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       accept, accept4 - accept a connection on a socket


       #include <sys/types.h>          /* See NOTES */
       #include <sys/socket.h>

       int accept(int sockfd, struct sockaddr *addr, socklen_t *addrlen);

       #define _GNU_SOURCE             /* See feature_test_macros(7) */
       #include <sys/socket.h>

       int accept4(int sockfd, struct sockaddr *addr,
                   socklen_t *addrlen, int flags);


       The  accept()  system  call  is  used  with  connection-based  socket  types (SOCK_STREAM,
       SOCK_SEQPACKET).  It extracts the  first  connection  request  on  the  queue  of  pending
       connections  for the listening socket, sockfd, creates a new connected socket, and returns
       a new file descriptor referring to that socket.  The newly created socket is  not  in  the
       listening state.  The original socket sockfd is unaffected by this call.

       The  argument  sockfd  is  a socket that has been created with socket(2), bound to a local
       address with bind(2), and is listening for connections after a listen(2).

       The argument addr is a pointer to a sockaddr structure.  This structure is filled in  with
       the address of the peer socket, as known to the communications layer.  The exact format of
       the address returned addr is determined by the socket's address family (see socket(2)  and
       the  respective  protocol  man  pages).   When addr is NULL, nothing is filled in; in this
       case, addrlen is not used, and should also be NULL.

       The addrlen argument is a value-result argument: the caller must initialize it to  contain
       the  size  (in  bytes)  of the structure pointed to by addr; on return it will contain the
       actual size of the peer 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  no  pending  connections  are  present  on  the queue, and the socket is not marked as
       nonblocking, accept() blocks the caller until a connection is present.  If the  socket  is
       marked  nonblocking  and  no  pending connections are present on the queue, accept() fails
       with the error EAGAIN or EWOULDBLOCK.

       In order to be notified of incoming connections on a socket,  you  can  use  select(2)  or
       poll(2).   A  readable  event will be delivered when a new connection is attempted and you
       may then call accept() to get a socket for that connection.  Alternatively,  you  can  set
       the socket to deliver SIGIO when activity occurs on a socket; see socket(7) for details.

       For certain protocols which require an explicit confirmation, such as DECNet, accept() can
       be  thought  of  as  merely  dequeuing  the  next  connection  request  and  not  implying
       confirmation.   Confirmation  can  be  implied  by  a normal read or write on the new file
       descriptor, and rejection can be implied by closing the new socket.  Currently only DECNet
       has these semantics on Linux.

       If  flags  is  0,  then  accept4()  is  the same as accept().  The following values can be
       bitwise ORed in flags to obtain different behavior:

       SOCK_NONBLOCK   Set the O_NONBLOCK file status flag on  the  new  open  file  description.
                       Using this flag saves extra calls to fcntl(2) to achieve the same result.

       SOCK_CLOEXEC    Set  the  close-on-exec (FD_CLOEXEC) flag on the new file descriptor.  See
                       the description of the O_CLOEXEC flag in open(2) for reasons why this  may
                       be useful.


       On  success,  these system calls return a nonnegative integer that is a descriptor for the
       accepted socket.  On error, -1 is returned, and errno is set appropriately.

   Error handling
       Linux accept() (and accept4()) passes already-pending network errors on the new socket  as
       an error code from accept().  This behavior differs from other BSD socket implementations.
       For reliable operation the application should detect the network errors  defined  for  the
       protocol  after  accept()  and treat them like EAGAIN by retrying.  In the case of TCP/IP,


              The  socket  is  marked  nonblocking and no connections are present to be accepted.
              POSIX.1-2001 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 descriptor is invalid.

              A connection has been aborted.

       EFAULT The addr argument is not in a writable part of the user address space.

       EINTR  The system call was interrupted  by  a  signal  that  was  caught  before  a  valid
              connection arrived; see signal(7).

       EINVAL Socket is not listening for connections, or addrlen is invalid (e.g., is negative).

       EINVAL (accept4()) invalid value in flags.

       EMFILE The per-process limit of open file descriptors has been reached.

       ENFILE The system limit on the total number of open files has been reached.

              Not  enough free memory.  This often means that the memory allocation is limited by
              the socket buffer limits, not by the system memory.

              The descriptor references a file, not a socket.

              The referenced socket is not of type SOCK_STREAM.

       EPROTO Protocol error.

       In addition, Linux accept() may fail if:

       EPERM  Firewall rules forbid connection.

       In addition, network errors for the new socket and as defined  for  the  protocol  may  be
       returned.   Various  Linux kernels can return other errors such as ENOSR, ESOCKTNOSUPPORT,
       EPROTONOSUPPORT, ETIMEDOUT.  The value ERESTARTSYS may be seen during a trace.


       The accept4() system call is available starting with Linux 2.6.28;  support  in  glibc  is
       available starting with version 2.10.


       accept(): POSIX.1-2001, SVr4, 4.4BSD, (accept() first appeared in 4.2BSD).

       accept4() is a nonstandard Linux extension.

       On  Linux,  the new socket returned by accept() does not inherit file status flags such as
       O_NONBLOCK and O_ASYNC  from  the  listening  socket.   This  behavior  differs  from  the
       canonical BSD sockets implementation.  Portable programs should not rely on inheritance or
       noninheritance of file status flags and always explicitly set all required  flags  on  the
       socket returned from accept().


       POSIX.1-2001  does not require the inclusion of <sys/types.h>, and this header file is not
       required on Linux.  However, some historical (BSD) implementations  required  this  header
       file, and portable applications are probably wise to include it.

       There  may  not  always be a connection waiting after a SIGIO is delivered or select(2) or
       poll(2) return a readability event because the connection might have been  removed  by  an
       asynchronous  network  error or another thread before accept() is called.  If this happens
       then the call will block waiting for the  next  connection  to  arrive.   To  ensure  that
       accept() never blocks, the passed socket sockfd needs to have the O_NONBLOCK flag set (see

   The socklen_t type
       The third argument of accept() was originally declared as an int  *  (and  is  that  under
       libc4  and  libc5  and on many other systems like 4.x BSD, SunOS 4, SGI); a POSIX.1g draft
       standard wanted to change it into a size_t *, and that is what it is for SunOS  5.   Later
       POSIX  drafts  have  socklen_t  *,  and  so  do  the Single UNIX Specification and glibc2.
       Quoting Linus Torvalds:

       "_Any_ sane library _must_ have "socklen_t" be the same size as int.  Anything else breaks
       any  BSD  socket  layer stuff.  POSIX initially did make it a size_t, and I (and hopefully
       others, but obviously not too many) complained to them very loudly indeed.   Making  it  a
       size_t  is completely broken, exactly because size_t very seldom is the same size as "int"
       on 64-bit architectures, for example.  And it has to be the same  size  as  "int"  because
       that's  what the BSD socket interface is.  Anyway, the POSIX people eventually got a clue,
       and created "socklen_t".  They shouldn't have touched it in the first place, but once they
       did  they felt it had to have a named type for some unfathomable reason (probably somebody
       didn't like losing face over having done the original stupid thing, so they silently  just
       renamed their blunder)."


       See bind(2).


       bind(2), connect(2), listen(2), select(2), socket(2), socket(7)


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