Practical usage of setjmp and longjmp in C
The combination of setjmp and longjmp is "super strength goto". Use with EXTREME care. However, as others have explained, a longjmp is very useful to get out of a nasty error situation, when you want to get me back to the beginning quickly, rather than having to trickle back an error message for 18 layers of functions.
However, just like goto, but worse, you have to be REALLY careful how you use this. A longjmp will just get you back to the beginning of the code. It won't affect all the other states that may have changed between the setjmp and getting back to where setjmp started. So allocations, locks, half-initialized data structures, etc, are still allocated, locked and half-initialized when you get back to where setjmp was called. This means, you have to really care for the places where you do this, that it's REALLY ok to call longjmp without causing MORE problems. Of course, if the next thing you do is "reboot" [after storing a message about the error, perhaps] - in an embedded system where you've discovered that the hardware is in a bad state, for example, then fine.
I have also seen setjmp/longjmp used to provide very basic threading mechanisms. But that's pretty special case - and definitely not how "standard" threads work.
Edit: One could of course add code to "deal with cleaning up", in the same way that C++ stores the exception points in the compiled code and then knows what gave an exception and what needs cleaning up. This would involve some sort of function pointer table and storing away "if we jump out from below here, call this function, with this argument". Something like this:
struct
{
void (*destructor)(void *ptr);
};
void LockForceUnlock(void *vlock)
{
LOCK* lock = vlock;
}
LOCK func_lock;
void func()
{
ref = add_destructor(LockForceUnlock, mylock);
Lock(func_lock)
...
func2(); // May call longjmp.
Unlock(func_lock);
remove_destructor(ref);
}
With this system, you could do "complete exception handling like C++". But it's quite messy, and relies on the code being well written.
Error handling
Suppose there is an error deep down in a function nested in many other functions and error handling makes sense only in the top level function.
It would be very tedious and awkward if all the functions in between had to return normally and evaluate return values or a global error variable to determine that further processing doesn't make sense or even would be bad.
That's a situation where setjmp/longjmp makes sense. Those situations are similar to situation where exception in other langages (C++, Java) make sense.
Coroutines
Besides error handling, I can think also of another situation where you need setjmp/longjmp in C:
It is the case when you need to implement coroutines.
Here is a little demo example. I hope it satisfies the request from Sivaprasad Palas for some example code and answers the question of TheBlastOne how setjmp/longjmp supports the implementation of corroutines (as much as I see it doesn't base on any non-standard or new behaviour).
EDIT:
It could be that it actually is undefined behaviour to do a longjmp down the callstack (see comment of MikeMB; though I have not yet had opportunity to verify that).
#include <stdio.h>
#include <setjmp.h>
jmp_buf bufferA, bufferB;
void routineB(); // forward declaration
void routineA()
{
int r ;
printf("(A1)\n");
r = setjmp(bufferA);
if (r == 0) routineB();
printf("(A2) r=%d\n",r);
r = setjmp(bufferA);
if (r == 0) longjmp(bufferB, 20001);
printf("(A3) r=%d\n",r);
r = setjmp(bufferA);
if (r == 0) longjmp(bufferB, 20002);
printf("(A4) r=%d\n",r);
}
void routineB()
{
int r;
printf("(B1)\n");
r = setjmp(bufferB);
if (r == 0) longjmp(bufferA, 10001);
printf("(B2) r=%d\n", r);
r = setjmp(bufferB);
if (r == 0) longjmp(bufferA, 10002);
printf("(B3) r=%d\n", r);
r = setjmp(bufferB);
if (r == 0) longjmp(bufferA, 10003);
}
int main(int argc, char **argv)
{
routineA();
return 0;
}
Following figure shows the flow of execution:
Warning note
When using setjmp/longjmp be aware that they have an effect on the validity of local variables often not considered.
Cf. my question about this topic.
I've written a Java-like exception handling mechanism in C using setjmp(), longjmp() and system functions. It catches custom exceptions but also signals like SIGSEGV. It features infinite nesting of exception handling blocks, which works accross function calls, and supports the two most common threading implementations. It allows you to define a tree hierarchy of exception classes that feature link-time inheritance, and the catch statement walks this tree to see if it needs to catch or pass on.
Here's a sample of how code looks using this:
try
{
*((int *)0) = 0; /* may not be portable */
}
catch (SegmentationFault, e)
{
long f[] = { 'i', 'l', 'l', 'e', 'g', 'a', 'l' };
((void(*)())f)(); /* may not be portable */
}
finally
{
return(1 / strcmp("", ""));
}
And here's part of the include file that contains a lot of logic:
#ifndef _EXCEPT_H
#define _EXCEPT_H
#include <stdlib.h>
#include <stdio.h>
#include <signal.h>
#include <setjmp.h>
#include "Lifo.h"
#include "List.h"
#define SETJMP(env) sigsetjmp(env, 1)
#define LONGJMP(env, val) siglongjmp(env, val)
#define JMP_BUF sigjmp_buf
typedef void (* Handler)(int);
typedef struct _Class *ClassRef; /* exception class reference */
struct _Class
{
int notRethrown; /* always 1 (used by throw()) */
ClassRef parent; /* parent class */
char * name; /* this class name string */
int signalNumber; /* optional signal number */
};
typedef struct _Class Class[1]; /* exception class */
typedef enum _Scope /* exception handling scope */
{
OUTSIDE = -1, /* outside any 'try' */
INTERNAL, /* exception handling internal */
TRY, /* in 'try' (across routine calls) */
CATCH, /* in 'catch' (idem.) */
FINALLY /* in 'finally' (idem.) */
} Scope;
typedef enum _State /* exception handling state */
{
EMPTY, /* no exception occurred */
PENDING, /* exception occurred but not caught */
CAUGHT /* occurred exception caught */
} State;
typedef struct _Except /* exception handle */
{
int notRethrown; /* always 0 (used by throw()) */
State state; /* current state of this handle */
JMP_BUF throwBuf; /* start-'catching' destination */
JMP_BUF finalBuf; /* perform-'finally' destination */
ClassRef class; /* occurred exception class */
void * pData; /* exception associated (user) data */
char * file; /* exception file name */
int line; /* exception line number */
int ready; /* macro code control flow flag */
Scope scope; /* exception handling scope */
int first; /* flag if first try in function */
List * checkList; /* list used by 'catch' checking */
char* tryFile; /* source file name of 'try' */
int tryLine; /* source line number of 'try' */
ClassRef (*getClass)(void); /* method returning class reference */
char * (*getMessage)(void); /* method getting description */
void * (*getData)(void); /* method getting application data */
void (*printTryTrace)(FILE*);/* method printing nested trace */
} Except;
typedef struct _Context /* exception context per thread */
{
Except * pEx; /* current exception handle */
Lifo * exStack; /* exception handle stack */
char message[1024]; /* used by ExceptGetMessage() */
Handler sigAbrtHandler; /* default SIGABRT handler */
Handler sigFpeHandler; /* default SIGFPE handler */
Handler sigIllHandler; /* default SIGILL handler */
Handler sigSegvHandler; /* default SIGSEGV handler */
Handler sigBusHandler; /* default SIGBUS handler */
} Context;
extern Context * pC;
extern Class Throwable;
#define except_class_declare(child, parent) extern Class child
#define except_class_define(child, parent) Class child = { 1, parent, #child }
except_class_declare(Exception, Throwable);
except_class_declare(OutOfMemoryError, Exception);
except_class_declare(FailedAssertion, Exception);
except_class_declare(RuntimeException, Exception);
except_class_declare(AbnormalTermination, RuntimeException); /* SIGABRT */
except_class_declare(ArithmeticException, RuntimeException); /* SIGFPE */
except_class_declare(IllegalInstruction, RuntimeException); /* SIGILL */
except_class_declare(SegmentationFault, RuntimeException); /* SIGSEGV */
except_class_declare(BusError, RuntimeException); /* SIGBUS */
#ifdef DEBUG
#define CHECKED \
static int checked
#define CHECK_BEGIN(pC, pChecked, file, line) \
ExceptCheckBegin(pC, pChecked, file, line)
#define CHECK(pC, pChecked, class, file, line) \
ExceptCheck(pC, pChecked, class, file, line)
#define CHECK_END \
!checked
#else /* DEBUG */
#define CHECKED
#define CHECK_BEGIN(pC, pChecked, file, line) 1
#define CHECK(pC, pChecked, class, file, line) 1
#define CHECK_END 0
#endif /* DEBUG */
#define except_thread_cleanup(id) ExceptThreadCleanup(id)
#define try \
ExceptTry(pC, __FILE__, __LINE__); \
while (1) \
{ \
Context * pTmpC = ExceptGetContext(pC); \
Context * pC = pTmpC; \
CHECKED; \
\
if (CHECK_BEGIN(pC, &checked, __FILE__, __LINE__) && \
pC->pEx->ready && SETJMP(pC->pEx->throwBuf) == 0) \
{ \
pC->pEx->scope = TRY; \
do \
{
#define catch(class, e) \
} \
while (0); \
} \
else if (CHECK(pC, &checked, class, __FILE__, __LINE__) && \
pC->pEx->ready && ExceptCatch(pC, class)) \
{ \
Except *e = LifoPeek(pC->exStack, 1); \
pC->pEx->scope = CATCH; \
do \
{
#define finally \
} \
while (0); \
} \
if (CHECK_END) \
continue; \
if (!pC->pEx->ready && SETJMP(pC->pEx->finalBuf) == 0) \
pC->pEx->ready = 1; \
else \
break; \
} \
ExceptGetContext(pC)->pEx->scope = FINALLY; \
while (ExceptGetContext(pC)->pEx->ready > 0 || ExceptFinally(pC)) \
while (ExceptGetContext(pC)->pEx->ready-- > 0)
#define throw(pExceptOrClass, pData) \
ExceptThrow(pC, (ClassRef)pExceptOrClass, pData, __FILE__, __LINE__)
#define return(x) \
{ \
if (ExceptGetScope(pC) != OUTSIDE) \
{ \
void * pData = malloc(sizeof(JMP_BUF)); \
ExceptGetContext(pC)->pEx->pData = pData; \
if (SETJMP(*(JMP_BUF *)pData) == 0) \
ExceptReturn(pC); \
else \
free(pData); \
} \
return x; \
}
#define pending \
(ExceptGetContext(pC)->pEx->state == PENDING)
extern Scope ExceptGetScope(Context *pC);
extern Context *ExceptGetContext(Context *pC);
extern void ExceptThreadCleanup(int threadId);
extern void ExceptTry(Context *pC, char *file, int line);
extern void ExceptThrow(Context *pC, void * pExceptOrClass,
void *pData, char *file, int line);
extern int ExceptCatch(Context *pC, ClassRef class);
extern int ExceptFinally(Context *pC);
extern void ExceptReturn(Context *pC);
extern int ExceptCheckBegin(Context *pC, int *pChecked,
char *file, int line);
extern int ExceptCheck(Context *pC, int *pChecked, ClassRef class,
char *file, int line);
#endif /* _EXCEPT_H */
There's also a C module that contains the logic for signal handling and some bookkeeping.
It was extremely tricky to implement I can tell you and I almost quit. I really pushed to make it as close to Java as possible; I found it surprising how far I got with just C.
Give me a shout if you're interested.
The theory is that you can use them for error handling so that you can jump out of deeply nested call chain without needing to deal with handling errors in every function in the chain.
Like every clever theory this falls apart when meeting reality. Your intermediate functions will allocate memory, grab locks, open files and do all kinds of different things that require cleanup. So in practice setjmp/longjmp are usually a bad idea except in very limited circumstances where you have total control over your environment (some embedded platforms).
In my experience in most cases whenever you think that using setjmp/longjmp would work, your program is clear and simple enough that every intermediate function call in the call chain can do error handling, or it's so messy and impossible to fix that you should do exit when you encounter the error.