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CONNECT(2) System Calls Manual CONNECT(2)

connect — initiate a connection on a socket

library “libc”

#include <sys/socket.h>

int
connect(int s, const struct sockaddr *name, socklen_t namelen);

The s argument is a socket. If it is of type SOCK_DGRAM, this call specifies the peer with which the socket is to be associated; this address is that to which datagrams are to be sent, and the only address from which datagrams are to be received. If the socket is of type SOCK_STREAM, this call attempts to make a connection to another socket. The other socket is specified by name, which is an address in the communications space of the socket. namelen indicates the amount of space pointed to by name, in bytes; the sa_len member of name is ignored. Each communications space interprets the name argument in its own way. Generally, stream sockets may successfully () only once; datagram sockets may use connect() multiple times to change their association. Datagram sockets may dissolve the association by connecting to an invalid address, such as a null address.

The connect() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The connect() system call fails if:

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The s argument is not a valid descriptor.
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The namelen argument is not a valid length for the address family.
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The s argument is a descriptor for a file, not a socket.
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The specified address is not available on this machine.
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Addresses in the specified address family cannot be used with this socket.
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The socket is already connected.
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Connection establishment timed out without establishing a connection.
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The attempt to connect was forcefully rejected.
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The connection was reset by the remote host.
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The network is not reachable from this host.
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The remote host is not reachable from this host.
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The address is already in use.
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The name argument specifies an area outside the process address space.
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The socket is non-blocking and the connection cannot be completed immediately. It is possible to select(2) for completion by selecting the socket for writing.
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The connection attempt was interrupted by the delivery of a signal. The connection will be established in the background, as in the case of .
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A previous connection attempt has not yet been completed.
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An attempt is made to connect to a broadcast address (obtained through the INADDR_BROADCAST constant or the INADDR_NONE return value) through a socket that does not provide broadcast functionality.
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An auto-assigned port number was requested but no auto-assigned ports are available. Increasing the port range specified by sysctl(3) MIB variables net.inet.ip.portrange.first and net.inet.ip.portrange.last may alleviate the problem.

The following errors are specific to connecting names in the UNIX domain. These errors may not apply in future versions of the UNIX IPC domain.

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A component of the path prefix is not a directory.
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A component of a pathname exceeded 255 characters, or an entire path name exceeded 1023 characters.
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The named socket does not exist.
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Search permission is denied for a component of the path prefix.
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Write access to the named socket is denied.
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Too many symbolic links were encountered in translating the pathname.
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Write access to the named socket is denied.

accept(2), getpeername(2), getsockname(2), select(2), socket(2), sysctl(3), sysctl(8)

The connect() system call appeared in 4.2BSD.

August 18, 2016 FreeBSD-12.0