23.2 Details of Non-Local Exits

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)

Preliminary: | MT-Safe | AS-Safe | AC-Safe | See POSIX Safety Concepts.

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)

Preliminary: | MT-Safe | AS-Unsafe plugin corrupt lock/hurd | AC-Unsafe corrupt lock/hurd | See POSIX Safety Concepts.

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 an 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.