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copied!<p>setjmp is used to place a <em>marker</em> to where the call of longjump should return, it returns 0 if it is called directly, it returns 1 if it's called because a longjmp to that setjmp is invoked.</p> <p>You have to think about setjmp like something that can be normally called and does not do anything (returning 0) in normal operation while returns 1 and it's indirectly called (and returns from there) when a long jump is called. I know what you mean about confusing because it's actually confusing..</p> <p>This is the example given by wikipedia:</p> <pre><code>#include <stdio.h> #include <setjmp.h> static jmp_buf buf; void second(void) { printf("second\n"); // prints longjmp(buf,1); // jumps back to where setjmp was called - making setjmp now return 1 } void first(void) { second(); printf("first\n"); // does not print } int main() { if ( ! setjmp(buf) ) { first(); // when executed, setjmp returns 0 } else { // when longjmp jumps back, setjmp returns 1 printf("main"); // prints } return 0; } </code></pre> <p>Are you able to understand it? When program is launched <code>setjmp</code> is executed in main and returns 0 (because it is directly called), so <code>first</code> is called, that calls <code>second</code> and then it arrives <code>longjmp</code> that switches context going back to where <code>setjmp</code> was used, but this time, since it goes back from a jump and it is indirectly called the function returns 1.</p> <p>The useful thing of setjmp/longjmp approach is that you can handle error situations without caring to keep a flag between function calls (especially when you have many, think about a recursive procedure for typechecking in a compiler). If something goes wrong in typechecking deep in call stack normally you have to return a flag and keep returning it to warn the caller that typechecking failed. With <strong>longjmp</strong> you just go out and handle error without caring about passing flags back. The only problem is that this forces a context switch that doesn't care about standard deallocation of stack/heap memory so you should handle it by yourself.</p>

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