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Here are the details on the functions and data structures used for performing non-local exits. These facilities are declared in setjmp.h.
Objects of type
jmp_bufhold the state information to be restored by a non-local exit. The contents of ajmp_bufidentify a specific place to return to.
When called normally,
setjmpstores information about the execution state of the program in state and returns zero. Iflongjmpis later used to perform a non-local exit to this state,setjmpreturns a nonzero value.
This function restores current execution to the state saved in state, and continues execution from the call to
setjmpthat established that return point. Returning fromsetjmpby means oflongjmpreturns the value argument that was passed tolongjmp, rather than0. (But if value is given as0,setjmpreturns1).
There are a lot of obscure but important restrictions on the use of
setjmp and longjmp. Most of these restrictions are
present because non-local exits require a fair amount of magic on the
part of the C compiler and can interact with other parts of the language
in strange ways.
The setjmp function is actually a macro without an actual
function definition, so you shouldn't try to `#undef' it or take
its address. In addition, calls to setjmp are safe in only the
following contexts:
Return points are valid only during the dynamic extent of the function
that called setjmp to establish them. If you longjmp to
a return point that was established in a function that has already
returned, unpredictable and disastrous things are likely to happen.
You should use a nonzero value argument to longjmp. While
longjmp refuses to pass back a zero argument as the return value
from setjmp, this is intended as a safety net against accidental
misuse and is not really good programming style.
When you perform a non-local exit, accessible objects generally retain
whatever values they had at the time longjmp was called. The
exception is that the values of automatic variables local to the
function containing the setjmp call that have been changed since
the call to setjmp are indeterminate, unless you have declared
them volatile.