Langue: en

Autres versions - même langue

Version: 338340 (ubuntu - 24/10/10)

Section: 2 (Appels système)

BSD mandoc


recv recvfrom recvmsg - receive a message from a socket


Lb libc


In sys/types.h In sys/socket.h Ft ssize_t Fn recv int s void *buf size_t len int flags Ft ssize_t Fn recvfrom int s void * restrict buf size_t len int flags struct sockaddr * restrict from socklen_t * restrict fromlen Ft ssize_t Fn recvmsg int s struct msghdr *msg int flags


The Fn recvfrom and Fn recvmsg system 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 Fa from is not a null pointer and the socket is not connection-oriented, the source address of the message is filled in. The Fa fromlen argument is a value-result argument, initialized to the size of the buffer associated with Fa from , and modified on return to indicate the actual size of the address stored there.

The Fn recv function is normally used only on a connected socket (see connect(2)) and is identical to Fn recvfrom with a null pointer passed as its Fa from argument.


If no messages are available at the socket, the receive call waits for a message to arrive, unless the socket is non-blocking (see fcntl(2)) in which case the value -1 is returned and the global variable errno is set to Er EAGAIN . The receive calls normally return any data available, up to the requested amount, rather than waiting for receipt of the full amount requested; this behavior is affected by the socket-level options SO_RCVLOWAT and SO_RCVTIMEO described in getsockopt(2).

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

The Fa flags argument to a Fn recv function is formed by or Ap ing one or more of the values:

MSG_OOB Ta process out-of-band data
MSG_PEEK Ta peek at incoming message
MSG_WAITALL Ta wait for full request or error
MSG_DONTWAIT Ta do not block


The Fn recvmsg system call uses a Fa msghdr structure to minimize the number of directly supplied arguments. This structure has the following form, as defined in In sys/socket.h :

 struct msghdr {
         void            *msg_name;      /* optional address */
         socklen_t        msg_namelen;   /* size of address */
         int              msg_iovlen;    /* # elements in msg_iov */
         socklen_t        msg_controllen;/* ancillary data buffer len */
         int              msg_flags;     /* flags on received message */

Here Fa msg_name and Fa msg_namelen specify the destination address if the socket is unconnected; Fa msg_name may be given as a null pointer if no names are desired or required. The Fa msg_iov and Fa msg_iovlen arguments describe scatter gather locations, as discussed in read(2). The Fa msg_control argument, which has length Fa msg_controllen , points to a buffer for other protocol control related messages or other miscellaneous ancillary data. The messages are of the form:

 struct cmsghdr {
         int        cmsg_level;  /* originating protocol */
         int        cmsg_type;   /* protocol-specific type */
 /* followed by
         u_char     cmsg_data[]; */

As an example, one could use this to learn of changes in the data-stream in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting a Fn recvmsg with no data buffer provided immediately after an Fn accept system call.

Open file descriptors are now passed as ancillary data for AF_UNIX domain sockets, with Fa cmsg_level set to SOL_SOCKET and Fa cmsg_type set to SCM_RIGHTS

Process credentials can also be passed as ancillary data for AF_UNIX domain sockets using a Fa cmsg_type of SCM_CREDS In this case, Fa cmsg_data should be a structure of type Fa cmsgcred , which is defined in In sys/socket.h as follows:

 struct cmsgcred {
         pid_t   cmcred_pid;             /* PID of sending process */
         uid_t   cmcred_uid;             /* real UID of sending process */
         uid_t   cmcred_euid;            /* effective UID of sending process */
         gid_t   cmcred_gid;             /* real GID of sending process */
         short   cmcred_ngroups;         /* number or groups */
         gid_t   cmcred_groups[CMGROUP_MAX];     /* groups */






The calls fail if:
The argument Fa s is an invalid descriptor.
The remote socket end is forcibly closed.
The argument Fa s does not refer to a socket.
The Fn recvmsg system call was used to receive rights (file descriptors) that were in flight on the connection. However, the receiving program did not have enough free file descriptor slots to accept them. In this case the descriptors are closed, any pending data can be returned by another call to Fn recvmsg .




The Fn recv function appeared in BSD 4.2