Format
#include <setjmp.h>
int setjmp(jmp_buf env);Language Level: ANSI
Threadsafe: Yes.
Description
The setjmp() function saves a stack environment that can subsequently be restored by the longjmp() function. The setjmp() and longjmp() functions provide a way to perform a non-local goto. They are often used in signal handlers.
A call to the setjmp() function causes it to save the current stack environment in env. A subsequent call to the longjmp() function restores the saved environment and returns control to a point corresponding to the setjmp() call. The values of all variables (except register variables) available to the function receiving control contain the values they had when the longjmp() function was called. The values of register variables are unpredictable. Nonvolatile auto variables that are changed between calls to the setjmp() function and the longjmp() function are also unpredictable.
Return Value
The setjmp() function returns the value 0 after saving the stack environment. If the setjmp() function returns as a result of a longjmp() call, it returns the value argument of the longjmp() function, or 1 if the value argument of the longjmp() function is 0. There is no error return value.
Example that uses setjmp()
This example saves the stack environment at the statement:
if(setjmp(mark) != 0) ...When the system first performs the if statement, it saves the environment in mark and sets the condition to FALSE because the setjmp() function returns a 0 when it saves the environment. The program prints the message:
setjmp has been calledThe subsequent call to function p() causes it to call the longjmp() function. Control is transferred to the point in the main() function immediately after the call to the setjmp() function using the environment saved in the mark variable. This time, the condition is TRUE because -1 is specified in the second parameter on the longjmp() function call as the return value to be placed on the stack. The example then performs the statements in the block, prints the message "longjmp() has been called", calls the recover() function, and leaves the program.
#include <stdio.h>
#include <setjmp.h>
#include <stdlib.h>
jmp_buf mark;
void p(void);
void recover(void);
int main(void)
{
if (setjmp(mark) != 0)
{
printf("longjmp has been called\n");
recover();
exit(1);
}
printf("setjmp has been called\n");
printf("Calling function p()\n");
p();
printf("This point should never be reached\n");
}
void p(void)
{
printf("Calling longjmp() from inside function p()\n");
longjmp(mark, -1);
printf("This point should never be reached\n");
}
void recover(void)
{
printf("Performing function recover()\n");
}
/*******************Output should be as follows: **********************
setjmp has been called
Calling function p()
Calling longjmp() from inside function p()
longjmp has been called
Performing function recover()
**********************************************************************/Related Information