Advanced Programming in the UNIX® Environment

ptg10805159

216 Process Environment Chapter 7

We callsetjmpfrom the location that we want to return to, which in this example

is in themainfunction. In this case,setjmpreturns 0 because we called it directly.In

the call tosetjmp,the argumentenvis of the special typejmp_buf.This data type is

some form of array that is capable of holding all the information required to restorethe

status of the stack to the state when we calllongjmp.Normally,theenvvariable is a

global variable, since we’ll need to reference it from another function.

When we encounter an error — say, in thecmd_addfunction — we calllongjmp

with two arguments. The first is the sameenvthat we used in a call tosetjmp,and the

second,val, is a nonzerovalue that becomes the return value fromsetjmp.The second

argument allows us to use morethan onelongjmpfor eachsetjmp.For example, we

couldlongjmpfromcmd_addwith avalof 1 and also calllongjmpfromget_token

with avalof 2. In themainfunction, the return value fromsetjmpis either 1 or 2, and

we can test this value, if we want, and determine whether thelongjmpwas from

cmd_addorget_token.

Let’s return to the example. Figure7.11shows both themain and cmd_add

functions. (The other two functions,do_lineandget_token,haven’t changed.)

#include "apue.h"

#include <setjmp.h>

#define TOK_ADD 5

jmp_buf jmpbuffer;

int

main(void)

{

char line[MAXLINE];

if (setjmp(jmpbuffer) != 0)

printf("error");

while (fgets(line, MAXLINE, stdin) != NULL)

do_line(line);

exit(0);

}

...

void

cmd_add(void)

{

int token;

token = get_token();

if (token < 0) /* an error has occurred */

longjmp(jmpbuffer, 1);

/* rest of processing for this command */

}

Figure 7.11 Example ofsetjmpandlongjmp