recvfrom()--Receive Data

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

int recvfrom(int socket_descriptor,
             char *buffer,
             int buffer_length,
             int flags,
             struct sockaddr *from_address,
             int *address_length)

The recvfrom() function is used to receive data through a connected or unconnected socket.

Parameters

socket_descriptor

(Input) The socket descriptor that is to be read from.

buffer

(Input) The pointer to the buffer in which the data that is to be read is stored.

buffer_length

(Input) The length of the buffer.

int flags

(Input) A flag value that controls the reception of the data. The flags value is either zero, or is obtained by performing an OR operation on one or more of the following constants:

MSG_OOB

Receive out-of-band data. Valid only for sockets with an address family of AF_INET and type SOCK_STREAM.

MSG_PEEK

Obtain a copy of the message without removing the message from the socket.

from_address

(Output) A pointer to a buffer of type struct sockaddr that contains the address from which the message was received.

The structure sockaddr is defined in <sys/socket.h>.

      struct sockaddr {
         u_short sa_family;
         char    sa_data[14];
      };

The sa_family field identifies the address family to which the address belongs, and sa_data is the address whose format is dependent on the address family.

address_length

(Input/output) This parameter is a value-result field. The caller passes a pointer to the length of the from_address parameter. On return from the call, address_length will contain the actual length of the address.

Return Value

recvfrom() returns an integer. Possible values are:

• -1 (unsuccessful)

• n (successful), where n is the number of bytes received.

Error Conditions

When recvfrom() fails, errno can be set to one of the following:

[EACCES]

Permission denied.

The socket pointed to by the socket_descriptor parameter is using a connection-oriented transport service, and a connect() was previously completed. The process, however, does not have the appropriate privileges to the objects that were needed to establish a connection. For example, the connect() required the use of an APPC device that the process was not authorized to.

[EBADF]

Descriptor not valid.

[ECONNABORTED]

Connection ended abnormally.

This error code indicates that the transport provider ended the connection abnormally because of one of the following:

• The retransmission limit has been reached for data that was being sent on the socket.

• A protocol error was detected.

[ECONNREFUSED]

The destination socket refused an attempted connect operation.

[ECONNRESET]

A connection with a remote socket was reset by that socket.

[EFAULT]

Bad address.

The system detected an address which was not valid while attempting to access the buffer, from_address, or address_length parameter.

[EINTR]

Interrupted function call.

[EINVAL]

Parameter not valid.

This error code indicates one of the following:

• The buffer_length parameter specifies a negative value.

• The flags parameter specifies a value that includes the MSG_OOB flag, but no OOB data was available to be received.

• The flags parameter specifies a value that includes the MSG_OOB flag, and the socket option SO_OOBINLINE has been set.

• The socket_descriptor parameter points to a socket that is using a connectionless transport service, is not a socket of type SOCK_RAW, and is not bound to an address.

[EIO]

Input/output error.

[ENOBUFS]

There is not enough buffer space for the requested operation.

[ENOTCONN]

Requested operation requires a connection.

This error code is returned only on sockets that use a connection-oriented transport service.

[ENOTSOCK]

The specified descriptor does not reference a socket.

[EOPNOTSUPP]

Operation not supported.

This error code indicates one of the following:

• The flags parameter specifies a value that includes the MSG_OOB flag, but the socket_descriptor parameter points to a connectionless socket.

• The flags parameter specifies a value that includes the MSG_OOB flag, but the socket_descriptor parameter points to a socket that does not have an address family of AF_INET.

[ETIMEDOUT]

A remote host did not respond within the timeout period.

A non-blocking connect() was previously issued that resulted in the connection establishment timing out. No connection is established. This error code is returned only on sockets that use a connection-oriented transport service.

[EUNATCH]

The protocol required to support the specified address family is not available at this time.

[EUNKNOWN]

Unknown system state.

[EWOULDBLOCK]

Operation would have caused the thread to be suspended.

Error Messages

CPE3418 E

Possible APAR condition or hardware failure.

CPF9872 E

Program or service program &1 in library &2 ended. Reason code &3.

CPFA081 E

Unable to set return value or error code.

Usage Notes

1. For sockets that use a connection-oriented transport service (for example, sockets with a type of SOCK_STREAM), a returned value of zero indicates one of the following:

   • The partner program has issued a close() for the socket.

   • The partner program has issued a shutdown() to disable writing to the socket.

   • The connection is broken and the error was returned on a previously issued socket function.

   • A shutdown() to disable reading was previously done on the socket.

2. If the socket is using a connection-oriented transport service, the from_address and address_length parameters are ignored.

3. For sockets that have a type of SOCK_SEQPACKET, each output operation by the partner program constitutes one record. If the record length is larger than the value specified by the buffer_length parameter, the remaining data from the record is discarded.

4. The following applies to sockets that use a connectionless transport service (for example, a socket with a type of SOCK_DGRAM):

   • If a connect() has been issued previously, then data can be received only from the address specified in the previous connect().

   • If the from_address parameter is set to NULL or address_length specifies a value of zero, the address from which data is received is discarded by the system.

   • If the length of the address to be returned exceeds the length of the from_address parameter, the returned address is truncated.

   • If the socket is using an address family of AF_UNIX, the address (which is a path name) is returned in the default coded character set identifier (CCSID) currently in effect for the job.

   • The entire message must be read in a single read operation. If the size of the message is too large to fit in the user supplied buffer, the remaining bytes of the message are discarded.

   • A returned value of zero indicates one of the following:

     • The partner program has sent a NULL message (a datagram with no user data).

     • A shutdown() to disable reading was previously done on the socket.

     • The buffer length specified was zero.

Related Information

• fcntl()--Retrieve or Change Descriptor Attributes

• ioctl()--Change Descriptor Attributes

• recv()--Receive Data

• recvmsg()--Receive Data or Descriptors or Both