Mmap

Truncating a Mapped File Can Trigger SIGBUS

A process can retain a valid mmap() address range after another operation shrinks the backing file, then receive SIGBUS when it touches a mapped page past the file’s new end. The mapping itself has not vanished. Its backing object no longer covers every page that the virtual mapping originally referenced. This boundary is easy to miss because mapping lifetime and file size are separate state. Closing the original file descriptor does not invalidate an established mapping, and shrinking the file does not act like munmap() on every process that maps it.