setjmp and longjmp in C

A non-local jump back to a saved point — C's closest thing to an exception.

#include <setjmp.h>
#include <stdio.h>

static jmp_buf recovery;

static void deep(int depth) {
    if (depth > 2) {
        longjmp(recovery, 42);
    }
    deep(depth + 1);
}

int attempt(void) {
    volatile int attempts = 0;
    int code = setjmp(recovery);

    if (code != 0) {
        printf("recovered with %d after %d attempt(s)\n", code, attempts);
        return code;
    }
    attempts++;
    deep(0);
    return 0;
}

How it works

1. setjmp returns zero when it saves, non-zero when jumped to.
2. longjmp unwinds without running any cleanup.
3. Locals that must survive it should be volatile.

Keywords and builtins used here

• attempt
• deep
• if
• int
• jmp_buf
• return
• static
• void
• volatile

The run, in numbers

Lines

24

Characters to type

366

Tokens

98

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 53 seconds.

Type this snippet

Step 2 of 2 in Escape hatches, step 12 of 13 in Unions, bitfields & undefined behavior.

← Previous Next →