Writing channel exit programs on IBM i

We can use the following information to help you write and compile channel-exit programs for IBM® i.

The exit is a program object written in the ILE C, ILE RPG, or ILE COBOL language. The exit program names and their libraries are named in the channel definition.

• The program must be made thread safe and created with the ILE C, ILE RPG, or ILE COBOL compiler. For ILE RPG you must specify the THREAD(*SERIALIZE) control specification, and for ILE COBOL you must specify SERIALIZE for the THREAD option of the PROCESS statement. The programs must also be bound to the threaded IBM MQ libraries: QMQM/LIBMQM_R in the case of ILE C and ILE RPG, and AMQ0STUB_R in the case of ILE COBOL. For additional information about making RPG or COBOL applications thread safe, refer to the appropriate Programmer's Guide for the language.
• IBM MQ for IBM i requires that the exit programs are enabled for teraspace support. (Teraspace is a form of shared memory introduced in OS/400 V4R4.) For the ILE RPG and COBOL compilers, any programs compiled on OS/400 V4R4 or later are so enabled. For C, the programs must be compiled with the TERASPACE(*YES *TSIFC) options specified on CRTCMOD or CRTBNDC commands.
• An exit returning a pointer to its own buffer space must ensure that the object pointed to exists beyond the time span of the channel-exit program. The pointer cannot be the address of a variable on the program stack, nor of a variable in the program heap. Instead, the pointer must be obtained from the system. An example is a user space created in the user exit. To ensure that any data area allocated by the channel-exit program is still available for the MCA when the program ends, the channel exit must run in the activation group of the caller or a named activation group. Do this by setting the ACTGRP parameter on CRTPGM to a user-defined value or *CALLER. If the program is created in this way, the channel-exit program can allocate dynamic memory and pass a pointer to this memory back to the MCA.