RECVFROM

The RECVFROM call receives data on a socket with descriptor S and stores it in a buffer. The RECVFROM call applies to both connected and unconnected sockets. The socket address is returned in the NAME structure. If a datagram packet is too long to fit in the supplied buffers, datagram sockets discard extra bytes.

For datagram protocols, RECVFROM returns the source address associated with each incoming datagram. For connection-oriented protocols like TCP, GETPEERNAME returns the address associated with the other end of the connection.

If NAME is nonzero, the call returns the address of the sender. The NBYTE parameter should be set to the size of the buffer.

On return, NBYTE contains the number of data bytes received.

For stream sockets, data is processed as streams of information with no boundaries separating the data. For example, if programs A and B are connected with a stream socket and program A sends 1000 bytes, each call to this function can return any number of bytes, up to the entire 1000 bytes. The number of bytes returned will be contained in RETCODE. Therefore, programs using stream sockets should place RECVFROM in a loop that repeats until all data has been received.

For raw sockets, RECVFROM adds a 20-byte header.

If data is not available for the socket, and the socket is in blocking mode, RECVFROM blocks the caller until data arrives. If data is not available and the socket is in nonblocking mode, RECVFROM returns a -1 and sets ERRNO to 35 (EWOULDBLOCK). See FCNTL or IOCTL for a description of how to set nonblocking mode.

Note: See EZACIC05 for a subroutine that will translate ASCII input data to EBCDIC.

The following requirements apply to this call:

Authorization:

Supervisor state or problem state, any PSW key.

Dispatchable unit mode:

Task.

Cross memory mode:

PASN = HASN.

Amode:

31-bit or 24-bit.

Note: See "Addressability mode (Amode) considerations" under CALL instruction API environmental restrictions and programming requirements.

ASC mode:

Primary address space control (ASC) mode.

Interrupt status:

Enabled for interrupts.

Locks:

Unlocked.

Control parameters:

All parameters must be addressable by the caller and in the primary address space.

Figure 1 shows an example of RECVFROM call instructions.

Figure 1. RECVFROM call instruction example

    WORKING-STORAGE SECTION.
        01  SOC-FUNCTION    PIC X(16)  VALUE IS 'RECVFROM'.
        01  S               PIC 9(4) BINARY.
        01  FLAGS           PIC 9(8) BINARY.
            88  NO-FLAG                VALUE IS 0.
            88  OOB                    VALUE IS 1.
            88  PEEK                   VALUE IS 2.
        01  NBYTE           PIC 9(8) BINARY.
        01  BUF             PIC X(length of buffer).

    * IPv4 socket address structure.
        01  NAME.
            03  FAMILY      PIC 9(4) BINARY.
            03  PORT        PIC 9(4) BINARY.
            03  IP-ADDRESS  PIC 9(8) BINARY.
            03  RESERVED    PIC X(8).

    * IPv6 socket address structure.
        01  NAME.
            03  FAMILY      PIC 9(4) BINARY.
            03  PORT        PIC 9(4) BINARY.
            03  FLOWINFO    PIC 9(8) BINARY.
            03  IP-ADDRESS.
                10 FILLER   PIC 9(16) BINARY.
                10 FILLER   PIC 9(16) BINARY.
            03  SCOPE-ID    PIC 9(8) BINARY.

        01  ERRNO           PIC 9(8) BINARY.
        01  RETCODE         PIC S9(8) BINARY.
 
    PROCEDURE DIVISION.
         CALL 'EZASOKET' USING SOC-FUNCTION S FLAGS
                         NBYTE BUF NAME ERRNO RETCODE.

For equivalent PL/I and assembler language declarations, see Converting parameter descriptions.