send(3) - glibc (send / sendto / sendmsg / recv family, readv / writev)

Companion page to socket-connect-bind-listen-accept. The raw syscall entries are in Network and IPC syscalls; generic vector I/O on descriptors is also covered in readv-writev-pread-pwrite.

Declaration and gates

ssize_t send(int socket, const void *buffer, size_t size, int flags);
ssize_t sendto(int socket, const void *buffer, size_t size, int flags,
               struct sockaddr *addr, socklen_t length);
ssize_t sendmsg(int socket, const struct msghdr *message, int flags);
ssize_t recv(int socket, void *buffer, size_t size, int flags);
ssize_t recvfrom(int socket, void *buffer, size_t size, int flags,
                 struct sockaddr *addr, socklen_t *length_ptr);
ssize_t recvmsg(int socket, struct msghdr *message, int flags);
ssize_t readv(int fd, const struct iovec *iov, int iovcnt);   /* unistd.h */
ssize_t writev(int fd, const struct iovec *iov, int iovcnt);  /* unistd.h */

Semantics

With flags == 0, send/recv are equivalent to write/read on the socket; the flags select socket-specific modes: MSG_OOB (out-of-band data), MSG_PEEK (input only; look without consuming), MSG_DONTROUTE (output only) (manual, Socket Data Options). sendto/recvfrom add the per-packet destination/source address used on datagram sockets; connect on a datagram socket fixes a default destination that AF_UNSPEC clears (manual, Sending Datagrams). sendmsg/recvmsg take a struct msghdr for scatter/gather buffers and ancillary data. A truncated datagram is lost: recvfrom returns the first size bytes and the rest is discarded (manual, Receiving Datagrams). A broken stream connection makes send/write raise SIGPIPE; if that is ignored or handled, they fail with EPIPE (manual, Sending Data). readv/writev are descriptor calls that also work on sockets. On nonblocking sockets a partial send returns the bytes accepted so far.

Gotchas and portability

• Successful send means queued without error, not delivered (manual, Sending Data).
• A sendto error can report a problem from a previous packet (manual, Sending Datagrams).
• musl src/network/send.c is a two-line alias: send(fd, buf, len, flags) calls sendto(fd, buf, len, flags, 0, 0).
• musl’s sendmsg/recvmsg wrappers take the same syscalls; no MSG_* flag differences were found in the cached musl 1.2.5 tree (unverified beyond the listed sources).

References

• glibc manual, nodes Transferring Data, Socket Data Options, Datagrams.
• man2: send.2, sendto.2, sendmsg.2, recv.2, recvfrom.2, recvmsg.2, readv.2, writev.2.
• Headers sys/socket.h, sys/uio.h; nm -D --with-symbol-versions on libc.so.6 (glibc 2.41).