ConnectAttach(), ConnectAttach_r()

Establish a connection between a process and a channel


#include <sys/neutrino.h>
#include <sys/netmgr.h>

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

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


The node descriptor of the node (e.g. ND_LOCAL_NODE for the local node) on which the process that owns the channel is running; see Node descriptors, below.
The process ID of the owner of the channel. If pid is zero, the calling process is assumed.
The channel ID, returned by ChannelCreate() , of the channel to connect to the process.
The lowest acceptable connection ID.
Treating a connection as a file descriptor can lead to unexpected behavior. Therefore, you should OR _NTO_SIDE_CHANNEL into index when you create a connection. If you do this, the connection ID is returned from a different space than file descriptors; the ID is greater than any valid file descriptor.

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

If flags contains _NTO_COF_CLOEXEC, the connection is closed when your process calls an exec*() function to start a new process.



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


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 on the node specified by nd. Any function that passes a node descriptor can use either the value 0 or the constant ND_LOCAL_NODE to refer to the local node.

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

  • In order to establish a connection, your process must have the PROCMGR_AID_CONNECTION ability enabled. For more information, see procmgr_ability() .
  • If a process wants other processes to communicate with it, it typically uses name_attach() to create a channel and associate a name with it, and the sender process uses name_open() 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 either MsgSendv() 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.

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.

If index has _NTO_SIDE_CHANNEL set, the index is ignored and the connection ID returned is the first available index in the _NTO_SIDE_CHANNEL space.

If a process creates multiple connections to the same channel, the system maintains a link count and shares internal kernel object resources for efficiency.

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() . If any threads are blocked on the channel (via MsgSendv()) at the time the connection is detached, the send fails and returns with an error.

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.

Node descriptors

The nd (node descriptor) is a temporary numeric description of a remote node.


The only difference between these functions is the way they indicate errors:

A connection ID that's used by the message primitives. If an error occurs, the function returns -1 and sets errno .
A connection ID that's used by the message primitives. This function does NOT set errno. If an error occurs, the function returns the negative of a value from the Errors section.


Insufficent resources.
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.
The process has no unused file descriptors, or no file descriptors greater than or equal to index are available.
The node indicated by nd doesn't exist.
The process indicated by pid is no longer valid. It's going away, has core dumped, or become a zombie.
The calling process doesn't have the required permission; see procmgr_ability() .
The process indicated by pid, or the channel indicated by chid, doesn't exist.


QNX Neutrino

Cancellation point Yes
Interrupt handler No
Signal handler Yes
Thread Yes