C/C++: Bus error (core dumped) — SIGBUS from misaligned access or a bad memory-mapped file

Meaning

SIGBUS differs from SIGSEGV: the address may be valid but the access is not allowed. On ARM and SPARC, misaligned reads of int or double raise it; on x86 and macOS it more often comes from mmap: the file was truncated, the disk filled up, or the file lives on a removed network share. Writing to a string literal on some platforms also produces it.

Common causes

• mmaped file truncated or deleted while the program reads it
• Disk full or quota exceeded when writing to an mmap'ed or sparse file
• Misaligned pointer cast (*(int*)(buf + 1)) on strict-alignment CPUs
• Writing into a string literal or read-only mapping
• Stack alignment problems in hand-written assembly

Detailed fix by platform

Code examples

#include <cstdint>
#include <cstring>

uint32_t read_u32(const unsigned char* p) {
    uint32_t v;
    std::memcpy(&v, p, sizeof v);      // safe for any alignment
    return v;                          // *(uint32_t*)p could raise SIGBUS on strict CPUs
}

How to diagnose

1. Mapping — Is a mmap'ed file involved?
2. Disk — Is the disk full?
3. Alignment — Any pointer cast to a wider type?

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