the oob data from the sender */
⋮
}