SAS/C Library Reference, Vol. 1

blkjmp -- Intercept Nonlocal gotos

SYNOPSIS

 #include <lcjmp.h>

 int blkjmp(jmp_buf env);

DESCRIPTION

blkjmp requests interception of calls to longjmp that could terminate the calling function. When you call blkjmp, it always returns 0. If a call to longjmp is later intercepted, the call to blkjmp is resumed and it then returns the integer argument that was passed to longjmp. The env variable is modified to indicate the target of the intercepted longjmp so that it can be reissued by the intercepting routine.

After a call to longjmp is intercepted, blkjmp must be reissued if continued interception is wanted.

Because exit is implemented as a longjmp to the caller of main, you can use blkjmp to intercept program exit.

RETURN VALUE

blkjmp normally returns 0; it returns a non-zero value if a call to longjmp has been intercepted (in which case, blkjmp returns the value of the second argument passed to longjmp).

CAUTION

Variables of storage class auto and register whose values are changed between the blkjmp and longjmp calls have indeterminate values on return to blkjmp.

EXAMPLE

This example demonstrates how blkjmp can be used to enable a function to release resources, even if terminated by a call to longjmp in a function it calls:

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

  jmp_buf env;

  static void get_resource(void), use_resource(void);

  int main()
  {
     int code;

     if (code = setjmp(env)) goto escape;
     get_resource();
     puts("get_resource returned normally.");
     exit(0);
        escape:
           printf("Executing escape routine for error %dn", code);
           exit(code);
  }

  static void get_resource(void)
  {
     int code;
     jmp_buf my_env;

        /* Allocate resource here.       */
     if (code = blkjmp(my_env)) goto release;
     puts("Resources allocated.");
        /* Free resource here.           */
     use_resource();
     puts("use_resource returned normally, "
          "get_resource is freeing resources.");
     return;
           /* Free resource here.        */
        release:
           printf("use_resource indicated error %dn", code);
           puts("Resources now freed, proceeding with longjmp.");
           longjmp(my_env, code);
  }

  static void use_resource(void)
  {
     puts("Entering use_resource");

        /* Attempt to use resource here. */
     puts("Error 3 detected, calling longjmp.");
     longjmp(env, 3);
     puts("This statement will not be executed.");
  }

RELATED FUNCTIONS

longjmp, setjmp, siglongjmp, sigsetjmp