Advanced Programming in the UNIX® Environment

ptg10805159

Section 7.10 setjmpandlongjmpFunctions 217

Whenmainis executed, we callsetjmp,which records whatever information it needs

to in the variablejmpbuffer and returns 0. We then calldo_line,which calls

cmd_add,and assume that an error of some form is detected. Beforethe call to

longjmpincmd_add,the stack looks like that in Figure7.10. Butlongjmpcauses the

stack to be ‘‘unwound’’back to themainfunction, throwing away the stack frames for

cmd_addanddo_line(Figure7.12). Callinglongjmpcauses thesetjmpinmainto

return, but this time it returns with a value of 1 (the second argument forlongjmp).

stack frame

formain

bottom of stack higher address

lower address

direction of

stack growth

Figure 7.12 Stack frame afterlongjmphas been called

Automatic, Register,and Volatile Variables

We’ve seen what the stack looks like after callinglongjmp.The next question is, ‘‘What

arethe states of the automatic variables and register variables in themainfunction?’’

When we return tomainas a result of thelongjmp, do these variables have values

corresponding to those when thesetjmpwas previously called (i.e., aretheir values

rolled back), or aretheir values left alone so that their values arewhatever they were

when do_line was called (which caused cmd_add to be called, which caused

longjmp to be called)? Unfortunately,the answer is ‘‘It depends.’’Most

implementations do not try to roll back these automatic variables and register variables,

but the standards say only that their values areindeterminate. If you have an automatic

variable that you don’t want rolled back, define it with thevolatile attribute.

Variables that aredeclared as global or static areleft alone whenlongjmpis executed.

Example

The program in Figure7.13 demonstrates the different behavior that can be seen with

automatic, global, register,static, and volatile variables after callinglongjmp.