Here are the details on the functions and data structures used for performing non-local exits. These facilities are declared in `setjmp.h'.
jmp_bufhold the state information to be restored by a non-local exit. The contents of a
jmp_bufidentify a specific place to return to.
setjmpstores information about the execution state of the program in state and returns zero. If
longjmpis later used to perform a non-local exit to this state,
setjmpreturns a nonzero value.
setjmpthat established that return point. Returning from
setjmpby means of
longjmpreturns the value argument that was passed to
longjmp, rather than
0. (But if value is given as
There are a lot of obscure but important restrictions on the use of
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.
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
Return points are valid only during the dynamic extent of the function
setjmp to establish them. If you
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 refuses to pass back a zero argument as the return value
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
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