28. Nonlocal jumps <setjmp.h>

The header <setjmp.h> defines the macro setjmp, and declares one function and one type, for bypassing the normal function call and return discipline. [1]

The type declared is

jmp_buf

which is an array type suitable for holding the information needed to restore a calling environment. The environment of a call to the setjmp macro consists of information sufficient for a call to the longjmp function to return execution to the correct block and invocation of that block, were it called recursively. It does not include the state of the floating-point status flags, of open files, or of any other component of the abstract machine.

It is unspecified whether setjmp is a macro or an identifier declared with external linkage. If a macro definition is suppressed in order to access an actual function, or a program defines an external identifier with the name setjmp, the behavior is undefined.

[1]These functions are useful for dealing with unusual conditions encountered in a low-level function of a program.

28.1. Save calling environment

28.1.1. The setjmp macro

Synopsis

#include <setjmp.h>
int setjmp(jmp_buf env);

Description

The setjmp macro saves its calling environment in its jmp_buf argument for later use by the longjmp function.

Returns

If the return is from a direct invocation, the setjmp macro returns the value zero. If the return is from a call to the longjmp function, the setjmp macro returns a nonzero value.

Environmental limits

An invocation of the setjmp macro shall appear only in one of the following contexts:

  • the entire controlling expression of a selection or iteration statement;
  • one operand of a relational or equality operator with the other operand an integer constant expression, with the resulting expression being the entire controlling expression of a selection or iteration statement;
  • the operand of a unary ! operator with the resulting expression being the entire controlling expression of a selection or iteration statement; or
  • the entire expression of an expression statement (possibly cast to void).

If the invocation appears in any other context, the behavior is undefined.

28.2. Restore calling environment

28.2.1. The longjmp function

Synopsis

#include <setjmp.h>
void longjmp(jmp_buf env, int val);

Description

The longjmp function restores the environment saved by the most recent invocation of the setjmp macro in the same invocation of the program with the corresponding jmp_buf argument. If there has been no such invocation, or if the function containing the invocation of the setjmp macro has terminated execution [2] in the interim, or if the invocation of the setjmp macro was within the scope of an identifier with variably modified type and execution has left that scope in the interim, the behavior is undefined.

All accessible objects have values, and all other components of the abstract machine [3] have state, as of the time the longjmp function was called, except that the values of objects of automatic storage duration that are local to the function containing the invocation of the corresponding setjmp macro that do not have volatile-qualified type and have been changed between the setjmp invocation and longjmp call are indeterminate.

Returns

After longjmp is completed, program execution continues as if the corresponding invocation of the setjmp macro had just returned the value specified by val. The longjmp function cannot cause the setjmp macro to return the value 0; if val is 0, the setjmp macro returns the value 1.

EXAMPLE The longjmp function that returns control back to the point of the setjmp invocation might cause memory associated with a variable length array object to be squandered.

#include <setjmp.h>

jmp_buf buf;
void g(int n);
void h(int n);
int n = 6;

void f(void)
{
  int x[n]; // valid: f is not terminated
  setjmp(buf);
  g(n);
}

void g(int n)
{
  int a[n]; // a may remain allocated
  h(n);
}

void h(int n)
{
  int b[n]; // b may remain allocated
  longjmp(buf, 2); // might cause memory loss
}
[2]For example, by executing a return statement or because another longjmp call has caused a transfer to a setjmp invocation in a function earlier in the set of nested calls.
[3]This includes, but is not limited to, the floating-point status flags and the state of open files.