Segmentation fault: reproduce the memory violation

Recognize the actual failure

A segmentation fault indicates an invalid memory access detected by the operating system. In C/C++ this can follow dereferencing an invalid pointer, indexing past a buffer, use-after-free or stack exhaustion. The printed crash location may be downstream of the original corruption.

Inspect safely before changing anything

cc -g -O0 -Wall -Wextra -fsanitize=address,undefined demo.c -o demo
./demo

Repair the cause rather than suppressing the warning

Use sanitizers when supported, otherwise a debugger with symbols and a minimized reproducer. Reduce input size and test the last safe index before the failure. Avoid enabling sanitizers on a production executable without understanding overhead and toolchain support. Do not interpret a compiler warning-free build as evidence of memory safety.

Reference: https://clang.llvm.org/docs/AddressSanitizer.html

Tiny boundary experiment

A crash report is evidence of an invalid access, not necessarily the site of its origin. Use a valid baseline first:

#include <stdio.h>
int main(void) {
    int values[3] = {10, 20, 30};
    for (size_t i = 0; i < sizeof values / sizeof values[0]; ++i)
        printf("%d\n", values[i]);
    return 0;
}

Compile with cc -std=c11 -g -O0 -Wall -Wextra -fsanitize=address,undefined demo.c -o demo where supported. Expected output is three lines: 10, 20, 30. In a disposable lab, change the loop condition to <=; a sanitizer can report a stack-buffer-overflow at index 3. Do not run intentionally invalid native samples against important data.

Trace the fault to ownership

For heap buffers, draw the allocation, every alias, the free operation and subsequent reads. If a pointer was freed but is still referenced, changing the crash location with a log statement does not repair the use-after-free. Validate with a sanitizer-enabled test build and then a normal release build. In a debugger, a stack trace, input fixture and compiler flags are necessary together; a stack trace alone may point after the corruption.