NAME

xpc_abort — conditions which cause XPC to abort

DESCRIPTION

The XPC API will aggressively defend itself against perceived misuse. Wherever XPC can definitively detect misuse of its APIs or data corruption, it will abort. For example, if the retain count of an object underflows by calling xpc_release(3) on it too many times, XPC will abort the process.

Many frameworks opt to keep the program limping along in such a state (and will perhaps print a warning message to the system log), but aborting upon detection provides unmistakable warning that there is a bug present in the application which should be fixed before shipping.

When XPC aborts a process, it will place information about the condition which triggered the abort in the Application Specific Information section of the crash report. The message will be human-readable, prefixed with "XPC API Misuse:", and the crash report will indicate the stack trace which caused the abort.

XPC will also abort if it detects unrecoverable data corruption in its internal state. The messages for these conditions will be prefixd with "Bug in libxpc:". If you come across such a crash, please file a bug and include the generated crash log, system log and steps to reproduce (if there are any identifiable steps).

Currently, the manner in which XPC aborts the process will result in termination due to SIGILL (illegal instruction). The exact signal raised may change from release to release (or platform to platform). But on OS X Lion, SIGILL may be used as a hint that the process was terminated intentionally.

DEBUGGING

When debugging in Xcode or at the gdb command prompt, the debugger acts as the exception handler for the process being debugged. As a result, if the process is aborted by XPC, no crash report will be generated, and thus it may not be obvious why the program was terminated.

As mentioned before, SIGILL is an indication that the process was terminated intentionally. If you observe the last frame in the crashing stack to be _xpc_api_misuse(), you may use the xpc_debugger_api_misuse_info() API from within the debugger to obtain a human-readable string describing why the process was aborted. For example:

Program received signal EXC_BAD_INSTRUCTION, Illegal instruction/operand.
0x000000010012b25e in _xpc_api_misuse ()
(gdb) p (char *)xpc_debugger_api_misuse_info()
$1 = 0x7fff5fbff908 "XPC API Misuse: Over-release of object."
(gdb)

This message indicates that xpc_release(3) was called too many times on an object.

IMPORTANT: The xpc_debugger_api_misuse_info() API can ONLY be called from within a debugger. It is not meant to be called directly from the program. Do not call it directly from your code, and do not rely on the address of the result for any reason.

SEE ALSO

xpc(3), xpc_object(3), xpc_objects(3)