Defect Report #152

Submission Date: 11 Jun 95
Submittor: DIN
Source: Jutta Degener
Question
This Defect Report was prepared with considerable help from Mark Brader, Clive Feather, Ronald Guilmette, and a person whose employment conditions require anonymity.
DIN-003:
Can longjmp be used to return from a signal handler invoked other than through abort or raise? The descriptions of signal and longjmp contradict each other.
According to subclause 7.7.1.1 The signal function:
If the signal occurs other than as the result of calling the abort or raise function, the behavior is undefined if the signal handler calls any function in the standard library other than the signal function itself (with a first argument of the signal number corresponding to the signal that caused the invocation of the handler) or refers to any object with static storage duration other than by assigning a value to a static storage duration variable of type volatile sig_atomic_t.
Since longjmp is a function, it cannot be called.
But subclause 7.6.2.1 The longjmp function, broadly guarantees the opposite:
As it bypasses the usual function call and return mechanisms, the longjmp function shall execute correctly in contexts of interrupts, signals and any of their associated functions.
Suggested Technical Corrigendum:
If the intent is to allow calls to exit and longjmp from signal handlers not invoked through calls to raise or abort, replace in subclause 7.6.2.1:
... other than the signal function itself ...
by:
... other than longjmp, exit, or the signal function itself ...
Alternatively, if the intent is to disallow calls to longjmp from signal handlers not invoked through calls to raise or abort, replace in subclause 7.7.1.1:
As it bypasses the usual function call and return mechanisms, the longjmp function shall execute correctly in contexts of interrupts, signals and any of their associated functions. However, if the longjmp function is invoked from a nested signal handler (that is, from a function invoked as a result of a signal raised during the handling of another signal), the behavior is undefined.
by:
If the longjmp function is invoked from a nested signal handler (that is, from a function invoked as a result of a signal raised during the handling of another signal), the behavior is undefined.
Response
The C Standard is clear enough as is. The longjmp function shall execute correctly when called from a non-nested signal handler invoked through calls to the raise or abort functions; if longjmp is called from a signal handler invoked by other means, or from a nested signal handler, the behavior is undefined.
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