SEND(2) SEND(2) NAME send, sendto, sendmsg - send a message from a socket C SYNOPSIS #include <sys/types.h> #include <sys/socket.h> int send(int s, const void *msg, int len, int flags); int sendto(int s, const void *msg, int len, int flags, const struct sockaddr *to, socklen_t tolen); int sendmsg(int s, const struct msghdr *msg, int flags); #if _XOPEN_SOURCE >= 500 ssize_t send(int s, const void *msg, size_t len, int flags); ssize_t sendto(int s, const void *msg, size_t len, int flags, const struct sockaddr *to, socklen_t tolen); ssize_t sendmsg(int s, const struct msghdr *msg, int flags); #elif _XOPEN_SOURCE < 500 ssize_t send(int s, const void *msg, size_t len, int flags); ssize_t sendto(int s, const void *msg, size_t len, int flags, const struct sockaddr *to, size_t tolen; ssize_t sendmsg(int s, const struct msghdr *msg, int flags); #endif DESCRIPTION Send, sendto, and sendmsg are used to transmit a message to another socket. Send may be used only when the socket is in a connected state, while sendto and sendmsg may be used when the socket is unconnected. The address of the target is given by to with tolen specifying its size. The length of the message is given by len. If the message is too long to pass atomically through the underlying protocol, then the error EMSGSIZE is returned, and the message is not transmitted. Usually no indication of failure to deliver is implicit in a send. Return values of -1 indicate some locally detected errors. Connected datagram sockets may receive error indications from a previous send. If no messages space is available at the socket to hold the message to be transmitted, then send normally blocks, unless the socket has been placed in non-blocking I/O mode. The select(2) call may be used to determine when it is possible to send more data. The flags parameter may include one or more of the following: #define MSG_OOB 0x1 /* process out-of-band data */ #define MSG_DONTROUTE 0x4 /* bypass routing, use direct interface */ #define MSG_DONT_BRKPG 0x100 /* don't break up pages for send */ The flag MSG_OOB is used to send "out-of-band" data on sockets that support this notion (e.g., SOCK_STREAM); the underlying protocol must also support "out-of-band" data. MSG_DONTROUTE is usually used only by diagnostic or routing programs. MSG_DONT_BRKPG is used to allow the user, on a per send basis to stop page flipping and the break up of large memory pages to 16k. This can be done automatically for all large pages transfers by setting the mtune/bsd/ip_nolgpg_brkup variable. NOTE that setting this can have a negative effect on performance. See recv(2) for a description of the msghdr structure. RETURN VALUE The call returns the number of characters sent, or -1 if an error occurred. ERRORS [EBADF] An invalid descriptor was specified. [ENOTSOCK] The argument s is not a socket. [EFAULT] An invalid user space address was specified for a parameter. [EMSGSIZE] (1) The socket requires that message be sent atomically, and the size of the message to be sent made this impossible; OR (2) In the sendmsg() call, the msg_iovlen member of the msghdr structure pointed to by msg is greater than MSG_MAXIOVLEN or less than or equal to 0. [EWOULDBLOCK] The socket is marked non-blocking and the requested operation would block. [ENOBUFS] The system was unable to allocate an internal buffer. The operation may succeed when buffers become available. [ENOBUFS] The output queue for a network interface was full. This generally indicates that the interface has stopped sending, but may be caused by transient congestion. [ECONNREFUSED] The remote port was invalid when using the send call on a connected datagram socket. [EISCONN] A sendto or sendmsg call was used on a connected socket. [EACCES] The requested operation specified a broadcast address as the destination but the SO_BROADCAST socket option was not enabled (see setsockopt(2)). [EHOSTUNREACH] The remote host was unreachable via the network. [ENETUNREACH] The remote network is unknown to the routing system. [EHOSTDOWN] The remote host was determined to be down, possibly due to a failure to resolve its MAC-level address (see arp(7P)). SEE ALSO fcntl(2), recv(2), select(2), getsockopt(2), socket(2), write(2) NOTES ABI-compliant versions of the above call can be obtained from libsocket.so. For each of these three functions, send, sendto and sendmsg , there are three types of functions in n32 and 64 bit C libraries for IRIX 6.5.19 and later versions. One is the normal type when _XOPEN_SOURCE is not defined; the second is XPG5 type when _XOPEN_SOURCE is set to >= 500; and the third is XPG4 type when _XOPEN_SOURCE set to < 500. send function: 1. For the normal case when _XOPEN_SOURCE is not defined, third argument type, will be an int and the normal send is used. 2. When _XOPEN_SOURCE is >= 500 or < 500, third argument type will be a size_t and XPG5 or XPG4 send is used. sendto function: 1. For the normal case when _XOPEN_SOURCE is not defined, third argument type will be an int and the sixth argument will be a pointer to socklen_t type, which is actually a pointer to an int, and the normal sendto is used. 2. When _XOPEN_SOURCE is set to >= 500, third argument type will be a size_t and the sixth argument will be of socklen_t type, which is actually a u_int32_t. 3. When _XOPEN_SOURCE is set to < 500, third argument type will be a size_t and the sixth argument will be of size_t type. sendmsg function: 1. For the normal case when _XOPEN_SOURCE is not defined, the normal struct msghdr will be used in the second argument. 2. When _XOPEN_SOURCE is set to >= 500, _XOPEN5 struct msghdr will be used in the second argument. 3. When _XOPEN_SOURCE is set to < 500, _XOPEN4 struct msghdr will be used in the second argument. In the AF_UNIX domain sendmsg function behaves in the following way: a. If _XOPEN_SOURCE is not defined, the msg_accrights field of struct msghdr that is used to specify access rights is limited to file descriptors, which each occupy the size of an int. b. If _XOPEN_SOURCE is defined, the msg_control field of struct msghdr that is used to specify ancillary data is limited to a cmsghdr structure followed by an array of file descriptors. The fields of cmsghdr structure are set to: cmsg_level set to SOL_SOCKET; cmsg_type set to SCM_RIGHTS; and cmsg_len set to data byte count, including the cmsghdr. Refer <sys/socket.h> for alternate definitions of socklen_t and struct msghdr. XPG5 type functions are not supported in o32 C library. The XPG5 type send, sendto and sendmsg functions are actually defined as static inline functions in <sys/socket.h>, and each call a new function _xpg5_send, _xpg5_sendto or _xpg5_sendmsg which are specific to IRIX 6.5.19 and later. Applications that call any of these XPG5 type functions should check for the existence of the new symbol as done in the following example. #include <sys/socket.h> #include <optional_sym.h> if (_MIPS_SYMBOL_PRESENT(_xpg5_send)) { send(s, &buf, len, flags); } else { ... } Because the static inline functions are defined in each source file that includes <sys/socket.h>, these static functions will have different addresses in case the inline expansion is not performed. This may cause problems if the address of the function is examined in programs. To avoid this problem, use -D_XPG5_SEND_USER_DEFINED compile option to disable the static inline definition of send in <sys/socket.h>, and define a user defined function in the following way: ssize_t send(int _s, void *_buf, size_t _len, int _flags) { return(_xpg5_send(_s, _buf, _len, _flags)); } Similarly use -D_XPG5_SENDTO_USER_DEFINED or -D_XPG5_SENDMSG_USER_DEFINED compile option to disable the xpg5 static inline definition of sendto or sendmsg in <sys/socket.h> Use the appropriate compile option always, when a user defined XPG5 function is required. Page 4