ConnectAttach(), ConnectAttach_r()

QNX SDP8.0C Library ReferenceAPIDeveloper

Establish a connection between a process and a channel

Synopsis:

#include <sys/neutrino.h>

int ConnectAttach( uint32_t reserved,
                   pid_t pid,
                   int chid,
                   unsigned index,
                   int flags );

int ConnectAttach_r( uint32_t reserved,
                     pid_t pid,
                     int chid,
                     unsigned index,
                     int flags );

Arguments:

reserved

Set this argument to zero.

pid

The process ID of the owner of the channel. If pid is zero, the calling process is assumed.

chid

The channel ID, returned by ChannelCreate(), of the channel to connect to the process.

index

If _NTO_SIDE_CHANNEL is not set, the lowest acceptable connection ID. The ID returned by the function will then be a file descriptor greater than or equal to this value. If _NTO_SIDE_CHANNEL is set, the connection is created as a side channel, meaning the ID returned is from a different space than file descriptors. In this case, any other bits in index are ignored.

Note:

Having a connection ID in the file descriptor range can lead to unexpected behavior. Therefore, you should pass in _NTO_SIDE_CHANNEL for index when you create a connection. The C library creates connections at various times without _NTO_SIDE_CHANNEL (e.g., during open()); however, it's unlikely that any applications would want to call it this way.

Once the ID is created, there's no difference in the use of the messaging primitives on it.

flags

Flags that affect the connection; a bitwise OR of zero or more of the following:

_NTO_COF_CLOEXEC — close the connection when the calling process calls an exec*() function to start a new process.
_NTO_COF_NOEVENT — prevent the server from sending sigevents over the channel to the client.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The ConnectAttach() and ConnectAttach_r() kernel calls establish a connection between the calling process and the channel specified by chid owned by the process specified by pid.

These functions are identical except in the way they indicate errors. See the Returns section for details.

Note:

If a process wants other processes to communicate with it, it typically will register a name with name_attach() or resmgr_attach() to create a channel and associate a name with it, and the sender process uses name_open() or one of open() or open_ext() to locate that name and create a connection to it.

The return value is a connection ID, which is a small int representing the connection. The system returns the first available connection ID starting at the value specified by the index argument. Any thread in the calling process can use the MsgSend*() or MsgSendv*() functions to send messages or MsgSendPulse() to send pulses over the connection. The connection ID is used directly as a POSIX file descriptor (fd) when communicating with I/O Resource managers such as a filesystem manager.

We recommend that you OR _NTO_SIDE_CHANNEL into index, to create the connection as a side channel. In this case, the index argument is ignored, and the connection ID returned is the first available index in the _NTO_SIDE_CHANNEL space, which is different than the file descriptor space.

If you don't OR _NTO_SIDE_CHANNEL into index, this behavior might result:

If file descriptor 0 is in use, file descriptor 1 isn't in use, and you call ConnectAttach() with 0 specified for index, a connection ID of 1 is returned.
File descriptor 1 (i.e., connection ID 1) is used as stdout, which is what printf() writes to. If your process makes any calls to printf(), NULL-terminated character strings are sent to the channel that you've connected to. Similar situations can happen with connection IDs 0 (stdin) and 2 (stderr).
Depending on how a child process is created, it may inherit the parent's file descriptors.
Since connections are treated like file descriptors, a connection created by the parent without _NTO_SIDE_CHANNEL in index and without _NTO_COF_CLOEXEC in flags, causes a child process to inherit that connection during process creation. This inheritance is done during process creation by duplicating file descriptors.

During duplication, an _IO_DUP message (with 0x115 as the first 2 bytes) is sent to the receiver on the other side of the connection. The receiver won't be expecting this message.

Connections created with _NTO_SIDE_CHANNEL are never inherited by child processes created by a fork() call or new process images executed by an exec*() call. This means the receiver won't get an unexpected _IO_DUP message.

If a client process creates multiple connections to the same channel, the system maintains a link count and shares internal kernel object resources for efficiency. The client should avoid using different flags (flags) when making multiple connections, or unexpected behavior could result.

Connections are owned by the process and may be used simultaneously by any thread in the process. You can detach a connection by calling ConnectDetach().

Note:

Connections and connection IDs persist until you call ConnectDetach(), even if the other process dies.

The connection is strictly local (i.e., it doesn't resolve across the network) and is resolved on the first use of the connection ID.

Blocking states

These calls don't block.

Returns:

A connection ID (coid) that's used by the message primitives. If an error occurs:

ConnectAttach() returns -1 and sets errno.
ConnectAttach_r() returns the negative of a value from the Errors section and doesn't set errno.

Errors:

EAGAIN

Insufficent resources.

EINVAL

One of the following:

The value of index is outside the range of valid connection IDs. This error applies only to file descriptors, not side channels (there's no limit on the range for side channels).
The combination of bits in the flags argument is invalid.

EMFILE

One of the following occurred:

The process has no unused file descriptors.
The number of connections already owned by the process is greater than or equal to the limit specified for RLIMIT_NOCONN_NP (see setrlimit()).
No file descriptors greater than or equal to index are available.

If you aren't creating a side channel, the following also applies:

There was no acceptable connection ID less than RLIMIT_NOFILE.

ENXIO

The process indicated by pid is no longer valid. It's going away, has core dumped, or become a zombie.

EPERM

The process that created the channel used the _NTO_CHF_PRIVATE flag to mark it as being private (see ChannelCreate() for more information), or the process doesn't have the necessary ability to connect (PROCMGR_AID_CHANNEL_CONNECT).

ESRCH

The process indicated by pid, or the channel indicated by chid, doesn't exist.

Classification:

QNX OS


Safety:
Cancellation point	No
Signal handler	Yes
Thread	Yes

Page updated: August 09, 2024