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 setjmp and longjmp provide this functionality.
A typical use of setjmp/longjmp is implementation of an exception mechanism that exploits the ability of longjmp to reestablish program or thread state, even across multiple levels of function calls. A less common use of setjmp is to create syntax similar to coroutines.
Sets up the local jmp_buf buffer and initializes it for the jump. This routine[1] saves the program's calling environment in the environment buffer specified by the env argument for later use by longjmp. If the return is from a direct invocation, setjmp returns 0. If the return is from a call to longjmp, setjmp returns a nonzero value. | ||
Restores the context of the environment buffer env that was saved by invocation of the setjmp routine in the same invocation of the program. Invoking longjmp from a nested signal handler is undefined. The value specified by value is passed from longjmp to setjmp. After longjmp is completed, program execution continues as if the corresponding invocation of setjmp had just returned. If the value passed to longjmp is 0, setjmp will behave as if it had returned 1; otherwise, it will behave as if it had returned value. |
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:
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.
jmp_buf | An array type suitable for holding the information needed to restore a calling environment. |
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.[2] 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 struct __jmp_bug_tag[1].
When a "non-local goto" is executed via setjmp/longjmp 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 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.[3] [4]
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.[5]
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.
static jmp_buf buf;
void second
void first
int main
When executed, the above program will output:
second main
Notice that although the first subroutine gets called, "first" is never printed, as second never returns control to first. Instead, "main" is printed when the conditional statement if (!setjmp(buf)) is checked a second time.
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:
if, switch or iteration statement! or comparison with an integer constantFollowing 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.
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), the "throw" as longjmp with the optional parameter as the exception, and the "catch" as the "else" block under "try".
static void first;static void second;
/* Use a file scoped static variable for the exception stack so we can access * it anywhere within this translation unit. */static jmp_buf exception_env;static int exception_type;
int main(void)
static void first
static void second
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
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.[5] 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.
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.
jmp_buf mainTask, childTask;
void call_with_cushion;void child;
int main " after first time that child yields
while (1) }
void call_with_cushion
void child
setjmp must be implemented as a macro, but POSIX explicitly states that it is undefined whether setjmp is a macro or a function.