Receive a message and its header from a socket
#include <sys/types.h> #include <sys/socket.h> ssize_t recvmsg( int s, struct msghdr * msg, int flags );
The recvmsg() routine receives a message from a socket, s, whether or not it's connection-oriented.
The recvmsg() call uses a msghdr structure to minimize the number of directly supplied parameters. This structure, defined in <sys/socket.h>, has the following form:
struct msghdr { caddr_t msg_name; /* optional address */ u_int msg_namelen; /* size of address */ struct iovec *msg_iov; /* scatter/gather array */ u_int msg_iovlen; /* # elements in msg_iov */ caddr_t msg_control; /* ancillary data, see below */ u_int msg_controllen; /* ancillary data buffer len */ int msg_flags; /* flags on received message */ };
The msg_name and msg_namelen parameters specify the address (source address for recvmsg(); destination address for sendmsg()) if the socket is unconnected; the msg_name parameter may be given as a null pointer if no names are desired or required.
The msg_iov and msg_iovlen parameters describe scatter-gather locations, as discussed in read().
The msg_control parameter, whose length is determined by msg_controllen, points to a buffer for other protocol-control related messages or for other miscellaneous ancillary data. The messages are of the form:
struct cmsghdr { u_int cmsg_len; /* data byte count, including hdr */ int cmsg_level; /* originating protocol */ int cmsg_type; /* protocol-specific type */ /* followed by u_char cmsg_data[]; */ };
With recvmsg() and recvmmsg(), the length in msg_controllen must not exceed SSIZE_MAX (see <limits.h>), or the function fails and sets errno to EOVERFLOW. With sendmsg() or sendmmsg(), msg_controllen must not exceed SSIZE_MAX - sizeof(_io_sock_sendto) - sizeof(socklen_t), or EOVERFLOW occurs.
The msg_flags field is set on return according to the message received:
The number of bytes received, or -1 if an error occurs (errno is set).
Safety: | |
---|---|
Cancellation point | Yes |
Interrupt handler | No |
Signal handler | No |
Thread | Yes |