.dSYM

Introduction

A .dSYM is a bundle, a folder that macOS shows as a single item, holding the debug information for a program, library or app built with Apple’s Xcode. On Apple’s systems the linker does not copy debug information into the program; it stays in the object files, and the dsymutil tool gathers it into a dSYM bundle named after the program, such as MyApp.app.dSYM.

Debuggers use a dSYM to show source lines and variables, and crash reporting services use it to turn the addresses in a crash report into function names and line numbers. App Store builds are usually archived with their dSYM files for this reason.

Structure

The bundle has the layout Contents/Info.plist, which identifies it, and Contents/Resources/DWARF/, which holds one file named after the program. That file is a Mach-O file of the debug symbols type (file type 10). It has no code; its __DWARF segment holds the DWARF debug sections, such as __debug_info and __debug_line, and its symbol table lists the program’s functions. For a universal program it holds one architecture per slice, like the program itself.

Each Mach-O program carries a UUID in an LC_UUID load command, and the dSYM carries the same UUID, so tools can match a crash or a running program to the right bundle. dwarfdump --uuid shows it.

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