recvfrom(2) System Calls Manual recvfrom(2)
NAME
recvfrom - Receives messages from sockets
SYNOPSIS
#include <sys/socket.h>
ssize_t recvfrom( int socket, void *buffer, size_t length, int flags, struct sockaddr *address, socklen_t
*address_len) ;
[XNS4.0] The definition of the recvfrom() function in XNS4.0 uses a size_t data type for the address_len parameter instead of a socklen_t
data type as specified in XNS5.0 (the previous definition).
[Tru64 UNIX] The following definition of the recvfrom() function does not conform to current standards and is supported only for backward
compatibility (see standards(5)):
int recvfrom( int socket, char *buffer, int length, int flags, struct sockaddr *address, int *address_len) ;
STANDARDS
Interfaces documented on this reference page conform to industry standards as follows:
recvfrom(): XNS5.0
Refer to the standards(5) reference page for more information about industry standards and associated tags.
PARAMETERS
Specifies the socket file descriptor. Specifies a pointer to the buffer to which the message should be written. Specifies the length (in
bytes) of the buffer pointed to by the buffer parameter. Points to a value that controls message reception. The parameter to control mes-
sage reception is formed by the logical OR of one or more of the following values: Peeks at the incoming message. Processes out-of-band
data. Requests that the function block wait until the full amount of data requested can be returned. The function may return a smaller
amount of data if a signal is caught, the connection is terminated, MSG_PEEK was specified, or an error is pending for the socket. Points
to a sockaddr structure, the format of which is determined by the domain and by the behavior requested for the socket. The sockaddr struc-
ture is an overlay for a sockaddr_in or sockaddr_un structure, depending on which of the supported address families is active.
[Tru64 UNIX] If the compile-time option _SOCKADDR_LEN is defined before the sys/socket.h header file is included, the sockaddr
structure takes 4.4BSD behavior, with a field for specifying the length of the socket address. Otherwise, the default 4.3BSD sock-
addr structure is used, with the length of the socket address assumed to be 14 bytes or less.
If _SOCKADDR_LEN is defined, the 4.3BSD sockaddr structure is defined with the name osockaddr. Specifies the length of the sockaddr
structure pointed to by the address parameter.
DESCRIPTION
The recvfrom() function permits an application program to receive messages from unconnected sockets. It is normally applied to unconnected
sockets because it includes parameters that permit a calling program to retrieve the source endpoint of received data.
To obtain the source address of the message, specify a nonzero value for the address parameter. The recvfrom() function is called with the
address_len parameter set to the size of the buffer specified by the address parameter. On return, this function modifies the address_len
parameter to the actual size in bytes of the address specified by the address parameter. The recvfrom() function returns the length of the
message written to the buffer pointed to by the buffer parameter. When a message is too long for the specified buffer, excess bytes may be
truncated depending on the type of socket that issued the message, and depending on which flags are set with the flags parameter.
When no message is available at the socket specified by the socket parameter, the recvfrom() function waits for a message to arrive, unless
the socket is nonblocking. When the socket is nonblocking, errno is set to [EWOULDBLOCK].
Use the select() and poll() functions to determine when more data arrives.
NOTES
[Tru64 UNIX] When compiled in the X/Open UNIX environment or the POSIX.1g socket environment, calls to the recvfrom() function are inter-
nally renamed by prepending _E to the function name. When you are debugging a module that includes the recvfrom() function and for which
_XOPEN_SOURCE_EXTENDED or _POSIX_PII_SOCKET has been defined, use _Erecvfrom to refer to the recvfrom() call. See standards(5) for further
information.
RETURN VALUES
Upon successful completion, the byte length of the written message is returned. If no messages are available and the peer has closed the
connection, the recv() function returns a value of 0. Otherwise, the function returns a value of -1 and sets errno to indicate the error.
ERRORS
If the recvfrom() function fails, errno may be set to one of the following values: The socket parameter is not a valid file descriptor. A
connection was forcibly closed by a peer. Nonexistent or protected address space is specified for the message buffer, sending address, or
address length.
[Tru64 UNIX] A valid message buffer was not specified. A signal interrupted recvfrom before any data was available. The MSG-OOB
flag is set and no out-of-band data is available. An I/O error occurred while reading from or writing to the file system. Insuffi-
cient resources are available in the system to complete the call. The system did not have sufficient memory to fulfill the request.
The available STREAMS resources were insufficient for the operation to complete. A receive is attempted on a connection-oriented
socket that is not connected. The socket parameter refers to a file, not a socket. The specified flags are not supported this
socket type. The connection timed out during connection establishment, or due to a transmission timeout on active connection. The
socket is nonblocking; no data is ready to be received.
The MSG_OOB flag is set, no out-of-band data is available, and either the socket is nonblocking or does not support blocking to
await out-of-band data.
RELATED INFORMATION
Functions: read(2), recv(2), recvmsg(2), select(2), send(2), sendmsg(2), sendto(2), shutdown(2), socket(2), write(2).
Standards: standards(5). delim off
recvfrom(2)