Setjmp.h

setjmp.h is a header defined in the C standard library to provide "non-local jumps": control flow that deviates from the usual subroutine call and return sequence. The complementary functions   and   provide this functionality.

A typical use of /  is implementation of an exception mechanism that exploits the ability of   to reestablish program or thread state, even across multiple levels of function calls. A less common use of  is to create syntax similar to coroutines.

Member functions
saves the current environment (the program state), at some point of program execution, into a platform-specific data structure that can be used at some later point of program execution by   to restore the program state to that saved by   into. This process can be imagined to be a "jump" back to the point of program execution where  saved the environment. The (apparent) return value from  indicates whether control reached that point normally (zero) or from a call to   (nonzero). This leads to a common idiom:.

POSIX.1 does not specify whether  and   save and restore the current set of blocked signals; if a program employs signal handling it should use POSIX's  /.

Member types
The C99 Rationale describes  as being an array type for backward compatibility; existing code refers to   storage locations by name (without the   address-of operator), which is only possible for array types. It notes that it can simply be a one-member-long array with its single member being the actual data; indeed, this is the approach employed by the GNU C library, which defines the type as.

Caveats and limitations
When a "non-local goto" is executed via /  in C++, normal "stack unwinding" does not occur. Therefore, any required cleanup actions will not occur either. This could include closing file descriptors, flushing buffers, or freeing heap-allocated memory.

If the function in which  was called returns, it is no longer possible to safely use   with the corresponding   object. This is because the stack frame is invalidated when the function returns. Calling  restores the stack pointer, which—because the function returned—would point to a non-existent and potentially overwritten or corrupted stack frame.

Similarly, C99 does not require that  preserve the current stack frame. This means that jumping into a function which was exited via a call to  is undefined.

Simple example
The example below shows the basic idea of setjmp. There,  calls , which in turn calls. Then,  jumps back into , skipping  's call of.

When executed, the above program will output:

second main

Notice that although the  subroutine gets called, " " is never printed. " " gets printed as the conditional statement  is executed a second time.

Exception handling
In this example,  is used to bracket exception handling, like   in some other languages. The call to  is analogous to a   statement, allowing an exception to return an error status directly to the. The following code adheres to the 1999 ISO C standard and Single UNIX Specification by invoking  in a limited range of contexts:
 * As the condition to an,   or iteration statement
 * As above in conjunction with a single  or comparison with an integer constant
 * As a statement (with the return value unused)

Following these rules can make it easier for the implementation to create the environment buffer, which can be a sensitive operation. More general use of  can cause undefined behaviour, such as corruption of local variables; conforming compilers and environments are not required to protect or even warn against such usage. However, slightly more sophisticated idioms such as  are common in literature and practice, and remain relatively portable. A simple conforming methodology is presented below, where an additional variable is maintained along with the state buffer. This variable could be elaborated into a structure incorporating the buffer itself.

In a more modern-looking example, the usual "try" block would be implemented as a setjmp (with some preparation code for multilevel jumps, as seen in first), the "throw" as longjmp with the optional parameter as the exception, and the "catch" as the "else" block under "try".

This program's output is: calling first entering first calling second entering second second failed, exception type: 3; remapping to type 1 first failed, exception type: 1

Cooperative multitasking
C99 provides that  is guaranteed to work only when the destination is a calling function, i.e., that the destination scope is guaranteed to be intact. Jumping to a function that has already terminated by  or   is undefined. However, most implementations of  do not specifically destroy local variables when performing the jump. Since the context survives until its local variables are erased, it could actually be restored by. In many environments (such as Really Simple Threads and TinyTimbers), idioms such as  can allow a called function to effectively pause-and-resume at a.

This is exploited by thread libraries to provide cooperative multitasking facilities without using  or other fiber facilities.

Considering that  to a child function will generally work unless sabotaged, and , as part of POSIX, is not required to be provided by C implementations, this mechanism may be portable where the   alternative fails.

Since no exception will be generated upon overflow of one of the multiple stacks in such a mechanism, it is essential to overestimate the space required for each context, including the one containing  and including space for any signal handlers that might interrupt regular execution. Exceeding the allocated space will corrupt the other contexts, usually with the outermost functions first. Unfortunately, systems requiring this kind of programming strategy are often also small ones with limited resources.