1ACCEPT(2) Linux Programmer's Manual ACCEPT(2)
2
6 accept, accept4 - accept a connection on a socket
7
9 #include <sys/socket.h>
10 int accept(int sockfd, struct sockaddr *restrict addr,
12 socklen_t *restrict addrlen);
13 #define _GNU_SOURCE /* See feature_test_macros(7) */
15 #include <sys/socket.h>
16 int accept4(int sockfd, struct sockaddr *restrict addr,
18 socklen_t *restrict addrlen, int flags);
19
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 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 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 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 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 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 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 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 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 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
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 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
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 EBADF sockfd is not an open file descriptor.
96 ECONNABORTED
98 A connection has been aborted.
99 EFAULT The addr argument is not in a writable part of the user address
101 space.
102 EINTR The system call was interrupted by a signal that was caught be‐
104 fore a valid connection arrived; see signal(7).
105 EINVAL Socket is not listening for connections, or addrlen is invalid
107 (e.g., is negative).
108 EINVAL (accept4()) invalid value in flags.
110 EMFILE The per-process limit on the number of open file descriptors has
112 been reached.
113 ENFILE The system-wide limit on the total number of open files has been
115 reached.
116 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 ENOTSOCK
123 The file descriptor sockfd does not refer to a socket.
124 EOPNOTSUPP
126 The referenced socket is not of type SOCK_STREAM.
127 EPERM Firewall rules forbid connection.
129 EPROTO Protocol error.
131 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
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
142 accept(): POSIX.1-2001, POSIX.1-2008, SVr4, 4.4BSD (accept() first ap‐
143 peared in 4.2BSD).
144 accept4() is a nonstandard Linux extension.
146 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
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 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 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
177 See bind(2).
178
180 bind(2), connect(2), listen(2), select(2), socket(2), socket(7)
181
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/.
187Linux 2021-08-27 ACCEPT(2)