The RECVFROM macro receives data for a socket and stores it in a buffer. RECVFROM returns the length of the incoming message or data stream.

If data is not available for the socket designated by descriptor S, and socket S is in blocking mode, the RECVFROM call blocks the caller until data arrives.

If data is not available and socket S is in nonblocking mode, RECVFROM returns a -1 and sets ERRNO to 35 (EWOULDBLOCK). Because RECVFROM returns the socket address in the NAME structure, it applies to any datagram socket, whether connected or unconnected. See FCNTL or IOCTL for a description of how to set nonblocking mode. If a datagram packet is too long to fit in the supplied buffer, datagram sockets discard extra bytes.

For stream sockets, the data is processed as streams of information with no boundaries separating data. For example, if applications A and B are connected with a stream socket and Application A sends 1000 bytes, each call to this function can return 1 byte, or 10 bytes, or the entire 1000 bytes. Applications using stream sockets should place RECVFROM in a loop that repeats until all of the data has been received.

Keyword

Description

S

Input parameter. A value or the address of a halfword binary number specifying the socket to receive the data.

NBYTE

Input parameter. A value or the address of a fullword binary number specifying the length of the input buffer. NBYTE must first be initialized to the size of the buffer associated with NAME. On return the NBYTE contains the number of bytes of data received.

BUF

On input, a buffer to be filled by completion of the call. The length of BUF must be at least as long as the value of NBYTE.

NAME

Initially, the IPv4 or IPv6 application provides a pointer to a structure that will contain the peer socket name on completion of the call. If the NAME parameter value is nonzero, the IPv4 or IPv6 source address of the message is specified with the address of who sent the datagram. Include the SYS1.MACLIB(BPXYSOCK) macro to get the assembler mappings for the socket address structure. The socket address structure mappings begin at the SOCKADDR label. The AF_INET socket address structure fields start at the SOCK_SIN label. The AF_INET6 socket address structure fields start at the SOCK_SIN6 label.

The IPv4 socket address structure contains the following fields:

Field

Description

FAMILY

Output parameter. A halfword binary number specifying the IPv4 addressing family. The value for the IPv4 socket descriptor (S parameter) is a decimal 2, indicating AF_INET.

PORT

Output parameter. A halfword binary number specifying the port number of the sending socket.

IPv4-ADDRESS

Output parameter. A fullword binary number specifying the 32-bit IPv4 Internet address of the sending socket.

RESERVED

Output parameter. An 8-byte reserved field. This field is required, but is not used.

The IPv6 socket structure contains the following fields:

Field

Description

NAMELEN

Output parameter. A 1-byte binary field specifying the length of this IPv6 socket address structure. Any value specified by the use of this field is ignored when processed as input and the field is set to 0 when processed as output.

FAMILY

Output parameter. A 1-byte binary field specifying the IPv6 addressing family. The value for IPv6 socket descriptor (S parameter) is a decimal 19, indicating AF_INET6.

PORT

Output parameter. A halfword binary number specifying the port number of the sending socket.

FLOW-INFO

Output parameter. A fullword binary field specifying the traffic class and flow label. This value of this field is undefined.

IPv6-ADDRESS

Output parameter. A 16-byte binary field that is set to the 128-bit IPv6 Internet address, in network byte order, of the sending socket.

SCOPE-ID

A fullword binary field which identifies a set of interfaces as appropriate for the scope of the address carried in the IPv6-ADDRESS field. For a link scope IPv6-ADDRESS, SCOPE-ID contains the link index for the IPv6-ADDRESS. For all other address scopes, SCOPE-ID is undefined.

ALET

Optional input parameter. A fullword binary field containing the ALET of BUF. The default is 0 (primary address space).

If a nonzero ALET is specified, the ALET must represent a valid entry in the dispatchable unit access list (DU-AL) for the task issuing this call. Note that ALETs can only be specified for synchronous socket calls (for example, ECB/REQAREA cannot be specified). An exception to this is an ALET representing a SCOPE=COMMON data space.

ERRNO

Output parameter. A fullword binary field. If RETCODE is negative, this field contains an error number. See Socket call error return codes for information about ERRNO return codes.

RETCODE

A fullword binary field that returns one of the following values:

Value

Description

0

A 0 return code indicates that the connection is closed and no data is available.

>0

A positive value indicates the number of bytes transferred by the RECVFROM call.

-1

Check ERRNO for an error code.

FLAGS

Input parameter. FLAGS can be a literal value or a fullword binary field.

Literal Value	Binary Value	Description
'MSG_OOB'	X'00000001'	Receive out-of-band data (stream sockets only). Out-of-band data can be read if the SO_OOBINLINE option is set for the socket regardless of whether the MSG_OOB flag is set.
'MSG_PEEK'	X'00000002'	Peek at the data, but do not destroy the data. If the peek flag is set, the next receive operation reads the same data.
'MSG_WAITALL'	X'00000040'	Requests that the function block until the full amount of requested data can be returned (stream sockets only). The function might return a smaller amount of data if the connection is terminated, if an error is pending, or if the SO_RCVTIMEO field is set and the timer expired for the socket.

ECB or REQAREA

Input parameter. This parameter is required if you are using APITYPE=3. It points to a 104-byte field containing:

For ECB

A 4-byte ECB posted by TCP/IP when the macro completes.

For REQAREA

A 4-byte user token (set by you) that is presented to your exit when the response to this function request is complete.

For ECB/REQAREA

The last 100 bytes is a storage field used by the interface to save the state information.

Note: This storage must not be modified until the macro function has completed and the ECB has been posted, or the asynchronous exit has been driven.

ERROR

Input parameter. The location in your program to receive control when the application programming interface (API) processing module cannot be loaded.

TASK

Input parameter. The location of the task storage area in your program.