Here are the details on the functions and data structures used for
performing non-local exits. These facilities are declared in
setjmp.h.
— Data Type: jmp_buf
Objects of type jmp_buf hold the state information to
be restored by a non-local exit. The contents of a jmp_buf
identify a specific place to return to.
— Macro: int setjmp (jmp_buf state)
When called normally, setjmp stores information about the
execution state of the program in state and returns zero. If
longjmp is later used to perform a non-local exit to this
state, setjmp returns a nonzero value.
— Function: void longjmp (jmp_buf state, int value)
This function restores current execution to the state saved in
state, and continues execution from the call to setjmp that
established that return point. Returning from setjmp by means of
longjmp returns the value argument that was passed to
longjmp, rather than 0. (But if value is given as
0, setjmp returns 1).
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:
As the test expression of a selection or iteration
statement (such as `if', `switch', or `while').
As one operand of a equality or comparison operator that appears as the
test expression of a selection or iteration statement. The other
operand must be an integer constant expression.
As the operand of a unary `!' operator, that appears as the
test expression of a selection or iteration statement.
By itself as an expression statement.
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.
Published under the terms of the GNU General Public License
