Ubuntu Manpages

recv, recvfrom, recvmsg

receive a message from a socket

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

ssize_t recv(int sockfd, void *buf, size_t len, int flags);

ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
                 struct sockaddr *src_addr, socklen_t *addrlen);

ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);

The recvfrom() and recvmsg() calls are used to receive messages from a socket, and may be used to receive data on a socket whether or not it is connection-oriented.

If src_addr is not NULL, and the underlying protocol provides the source address, this source address is filled in. When src_addr is NULL, nothing is filled in; in this case, addrlen is not used, and should also be NULL. The argument addrlen is a value-result argument, which the caller should initialize before the call to the size of the buffer associated with src_addr, and modified on return to indicate the actual size of the source address. The returned address is truncated if the buffer provided is too small; in this case, addrlen will return a value greater than was supplied to the call.

The recv() call is normally used only on a connected socket (see connect(2)) and is identical to recvfrom() with a NULL src_addr argument.

All three routines return the length of the message on successful completion. If a message is too long to fit in the supplied buffer, excess bytes may be discarded depending on the type of socket the message is received from.

If no messages are available at the socket, the receive calls wait for a message to arrive, unless the socket is nonblocking (see fcntl(2)), in which case the value -1 is returned and the external variable errno is set to EAGAIN or EWOULDBLOCK. The receive calls normally return any data available, up to the requested amount, rather than waiting for receipt of the full amount requested.

The select(2) or poll(2) call may be used to determine when more data arrives.

The flags argument to a recv() call is formed by ORing one or more of the following values:

Set the close-on-exec flag for the file descriptor received via a UNIX domain file descriptor using the SCM_RIGHTS operation (described in unix(7)). This flag is useful for the same reasons as the O_CLOEXEC flag of open(2).
Enables nonblocking operation; if the operation would block, the call fails with the error EAGAIN or EWOULDBLOCK (this can also be enabled using the O_NONBLOCK flag with the F_SETFL fcntl(2)).
This flag specifies that queued errors should be received from the socket error queue. The error is passed in an ancillary message with a type dependent on the protocol (for IPv4 IP_RECVERR). The user should supply a buffer of sufficient size. See cmsg(3) and ip(7) for more information. The payload of the original packet that caused the error is passed as normal data via msg_iovec. The original destination address of the datagram that caused the error is supplied via msg_name.
For local errors, no address is passed (this can be checked with the cmsg_len member of the cmsghdr). For error receives, the MSG_ERRQUEUE is set in the msghdr. After an error has been passed, the pending socket error is regenerated based on the next queued error and will be passed on the next socket operation.

The error is supplied in a sock_extended_err structure: 

#define SO_EE_ORIGIN_NONE    0
#define SO_EE_ORIGIN_LOCAL   1
#define SO_EE_ORIGIN_ICMP    2
#define SO_EE_ORIGIN_ICMP6   3
struct sock_extended_err
{
    uint32_t ee_errno;   /* error number */
    uint8_t  ee_origin;  /* where the error originated */
    uint8_t  ee_type;    /* type */
    uint8_t  ee_code;    /* code */
    uint8_t  ee_pad;     /* padding */
    uint32_t ee_info;    /* additional information */
    uint32_t ee_data;    /* other data */
    /* More data may follow */
};
struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);

ee_errno contains the errno number of the queued error. ee_origin is the origin code of where the error originated. The other fields are protocol-specific. The macro SOCK_EE_OFFENDER returns a pointer to the address of the network object where the error originated from given a pointer to the ancillary message. If this address is not known, the sa_family member of the sockaddr contains AF_UNSPEC and the other fields of the sockaddr are undefined. The payload of the packet that caused the error is passed as normal data.
For local errors, no address is passed (this can be checked with the cmsg_len member of the cmsghdr). For error receives, the MSG_ERRQUEUE is set in the msghdr. After an error has been passed, the pending socket error is regenerated based on the next queued error and will be passed on the next socket operation.
This flag requests receipt of out-of-band data that would not be received in the normal data stream. Some protocols place expedited data at the head of the normal data queue, and thus this flag cannot be used with such protocols.
This flag causes the receive operation to return data from the beginning of the receive queue without removing that data from the queue. Thus, a subsequent receive call will return the same data.
For raw (AF_PACKET), Internet datagram (since Linux 2.4.27/2.6.8), netlink (since Linux 2.6.22) and UNIX datagram (since Linux 3.4) sockets: return the real length of the packet or datagram, even when it was longer than the passed buffer. Not implemented for UNIX domain (unix(7)) sockets.

For use with Internet stream sockets, see tcp(7).

This flag requests that the operation block until the full request is satisfied. However, the call may still return less data than requested if a signal is caught, an error or disconnect occurs, or the next data to be received is of a different type than that returned.

The recvmsg() call uses a msghdr structure to minimize the number of directly supplied arguments. This structure is defined as follows in <sys/socket.h>: 

struct iovec {                    /* Scatter/gather array items */
    void  *iov_base;              /* Starting address */
    size_t iov_len;               /* Number of bytes to transfer */
};
struct msghdr {
    void         *msg_name;       /* optional address */
    socklen_t     msg_namelen;    /* size of address */
    struct iovec *msg_iov;        /* scatter/gather array */
    size_t        msg_iovlen;     /* # elements in msg_iov */
    void         *msg_control;    /* ancillary data, see below */
    size_t        msg_controllen; /* ancillary data buffer len */
    int           msg_flags;      /* flags on received message */
};

Here msg_name and msg_namelen specify the source address if the socket is unconnected; msg_name may be given as a NULL pointer if no names are desired or required. The fields msg_iov and msg_iovlen describe scatter-gather locations, as discussed in readv(2). The field msg_control, which has length msg_controllen, points to a buffer for other protocol control-related messages or miscellaneous ancillary data. When recvmsg() is called, msg_controllen should contain the length of the available buffer in msg_control; upon return from a successful call it will contain the length of the control message sequence.

The messages are of the form: 

struct cmsghdr {
    socklen_t     cmsg_len;     /* data byte count, including hdr */
    int           cmsg_level;   /* originating protocol */
    int           cmsg_type;    /* protocol-specific type */
/* followed by
    unsigned char cmsg_data[]; */
};

Ancillary data should be accessed only by the macros defined in cmsg(3).

As an example, Linux uses this ancillary data mechanism to pass extended errors, IP options, or file descriptors over UNIX domain sockets.

The msg_flags field in the msghdr is set on return of recvmsg(). It can contain several flags:

indicates end-of-record; the data returned completed a record (generally used with sockets of type SOCK_SEQPACKET).
indicates that the trailing portion of a datagram was discarded because the datagram was larger than the buffer supplied.
indicates that some control data were discarded due to lack of space in the buffer for ancillary data.
is returned to indicate that expedited or out-of-band data were received.
indicates that no data was received but an extended error from the socket error queue.

These calls return the number of bytes received, or -1 if an error occurred. In the event of an error, errno is set to indicate the error. The return value will be 0 when the peer has performed an orderly shutdown.

These are some standard errors generated by the socket layer. Additional errors may be generated and returned from the underlying protocol modules; see their manual pages.

The socket is marked nonblocking and the receive operation would block, or a receive timeout had been set and the timeout expired before data was received. POSIX.1-2001 allows either error to be returned for this case, and does not require these constants to have the same value, so a portable application should check for both possibilities.
The argument sockfd is an invalid descriptor.
A remote host refused to allow the network connection (typically because it is not running the requested service).
The receive buffer pointer(s) point outside the process's address space.
The receive was interrupted by delivery of a signal before any data were available; see signal(7).
Invalid argument passed.
Could not allocate memory for recvmsg().
The socket is associated with a connection-oriented protocol and has not been connected (see connect(2) and accept(2)).
The argument sockfd does not refer to a socket.

4.4BSD (these function calls first appeared in 4.2BSD), POSIX.1-2001.

POSIX.1-2001 describes only the MSG_OOB, MSG_PEEK, and MSG_WAITALL flags.

The prototypes given above follow glibc2. The Single UNIX Specification agrees, except that it has return values of type ssize_t (while 4.x BSD and libc4 and libc5 all have int). The flags argument is int in 4.x BSD, but unsigned int in libc4 and libc5. The len argument is int in 4.x BSD, but size_t in libc4 and libc5. The addrlen argument is int * in 4.x BSD, libc4 and libc5. The present socklen_t * was invented by POSIX. See also accept(2).

According to POSIX.1-2001, the msg_controllen field of the msghdr structure should be typed as socklen_t, but glibc currently types it as size_t.

See recvmmsg(2) for information about a Linux-specific system call that can be used to receive multiple datagrams in a single call.

An example of the use of recvfrom() is shown in getaddrinfo(3).

fcntl(2), getsockopt(2), read(2), recvmmsg(2), select(2), shutdown(2), socket(2), cmsg(3), sockatmark(3), socket(7)

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