Exception Generation and Handling

To generate exception, i used following 2 functions contained in header file setjmp.h:

1) setjmp()
2) longjmp()

These two routines require a datatype defined in setjmp.h:

3) jmp_buf

i) It specifies the buffer used by the routines to save and restore the program environment.
ii) The jmp_buf type is defined as: typedef char jmp_buf[_JBLEN];
iii) Example: jmp_buf env; where env- Variable in which environment is stored.

Description of 2 routines used:

1)int setjmp(env)

i)Sets up the local jmp_buf buffer and initializes it for the jump.
ii)Saves the program's calling environment in the environment buffer for later use by longjmp.
iii)If the return is from a direct invocation, setjmp returns 0.
iv)If the return is from a call to longjmp, setjmp returns a nonzero value.

2)longjmp(env,1)

i)Restores stack environment and execution locale previously saved in env by setjmp.


Generation of exception can follow the construct:

#include

jmp_buf env; <-----jump environment (must be global)
bit error_flag;

void trigger (void)
{
. . .
/* processing code here */
/* . . .
if (error_flag != 0)
{
longjmp (env, 1); <----return 1 to setjmp
}
. . .
}

Here is a simple program to generate and handle exception using setjmp() and longjmp() functions using above construct.

#include
#include

static jmp_buf buf;

void exception_handler()
{
printf("'if'detected\n"); <-----prints
longjmp(buf,1); <-----jumps back to where setjmp was called - making setjmp return 1
}

int main()
{
if (!setjmp(buf) )
{
exception_handler(); <-----when executed, setjmp returns 0
}
else
{ <-----when longjmp jumps back, setjmp returns 1
printf("Back to main block"); <-----prints
}

return 0;
}

OUTPUT is as follows:

[swati@localhost ~]$ gcc exc.c
[swati@localhost ~]$ ./a.out
'if'detected
Back to main block