C library macro - setjmp()

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The C library macro setjmp(), saves the current environment into the variable environment for later use by the function longjmp(). If this macro returns directly from the macro invocation, it returns zero but if it returns from a longjmp() function call, then it returns the value passed to longjmp as a second parameter.

This function is used to implement the exception handling in C.

Syntax

Following is the syntax of the C library macro setjmp() −

setjmp(jmp_buf environment)

Parameters

This function accepts only a single parameter −

• environment − This is the object of type jmp_buf where the environment information is stored.

Return Value

This macro may return more than once. First time, on its direct invocation, it always returns zero. When longjmp is called with the information set to the environment, the macro returns again; now it returns the value passed to longjmp as second parameter.

Example 1

Following is the basic C program that shows the usage of setjmp() function.

#include <setjmp.h>
#include <stdio.h> 
jmp_buf buf; 
void func() 
{ 
   printf("Welcome to Tutorialspoint\n"); 

   // Jump to the point setup by setjmp 
   longjmp(buf, 1); 

   printf("Python Tutorial\n"); 
} 

int main() 
{ 
   // Setup jump position using buf and return 0 
   if (setjmp(buf))
   printf("Java Tutorial\n"); 
   else { 
   printf("The else-statement printed\n"); 
   func(); 
	} 
   return 0; 
}

Output

The above code produces following result −

The else-statement printed
Welcome to Tutorialspoint
Java Tutorial

Example 2

In this example, we demonstrate the standard call and return sequences from the functions − longjmp() and setjmp().

#include <stdio.h>
#include <setjmp.h>
#include <stdnoreturn.h>
 
jmp_buf my_jump_buffer;
 
noreturn void foo(int status) 
{
   printf("foo(%d) called\n", status);
   longjmp(my_jump_buffer, status + 1); 
}
 
int main(void)
{
   volatile int count = 0; 
   if (setjmp(my_jump_buffer) != 5) 
       foo(++count);
   return 0;
}

Output

On execution of above code, we get the following result −

foo(1) called
foo(2) called
foo(3) called
foo(4) called

Example 3

Following is the another illustration of setjmp() function.

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

int a(char *s, jmp_buf env) {
   int i;

   i = setjmp(env);
   printf("Setjmp returned %d\n", i); 
   printf("S = %s\n", s);
   return i;
}

int b(int i, jmp_buf env) {
   printf("In b: i = %d, Calling longjmp\n", i);
   longjmp(env, i);
}

int main() {
   jmp_buf env;

   if (a("Tutorial", env) != 0)
       exit(0); 
   b(3, env);
   return 0;
}

Output

After executing the above code, we get the following result −

Setjmp returned 0
S = Tutorial
In b: i = 3, Calling longjmp
Setjmp returned 3
S = UHH