xref: /dragonfly/lib/libc/sys/accept.2 (revision dc71b7ab) 
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28.\"     @(#)accept.2	8.2 (Berkeley) 12/11/93
29.\" $FreeBSD: src/lib/libc/sys/accept.2,v 1.10.2.11 2002/05/09 02:24:40 silby Exp $
30.\" $DragonFly: src/lib/libc/sys/accept.2,v 1.4 2006/06/25 10:55:51 corecode Exp $
31.\"
32.Dd December 11, 1993
33.Dt ACCEPT 2
34.Os
35.Sh NAME
36.Nm accept
37.Nd accept a connection on a socket
38.Sh LIBRARY
39.Lb libc
40.Sh SYNOPSIS
41.In sys/types.h
42.In sys/socket.h
43.Ft int
44.Fn accept "int s" "struct sockaddr *addr" "socklen_t *addrlen"
45.Sh DESCRIPTION
46The argument
47.Fa s
48is a socket that has been created with
49.Xr socket 2 ,
50bound to an address with
51.Xr bind 2 ,
52and is listening for connections after a
53.Xr listen 2 .
54The
55.Fn accept
56call
57extracts the first connection request
58on the queue of pending connections, creates
59a new socket with the same properties as
60.Fa s ,
61and allocates a new file descriptor
62for the socket.  If no pending connections are
63present on the queue, and the socket is not marked
64as non-blocking,
65.Fn accept
66blocks the caller until a connection is present.
67If the socket is marked non-blocking and no pending
68connections are present on the queue,
69.Fn accept
70returns an error as described below.
71The accepted socket
72may not be used
73to accept more connections.  The original socket
74.Fa s
75remains open.
76.Pp
77The argument
78.Fa addr
79is a result parameter that is filled-in with
80the address of the connecting entity,
81as known to the communications layer.
82The exact format of the
83.Fa addr
84parameter is determined by the domain in which the communication
85is occurring.
86To ensure that the returned address fits,
87.Fa *addr
88should have a size of at least
89.Ft sizeof(struct sockaddr_storage) .
90The
91.Fa addrlen
92is a value-result parameter; it should initially contain the
93amount of space pointed to by
94.Fa addr ;
95on return it will contain the actual length (in bytes) of the
96address returned.
97This call
98is used with connection-based socket types, currently with
99.Dv SOCK_STREAM .
100.Pp
101It is possible to
102.Xr select 2
103a socket for the purposes of doing an
104.Fn accept
105by selecting it for read.
106.Pp
107For certain protocols which require an explicit confirmation,
108such as
109.Tn ISO
110or
111.Tn DATAKIT ,
112.Fn accept
113can be thought of
114as merely dequeueing the next connection
115request and not implying confirmation.
116Confirmation can be implied by a normal read or write on the new
117file descriptor, and rejection can be implied by closing the
118new socket.
119.Pp
120For some applications, performance may be enhanced by using an
121.Xr accept_filter 9
122to pre-process incoming connections.
123.Sh RETURN VALUES
124The call returns \-1 on error.  If it succeeds, it returns a non-negative
125integer that is a descriptor for the accepted socket.
126.Sh ERRORS
127The
128.Fn accept
129will fail if:
130.Bl -tag -width Er
131.It Bq Er EBADF
132The descriptor is invalid.
133.It Bq Er EINTR
134The
135.Fn accept
136operation was interrupted.
137.It Bq Er EMFILE
138The per-process descriptor table is full.
139.It Bq Er ENFILE
140The system file table is full.
141.It Bq Er ENOTSOCK
142The descriptor references a file, not a socket.
143.It Bq Er EINVAL
144.Xr listen 2
145has not been called on the socket descriptor.
146.It Bq Er EFAULT
147The
148.Fa addr
149parameter is not in a writable part of the
150user address space.
151.It Bq Er EWOULDBLOCK
152The socket is marked non-blocking and no connections
153are present to be accepted.
154.It Bq Er ECONNABORTED
155A connection arrived, but it was closed while waiting
156on the listen queue.
157.El
158.Sh SEE ALSO
159.Xr bind 2 ,
160.Xr connect 2 ,
161.Xr getpeername 2 ,
162.Xr listen 2 ,
163.Xr select 2 ,
164.Xr socket 2 ,
165.Xr accept_filter 9
166.Sh HISTORY
167The
168.Fn accept
169function appeared in
170.Bx 4.2 .
171