1ACCEPT(2)                  Linux Programmer's Manual                 ACCEPT(2)
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4

NAME

6       accept, accept4 - accept a connection on a socket
7

SYNOPSIS

9       #include <sys/socket.h>
10
11       int accept(int sockfd, struct sockaddr *restrict addr,
12                  socklen_t *restrict addrlen);
13
14       #define _GNU_SOURCE             /* See feature_test_macros(7) */
15       #include <sys/socket.h>
16
17       int accept4(int sockfd, struct sockaddr *restrict addr,
18                  socklen_t *restrict addrlen, int flags);
19

DESCRIPTION

21       The  accept()  system  call  is used with connection-based socket types
22       (SOCK_STREAM, SOCK_SEQPACKET).  It extracts the  first  connection  re‐
23       quest  on  the  queue  of pending connections for the listening socket,
24       sockfd, creates a new connected socket, and returns a new file descrip‐
25       tor  referring  to that socket.  The newly created socket is not in the
26       listening state.  The original socket  sockfd  is  unaffected  by  this
27       call.
28
29       The  argument  sockfd is a socket that has been created with socket(2),
30       bound to a local address with bind(2), and is listening for connections
31       after a listen(2).
32
33       The argument addr is a pointer to a sockaddr structure.  This structure
34       is filled in with the address of the peer socket, as known to the  com‐
35       munications  layer.   The  exact format of the address returned addr is
36       determined by the socket's address family (see socket(2)  and  the  re‐
37       spective protocol man pages).  When addr is NULL, nothing is filled in;
38       in this case, addrlen is not used, and should also be NULL.
39
40       The addrlen argument is a value-result argument: the caller  must  ini‐
41       tialize  it  to contain the size (in bytes) of the structure pointed to
42       by addr; on return it will contain the actual size of the peer address.
43
44       The returned address is truncated if the buffer provided is too  small;
45       in  this case, addrlen will return a value greater than was supplied to
46       the call.
47
48       If no pending connections are present on the queue, and the  socket  is
49       not  marked  as nonblocking, accept() blocks the caller until a connec‐
50       tion is present.  If the socket is marked nonblocking  and  no  pending
51       connections are present on the queue, accept() fails with the error EA‐
52       GAIN or EWOULDBLOCK.
53
54       In order to be notified of incoming connections on a  socket,  you  can
55       use  select(2),  poll(2), or epoll(7).  A readable event will be deliv‐
56       ered when a new connection is attempted and you may then call  accept()
57       to  get  a  socket for that connection.  Alternatively, you can set the
58       socket to deliver SIGIO when activity occurs on a socket; see socket(7)
59       for details.
60
61       If  flags  is 0, then accept4() is the same as accept().  The following
62       values can be bitwise ORed in flags to obtain different behavior:
63
64       SOCK_NONBLOCK   Set the O_NONBLOCK file status flag on  the  open  file
65                       description  (see  open(2)) referred to by the new file
66                       descriptor.  Using this flag saves extra calls  to  fc‐
67                       ntl(2) to achieve the same result.
68
69       SOCK_CLOEXEC    Set the close-on-exec (FD_CLOEXEC) flag on the new file
70                       descriptor.  See the description of the O_CLOEXEC  flag
71                       in open(2) for reasons why this may be useful.
72

RETURN VALUE

74       On  success,  these  system  calls return a file descriptor for the ac‐
75       cepted socket (a nonnegative integer).  On error, -1 is returned, errno
76       is set to indicate the error, and addrlen is left unchanged.
77
78   Error handling
79       Linux accept() (and accept4()) passes already-pending network errors on
80       the new socket as an error code from accept().  This  behavior  differs
81       from  other BSD socket implementations.  For reliable operation the ap‐
82       plication should detect the network errors defined for the protocol af‐
83       ter  accept()  and  treat them like EAGAIN by retrying.  In the case of
84       TCP/IP, these are ENETDOWN,  EPROTO,  ENOPROTOOPT,  EHOSTDOWN,  ENONET,
85       EHOSTUNREACH, EOPNOTSUPP, and ENETUNREACH.
86

ERRORS

88       EAGAIN or EWOULDBLOCK
89              The  socket is marked nonblocking and no connections are present
90              to be accepted.  POSIX.1-2001 and POSIX.1-2008 allow either  er‐
91              ror  to be returned for this case, and do not require these con‐
92              stants to have the same value, so a portable application  should
93              check for both possibilities.
94
95       EBADF  sockfd is not an open file descriptor.
96
97       ECONNABORTED
98              A connection has been aborted.
99
100       EFAULT The  addr argument is not in a writable part of the user address
101              space.
102
103       EINTR  The system call was interrupted by a signal that was caught  be‐
104              fore a valid connection arrived; see signal(7).
105
106       EINVAL Socket  is  not listening for connections, or addrlen is invalid
107              (e.g., is negative).
108
109       EINVAL (accept4()) invalid value in flags.
110
111       EMFILE The per-process limit on the number of open file descriptors has
112              been reached.
113
114       ENFILE The system-wide limit on the total number of open files has been
115              reached.
116
117       ENOBUFS, ENOMEM
118              Not enough free memory.  This often means that the memory  allo‐
119              cation is limited by the socket buffer limits, not by the system
120              memory.
121
122       ENOTSOCK
123              The file descriptor sockfd does not refer to a socket.
124
125       EOPNOTSUPP
126              The referenced socket is not of type SOCK_STREAM.
127
128       EPERM  Firewall rules forbid connection.
129
130       EPROTO Protocol error.
131
132       In addition, network errors for the new socket and as defined  for  the
133       protocol  may  be returned.  Various Linux kernels can return other er‐
134       rors such as ENOSR, ESOCKTNOSUPPORT, EPROTONOSUPPORT,  ETIMEDOUT.   The
135       value ERESTARTSYS may be seen during a trace.
136

VERSIONS

138       The accept4() system call is available starting with Linux 2.6.28; sup‐
139       port in glibc is available starting with version 2.10.
140

CONFORMING TO

142       accept(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (accept() first  ap‐
143       peared in 4.2BSD).
144
145       accept4() is a nonstandard Linux extension.
146
147       On  Linux,  the  new  socket returned by accept() does not inherit file
148       status flags such as O_NONBLOCK and O_ASYNC from the listening  socket.
149       This  behavior  differs  from the canonical BSD sockets implementation.
150       Portable programs should not rely on inheritance or  noninheritance  of
151       file  status  flags and always explicitly set all required flags on the
152       socket returned from accept().
153

NOTES

155       There may not always be a connection waiting after a SIGIO is delivered
156       or  select(2),  poll(2), or epoll(7) return a readability event because
157       the connection might have been removed by an asynchronous network error
158       or another thread before accept() is called.  If this happens, then the
159       call will block waiting for the next connection to arrive.   To  ensure
160       that  accept() never blocks, the passed socket sockfd needs to have the
161       O_NONBLOCK flag set (see socket(7)).
162
163       For certain protocols which require an explicit confirmation,  such  as
164       DECnet, accept() can be thought of as merely dequeuing the next connec‐
165       tion request and not implying confirmation.  Confirmation  can  be  im‐
166       plied  by a normal read or write on the new file descriptor, and rejec‐
167       tion can be implied by closing the new socket.  Currently, only  DECnet
168       has these semantics on Linux.
169
170   The socklen_t type
171       In the original BSD sockets implementation (and on other older systems)
172       the third argument of accept() was declared as an  int *.   A  POSIX.1g
173       draft  standard wanted to change it into a size_t *C; later POSIX stan‐
174       dards and glibc 2.x have socklen_t * .
175

EXAMPLES

177       See bind(2).
178

SEE ALSO

180       bind(2), connect(2), listen(2), select(2), socket(2), socket(7)
181

COLOPHON

183       This page is part of release 5.13 of the Linux  man-pages  project.   A
184       description  of  the project, information about reporting bugs, and the
185       latest    version    of    this    page,    can     be     found     at
186       https://www.kernel.org/doc/man-pages/.
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190Linux                             2021-08-27                         ACCEPT(2)
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