SEND(2) | System Calls Manual | SEND(2) |
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);
int
sendmmsg(int s, struct mmsghdr *mmsg, unsigned int vlen, unsigned int flags);
The sendmmsg() call be used to send multiple messages in the same call using an array of mmsghdr elements with the following form, as defined in <sys/socket.h>:
struct mmsghdr { struct msghdr msg_hdr; /* the message to be sent */ unsigned int msg_len; /* number of bytes transmitted */ };
The msg_len member contains the number of bytes sent for each msg_hdr member. The array has vlen elements, which is limited to 1024. If there is an error, a number fewer than vlen may be returned, and the error may be retrieved using getsockopt(2) with SO_ERROR.
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, the error EMSGSIZE is returned, and the message is not transmitted.
No indication of failure to deliver is implicit in a send(). Locally detected errors are indicated by a return value of -1.
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) or poll(2) call may be used to determine when it is possible to send more data. Unfortunately this does not work when the interface queue which is used to send the message is full, and the call returns ENOBUFS.
The flags parameter may include one or more of the following:
#define MSG_OOB 0x0001 /* process out-of-band data */ #define MSG_PEEK 0x0002 /* peek at incoming message */ #define MSG_DONTROUTE 0x0004 /* bypass routing, use direct interface */ #define MSG_EOR 0x0008 /* data completes record */ #define MSG_NOSIGNAL 0x0400 /* do not generate SIGPIPE on EOF */
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_EOR is used to indicate a record mark for protocols which support the concept. MSG_DONTROUTE is usually used only by diagnostic or routing programs.
See recv(2) for a description of the msghdr structure. MSG_NOSIGNAL is used to prevent SIGPIPE generation when writing a socket that may be closed.
An alternative reason: 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.
sendto() will also fail if:
sendmsg() and sendmmsg() will also fail if:
June 22, 2012 | NetBSD 7.2 |