C/C++: Segmentation fault (core dumped) — SIGSEGV from a null, dangling or out-of-bounds pointer

Meaning

A segfault is a symptom, not a cause: the real bug can be far from the crash. Compile with -g -fsanitize=address, run under gdb and use bt to see the exact line. Common reasons are dereferencing nullptr, using memory after free, writing past an array, and returning a pointer to a local variable.

Common causes

• Dereferencing a null pointer (failed fopen/malloc/find result not checked)
• Use after free or after the owner went out of scope
• Array or vector index out of range (v[10] on a 3-element vector)
• Returning a pointer or reference to a local variable
• Uninitialised pointer or infinite recursion

Detailed fix by platform

Code examples

#include <cstdio>

int main() {
    FILE* f = std::fopen("missing.txt", "r");
    // if (!f) { std::perror("fopen"); return 1; }    // the missing check
    char buf[64];
    std::fgets(buf, sizeof buf, f);                    // Segmentation fault: f == nullptr
}

How to diagnose

1. Backtrace — Which line does gdb bt show?
2. Sanitizer — What does AddressSanitizer report?
3. Pointer — Could it be null, freed or out of range?

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