# accept

## NAME



           accept, accept4 - accept a connection on a socket
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## SYNOPSIS



           **#include <sys/socket.h>**
    
           **int accept(int** _sockfd_**, struct sockaddr *_Nullable restrict** _addr_**,**
                      **socklen_t *_Nullable restrict** _addrlen_**);**
    
           **#define _GNU_SOURCE** /* See feature_test_macros(7) */
           **#include <sys/socket.h>**
    
           **int accept4(int** _sockfd_**, struct sockaddr *_Nullable restrict** _addr_**,**
                      **socklen_t *_Nullable restrict** _addrlen_**, int** _flags_**);**
    


## DESCRIPTION



           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), poll(2), or epoll(7).  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.
    
           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 open file
                  description (see open(2)) referred to by the new file
                  descriptor.  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.
    


## RETURN VALUE



           On success, these system calls return a file descriptor for the
           accepted socket (a nonnegative integer).  On error, -1 is
           returned, _errno_ is set to indicate the error, and _addrlen_ is left
           unchanged.
    
       **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, these are **ENETDOWN** ,
           **EPROTO** , **ENOPROTOOPT** , **EHOSTDOWN** , **ENONET** , **EHOSTUNREACH** , **EOPNOTSUPP** ,
           and **ENETUNREACH**.
    


## ERRORS



           **EAGAIN** or **EWOULDBLOCK**
                  The socket is marked nonblocking and no connections are
                  present to be accepted.  POSIX.1-2001 and POSIX.1-2008
                  allow either error to be returned for this case, and do not
                  require these constants to have the same value, so a
                  portable application should check for both possibilities.
    
           **EBADF** _sockfd_ is not an open file descriptor.
    
           **ECONNABORTED**
                  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 on the number of open file
                  descriptors has been reached.
    
           **ENFILE** The system-wide limit on the total number of open files has
                  been reached.
    
           **ENOBUFS**
           **ENOMEM** Not enough free memory.  This often means that the memory
                  allocation is limited by the socket buffer limits, not by
                  the system memory.
    
           **ENOTSOCK**
                  The file descriptor _sockfd_ does not refer to a socket.
    
           **EOPNOTSUPP**
                  The referenced socket is not of type **SOCK_STREAM**.
    
           **EPERM** Firewall rules forbid connection.
    
           **EPROTO** Protocol error.
    
           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.
    


## VERSIONS



           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**().
    


## STANDARDS



           **accept**()
                  POSIX.1-2008.
    
           **accept4**()
                  Linux.
    


## NOTES



           There may not always be a connection waiting after a **SIGIO** is
           delivered or select(2), poll(2), or epoll(7) 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 socket(7)).
    
           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.
    
       **The socklen_t type**
           In the original BSD sockets implementation (and on other older
           systems) the third argument of **accept**() was declared as an _int *_.
           A POSIX.1g draft standard wanted to change it into a _size_t *_ ;
           later POSIX standards and glibc 2.x have _socklen_t *_ .
    


## EXAMPLES



           See bind(2).
    


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