C语言 recv()函数recvfrom()函数recvmsg()函数

Posted 2022-08-27 Dontla

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RECV(2)                                                              Linux Programmer's Manual                                                             RECV(2)

NAME
       recv, recvfrom, recvmsg - receive a message from a socket	//接收来自套接字的消息

SYNOPSIS
       #include <sys/types.h>
       #include <sys/socket.h>

       ssize_t recv(int sockfd, void *buf, size_t len, int flags);

       ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
                        struct sockaddr *src_addr, socklen_t *addrlen);

       ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);

DESCRIPTION
       The  recv(),  recvfrom(), and recvmsg() calls are used to receive messages from a socket.  They may be used to receive data on both connectionless and con‐
       nection-oriented sockets.  This page first describes common features of all three system calls, and then describes the differences between the calls.
       //recv()、recvfrom() 和 recvmsg() 调用用于接收来自套接字的消息。 
       //它们可用于接收无连接和面向连接的套接字上的数据。 
       //本页首先描述所有三个系统调用的共同特征，然后描述调用之间的区别。

       All three calls return the length of the message on successful completion.  If a message is too long to fit in the supplied buffer,  excess  bytes  may  be
       discarded depending on the type of socket the message is received from.
       //成功完成后，所有三个调用都会返回消息的长度。 
       //如果消息太长而无法放入提供的缓冲区，则可能会丢弃多余的字节，具体取决于接收消息的套接字类型。

       If  no messages are available at the socket, the receive calls wait for a message to arrive, unless the socket is nonblocking (see fcntl(2)), in which case
       the value -1 is returned and the external variable errno is set to EAGAIN or EWOULDBLOCK.  The receive calls normally return any data available, up to  the
       requested amount, rather than waiting for receipt of the full amount requested.
       //如果套接字上没有可用的消息，则接收调用会等待消息到达，除非套接字是非阻塞的（请参阅 fcntl(2)），在这种情况下返回值 -1 并将外部变量 errno 设置为 EAGAIN 或 EWOULDBLOCK。 
       //接收调用通常会返回任何可用的数据，直到请求的数量，而不是等待接收请求的全部数量。

       An application can use select(2), poll(2), or epoll(7) to determine when more data arrives on a socket.
       //应用程序可以使用 select(2)、poll(2) 或 epoll(7) 来确定更多数据何时到达套接字。

   The flags argument
       The flags argument is formed by ORing one or more of the following values:
       //flags 参数由 ORing 以下一个或多个值形成：

       MSG_CMSG_CLOEXEC (recvmsg() only; since Linux 2.6.23)
              Set the close-on-exec flag for the file descriptor received via a UNIX domain file descriptor using the SCM_RIGHTS operation (described in unix(7)).
              This flag is useful for the same reasons as the O_CLOEXEC flag of open(2).

       MSG_DONTWAIT (since Linux 2.2)
              Enables nonblocking operation; if the operation would block, the call fails with the error EAGAIN or EWOULDBLOCK.  This provides similar behavior to
              setting  the  O_NONBLOCK  flag  (via the fcntl(2) F_SETFL operation), but differs in that MSG_DONTWAIT is a per-call option, whereas O_NONBLOCK is a
              setting on the open file description (see open(2)), which will affect all threads in the calling process and as well as other  processes  that  hold
              file descriptors referring to the same open file description.

       MSG_ERRQUEUE (since Linux 2.2)
              This  flag  specifies  that  queued  errors should be received from the socket error queue.  The error is passed in an ancillary message with a type
              dependent on the protocol (for IPv4 IP_RECVERR).  The user should supply a buffer of sufficient size.  See cmsg(3) and ip(7) for  more  information.
              The  payload  of the original packet that caused the error is passed as normal data via msg_iovec.  The original destination address of the datagram
              that caused the error is supplied via msg_name.

              For local errors, no address is passed (this can be checked with the cmsg_len member of the cmsghdr).  For error receives, the MSG_ERRQUEUE  is  set
              in  the  msghdr.   After  an error has been passed, the pending socket error is regenerated based on the next queued error and will be passed on the
              next socket operation.

              The error is supplied in a sock_extended_err structure:

                  #define SO_EE_ORIGIN_NONE    0
                  #define SO_EE_ORIGIN_LOCAL   1
                  #define SO_EE_ORIGIN_ICMP    2
                  #define SO_EE_ORIGIN_ICMP6   3

                  struct sock_extended_err
                  
                      uint32_t ee_errno;   /* error number */
                      uint8_t  ee_origin;  /* where the error originated */
                      uint8_t  ee_type;    /* type */
                      uint8_t  ee_code;    /* code */
                      uint8_t  ee_pad;     /* padding */
                      uint32_t ee_info;    /* additional information */
                      uint32_t ee_data;    /* other data */
                      /* More data may follow */
                  ;

                  struct sockaddr *SO_EE_OFFENDER(struct sock_extended_err *);

              ee_errno contains the errno number of the queued error.  ee_origin is the origin code of where the error originated.  The other fields are protocol-
              specific.   The macro SOCK_EE_OFFENDER returns a pointer to the address of the network object where the error originated from given a pointer to the
              ancillary message.  If this address is not known, the sa_family member of the sockaddr contains AF_UNSPEC and the other fields of the  sockaddr  are
              undefined.  The payload of the packet that caused the error is passed as normal data.

              For  local  errors, no address is passed (this can be checked with the cmsg_len member of the cmsghdr).  For error receives, the MSG_ERRQUEUE is set
              in the msghdr.  After an error has been passed, the pending socket error is regenerated based on the next queued error and will  be  passed  on  the
              next socket operation.

       MSG_OOB
              This  flag  requests  receipt  of out-of-band data that would not be received in the normal data stream.  Some protocols place expedited data at the
              head of the normal data queue, and thus this flag cannot be used with such protocols.

       MSG_PEEK
              This flag causes the receive operation to return data from the beginning of the receive queue without removing that data from the  queue.   Thus,  a
              subsequent receive call will return the same data.

       MSG_TRUNC (since Linux 2.2)
              For raw (AF_PACKET), Internet datagram (since Linux 2.4.27/2.6.8), netlink (since Linux 2.6.22), and UNIX datagram (since Linux 3.4) sockets: return
              the real length of the packet or datagram, even when it was longer than the passed buffer.

              For use with Internet stream sockets, see tcp(7).

       MSG_WAITALL (since Linux 2.2)
              This flag requests that the operation block until the full request is satisfied.  However, the call may still return less data than requested  if  a
              signal is caught, an error or disconnect occurs, or the next data to be received is of a different type than that returned.

   recvfrom()
       recvfrom() places the received message into the buffer buf.  The caller must specify the size of the buffer in len.
       //recvfrom() 将接收到的消息放入缓冲区 buf。 调用者必须在 len 中指定缓冲区的大小。

       If  src_addr is not NULL, and the underlying protocol provides the source address of the message, that source address is placed in the buffer pointed to by
       src_addr.  In this case, addrlen is a value-result argument.  Before the call, it should be initialized to the size of the buffer associated with src_addr.
       Upon  return,  addrlen is updated to contain the actual size of the source address.  The returned address is truncated if the buffer provided is too small;
       in this case, addrlen will return a value greater than was supplied to the call.

       If the caller is not interested in the source address, src_addr and addrlen should be specified as NULL.

   recv()
       The recv() call is normally used only on a connected socket (see connect(2)).  It is equivalent to the call:
       //recv() 调用通常仅在连接的套接字上使用（请参阅 connect(2)）。 相当于调用：

           recvfrom(fd, buf, len, flags, NULL, 0));

   recvmsg()
       The recvmsg() call uses a msghdr structure to minimize the number of directly supplied arguments.  This structure is defined as follows in <sys/socket.h>:

           struct iovec                     /* Scatter/gather array items */
               void  *iov_base;              /* Starting address */
               size_t iov_len;               /* Number of bytes to transfer */
           ;

           struct msghdr 
               void         *msg_name;       /* optional address */
               socklen_t     msg_namelen;    /* size of address */
               struct iovec *msg_iov;        /* scatter/gather array */
               size_t        msg_iovlen;     /* # elements in msg_iov */
               void         *msg_control;    /* ancillary data, see below */
               size_t        msg_controllen; /* ancillary data buffer len */
               int           msg_flags;      /* flags on received message */
           ;

       The msg_name field points to a caller-allocated buffer that is used to return the source address if the socket  is  unconnected.   The  caller  should  set
       msg_namelen  to  the size of this buffer before this call; upon return from a successful call, msg_namelen will contain the length of the returned address.
       If the application does not need to know the source address, msg_name can be specified as NULL.

       The fields msg_iov and msg_iovlen describe scatter-gather locations, as discussed in readv(2).

       The field msg_control, which has length msg_controllen, points to a buffer for other protocol control-related messages  or  miscellaneous  ancillary  data.
       When  recvmsg() is called, msg_controllen should contain the length of the available buffer in msg_control; upon return from a successful call it will con‐
       tain the length of the control message sequence.

       The messages are of the form:

           struct cmsghdr 
               socklen_t     cmsg_len;     /* data byte count, including hdr */
               int           cmsg_level;   /* originating protocol */
               int           cmsg_type;    /* protocol-specific type */
           /* followed by
               unsigned char cmsg_data[]; */
           ;

       Ancillary data should be accessed only by the macros defined in cmsg(3).

       As an example, Linux uses this ancillary data mechanism to pass extended errors, IP options, or file descriptors over UNIX domain sockets.

       The msg_flags field in the msghdr is set on return of recvmsg().  It can contain several flags:

       MSG_EOR
              indicates end-of-record; the data returned completed a record (generally used with sockets of type SOCK_SEQPACKET).

       MSG_TRUNC
              indicates that the trailing portion of a datagram was discarded because the datagram was larger than the buffer supplied.

       MSG_CTRUNC
              indicates that some control data were discarded due to lack of space in the buffer for ancillary data.

       MSG_OOB
              is returned to indicate that expedited or out-of-band data were received.

       MSG_ERRQUEUE
              indicates that no data was received but an extended error from the socket error queue.

RETURN VALUE
       These calls return the number of bytes received, or -1 if an error occurred.  In the event of an error, errno is set to indicate the error.

       When a stream socket peer has performed an orderly shutdown, the return value will be 0 (the traditional "end-of-file" return).

       Datagram sockets in various domains (e.g., the UNIX and Internet domains) permit zero-length datagrams.  When such a datagram is received, the return value
       is 0.

       The value 0 may also be returned if the requested number of bytes to receive from a stream socket was 0.

ERRORS
       These  are  some  standard errors generated by the socket layer.  Additional errors may be generated and returned from the underlying protocol modules; see
       their manual pages.

       EAGAIN or EWOULDBLOCK
              The socket is marked nonblocking and the receive operation would block, or a receive timeout had been set and the timeout expired  before  data  was
              received.   POSIX.1  allows  either  error  to be returned for this case, and does not require these constants to have the same value, so a portable
              application should check for both possibilities.

       EBADF  The argument sockfd is an invalid descriptor.

       ECONNREFUSED
              A remote host refused to allow the network connection (typically because it is not running the requested service).

       EFAULT The receive buffer pointer(s) point outside the process's address space.

       EINTR  The receive was interrupted by delivery of a signal before any data were available; see signal(7).

       EINVAL Invalid argument passed.

       ENOMEM Could not allocate memory for recvmsg().

       ENOTCONN
              The socket is associated with a connection-oriented protocol and has not been connected (see connect(2) and accept(2)).

       ENOTSOCK
              The file descriptor sockfd does not refer to a socket.

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, 4.4BSD (these interfaces first appeared in 4.2BSD).

       POSIX.1 describes only the MSG_OOB, MSG_PEEK, and MSG_WAITALL flags.

NOTES
       The socklen_t type was invented by POSIX.  See also accept(2).

       According to POSIX.1, the msg_controllen field of the msghdr structure should be typed as socklen_t, but glibc currently types it as size_t.

       See recvmmsg(2) for information about a Linux-specific system call that can be used to receive multiple datagrams in a single call.

EXAMPLE
       An example of the use of recvfrom() is shown in getaddrinfo(3).

SEE ALSO
       fcntl(2), getsockopt(2), read(2), recvmmsg(2), select(2), shutdown(2), socket(2), cmsg(3), sockatmark(3), socket(7)

COLOPHON
       This page is part of release 4.04 of the Linux man-pages project.  A description of the project, information about reporting bugs, and the  latest  version
       of this page, can be found at http://www.kernel.org/doc/man-pages/.

Linux                                                                       2015-12-28                                                                     RECV(2)
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