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In C, catching a page fault using signals like SIGSEGV (Segmentation Fault) is a powerful technique, especially when combined with setjmp and longjmp for error recovery. These functions allow us to implement non-local jumps in the program, saving and restoring the state of the execution context.

This approach is particularly useful when a segmentation fault occurs, and you want to avoid a complete program crash. In this blog, I’ll explain how setjmp and longjmp work, how they can be used with signal handling to catch page faults, and I’ll include some benchmarking code to measure the performance of these functions.

How setjmp and longjmp Work

• setjmp: Saves the current program state, including registers, program counter, and stack pointer, into a jmp_buf buffer. When called, it returns 0.
• longjmp: Jumps back to the state saved by setjmp. When it jumps back, setjmp will return the value passed to longjmp instead of 0.

The combination of these functions allows a program to handle errors and recover from certain kinds of faults, such as illegal memory access or page faults.

Example Code: Using SIGSEGV to Catch Page Faults

Here’s a basic example of using SIGSEGV to catch a page fault and recover the program by using setjmp and longjmp.

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

jmp_buf buf;

void sigsegv_handler(int signum) {
    printf("Caught SIGSEGV, returning to safe point\n");
    longjmp(buf, 1);  // Jump back to checkpoint
}

int main() {
    signal(SIGSEGV, sigsegv_handler);

    if (setjmp(buf) == 0) {
        // Set a checkpoint
        printf("Setting checkpoint\n");

        // Cause a segmentation fault
        int *p = (int *)0xdeadbeef;
        *p = 42;
    } else {
        // We are back here after the fault
        printf("Recovered from segmentation fault!\n");
    }

    return 0;
}

Explanation:

• We use signal(SIGSEGV, sigsegv_handler) to register the signal handler.
• In the signal handler, we use longjmp(buf, 1) to jump back to the saved state.
• The program deliberately causes a segmentation fault by accessing invalid memory, but instead of crashing, the handler recovers by jumping back to the checkpoint set by setjmp.

Benchmarking the Performance of setjmp and longjmp

To understand the cost of using setjmp and longjmp, we can measure their performance over multiple iterations. Below is a benchmarking example that tests both setjmp/longjmp and setjmp alone.

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

jmp_buf buf;

void benchmark_setjmp(long iterations) {
    struct timespec start, end;
    double elapsed_time;

    // Benchmark setjmp/longjmp
    clock_gettime(CLOCK_MONOTONIC, &start);
    for (long i = 0; i < iterations; i++) {
        if (setjmp(buf) == 0) {
            longjmp(buf, 1);  // Return to checkpoint
        }
    }
    clock_gettime(CLOCK_MONOTONIC, &end);
    elapsed_time = (end.tv_sec - start.tv_sec) * 1e9 + (end.tv_nsec - start.tv_nsec);
    printf("Benchmarking setjmp/longjmp over %ld iterations, each 1 took %.3f ns\n", iterations, elapsed_time / iterations);

    // Benchmark setjmp alone
    clock_gettime(CLOCK_MONOTONIC, &start);
    for (long i = 0; i < iterations; i++) {
        setjmp(buf);
    }
    clock_gettime(CLOCK_MONOTONIC, &end);
    elapsed_time = (end.tv_sec - start.tv_sec) * 1e9 + (end.tv_nsec - start.tv_nsec);
    printf("Benchmarking setjmp over %ld iterations, each 1 took %.3f ns\n", iterations, elapsed_time / iterations);
}

int main() {
    long iterations = 1000000;
    benchmark_setjmp(iterations);
    return 0;
}

Performance Results:
After running this code with 1 million iterations, here are the results:

Benchmarking setjmp/longjmp over 1000000 iterations, each 1 took 12.293 ns
Benchmarking setjmp over 1000000 iterations, each 1 took 4.478 ns

This shows that while longjmp adds overhead, the performance is still acceptable for many use cases where non-local jumps are required for error handling.

Practical Considerations

Using setjmp and longjmp comes with caveats:

1. Signal Safety: These functions should be used carefully within signal handlers. Specifically, non-local jumps can complicate the flow of the program and leave resources in an inconsistent state.
2. Portability: Behavior might vary slightly between platforms. On some systems, sigsetjmp and siglongjmp are preferable for more precise control over signal masks【16†source】【17†source】.

Conclusion

By combining setjmp, longjmp, and signals, you can catch and recover from page faults in your C programs. While setjmp/longjmp may add some overhead, they provide a powerful mechanism for handling errors gracefully without crashing the program. If you need to benchmark or optimize this approach, the above code can be a great starting point to understand the performance trade-offs.