Setjmp.h

Member functions

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

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

Member types

The C99 Rationale describes jmp_buf as being an array type for backward compatibility; existing code refers to jmp_buf storage locations by name (without the & address-of operator), which is only possible for array types.

Caveats and limitations

When a "non-local goto" is executed via setjmp/longjmp, 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 setjmp was called returns, it is no longer possible to safely use longjmp with the corresponding jmp_buf object. This is because the stack frame is invalidated when the function returns. Calling longjmp 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 longjmp preserve the current stack frame. This means that jumping into a function which was exited via a call to longjmp is undefined. However, most implementations of longjmp leave the stack frame intact, allowing setjmp and longjmp to be used to jump back-and-forth between two or more functions—a feature exploited for multitasking.

Compared to mechanisms in higher-level programming languages such as Python, Java, C++, C#, and even pre-C languages such as Algol 60, the technique of using setjmp/longjmp to implement an exception mechanism is cumbersome. These languages provide more powerful exception handling techniques, while languages such as Scheme, Smalltalk, and Haskell provide even more general continuation-handling constructs.

Simple example

The example below shows the basic idea of setjmp. There, main() calls first(), which in turn calls second(). Then, second() jumps back into main(), skipping first()'s call of printf().

When executed, the above program will output:

Notice that although the first() subroutine gets called, "first" is never printed. "main" gets printed as the conditional statement if (!setjmp(buf)) is executed a second time.

Exception handling

In this example, setjmp is used to bracket exception handling, like try in some other languages. The call to longjmp is analogous to a throw statement, allowing an exception to return an error status directly to the setjmp. The following code adheres to the 1999 ISO C standard and Single UNIX Specification by invoking setjmp in a limited range of contexts:

As the condition to an if, switch 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 setjmp 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 switch ((exception_type = setjmp(env))) { } 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.

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 longjmp 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 return or longjmp is undefined. However, most implementations of longjmp 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 setjmp. In many environments (such as Really Simple Threads and TinyTimbers), idioms such as if(!setjmp(child_env)) longjmp(caller_env); can allow a called function to effectively pause-and-resume at a setjmp.

This is exploited by thread libraries to provide cooperative multitasking facilities without using setcontext or other fiber facilities. Whereas setcontext is a library service which can create an execution context in heap-allocated memory and can support other services such as buffer overflow protection, abuse of setjmp is implemented by the programmer, who may reserve memory on the stack and fail to notify the library or operating system of the new operating context. On the other hand, a library's implementation of setcontext may internally use setjmp in a fashion similar to this example to save and restore a context, after it has been initialised somehow.

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