Clash Royale CLAN TAG

Why a segmentation fault occurs calling a function inside setjmp()?

I do not understand why in the function middleFunc(), a segmentation fault is raisen when entry_point(arg) is invoked inside the if ( setjmp(middle) ) statement.

middleFunc()

entry_point(arg)

if ( setjmp(middle) )

#include <stdio.h>
#include <setjmp.h>


jmp_buf start,middle,end;


void finalFunc(void *v)
{
printf("hellon");
return ;
}


void middleFunc(void (*entry_point)(void *), void *arg)
{
//just debug : this does not cause segmentation fault
entry_point(arg);

if ( setjmp(middle) ){
//this casues the segmentation fault
entry_point(arg);
//once the entry point (finalFunc) is executed go to jmp_buffer end
longjmp(end,1);
}
else {
longjmp(start,1);
}
}

int main(){

if (setjmp(end)){
//exit since finalFunc has been executed
return 0;
}

if (setjmp(start)){
//the middleFunc has previously set the jmp_buffer middle
longjmp(middle,1);
}

else{
int x = 1;
middleFunc(finalFunc,(void*)&x);
}

}


1 Answer
1

In your code the behavior is undefined. You are not allowed to long-jump to middle after middleFunc finished execution (either by normal completion or by another longjmp).

middle

middleFunc

longjmp

7.13.2.1 The longjmp function

2 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 execution²⁴⁸⁾ in the interim [...] the behavior is undefined.

longjmp

setjmp

jmp_buf

setjmp

248) 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.

return

longjmp

setjmp

In your code middleFunc sets up middle and after that immediately exits to main by doing longjmp(start,1). After that jump middle is no longer valid. You are no longer allowed to jump to middle from anywhere. setjmp/longjmp mechanism only supports jumps up the call stack. You cannot do side-jumps or down-jumps. Only up-jumps are supported.

middleFunc

middle

main

longjmp(start,1)

middle

middle

setjmp/longjmp

From the practical point of view, you are attempting to jump into a "dead" function invocation and somehow expecting that function parameter values are still valid (like, preserved from the previous invocation or something). But they are not. setjmp/longjmp do not preserve/restore parameter values. Value of entry_point in that "dead" invocation is probably some garbage. When you attempt to make a call through entry_point, the code coredumps.

setjmp/longjmp

entry_point

entry_point

P.S. It is true that side-jumping with setjmp/longjmp is sometimes used to implement co-routines. However, such usage falls outside the boundaries of standard library specification. And in any case such usage will never expect preservation of parameter values.

setjmp/longjmp

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