How to inspect Mach-O files

clang main.c produces an a.out, which on macOS is a binary in the Mach-O (“Mach object”) format:

$ clang main.c
$ file a.out
a.out: Mach-O 64-bit executable x86_64

clang produces Mach-O files when run on macOS because the executable format in macOS is Mach-O. By contrast, on Linux, clang produces ELF files (“Executable and Linkable Format”), because Linux’s executable format is ELF. This is documented in man pages. On macOS, the page for the execve system call says:

execve() transforms the calling process into a new process. The new process is constructed from an ordinary file … This file is either an executable object file, or a file of data for an interpreter. An executable object file consists of … see a.out(5).

The page for a.out says

The object files produced by the assembler and link editor are in Mach-O (Mach object) file format.

Since Mach-O files are just ordinary files, we can dig into the bits-and-bytes. But we can also inspect Mach-O files with a tool called otool (“object tool”). For example, we can see what dynamic libraries our a.out requires:

$ otool -L a.out
	/usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1238.60.2)

A .dylib is a Mach-O dynamic module/library. Our clang decided that our program should depend on a dynamic library at /usr/lib/libSystem.B.dylib. This provides the implementations of many things used by C programs, such as stdio functions.

Dynamic libraries can themselves require dynamic libraries. The big dylib at /usr/lib/libSystem.B.dylib requires a bunch more dylibs:

$ otool -L /usr/lib/libSystem.B.dylib
	/usr/lib/system/libsystem_asl.dylib (compatibility version 1.0.0, current version 349.50.5)
	/usr/lib/system/libsystem_blocks.dylib (compatibility version 1.0.0, current version 67.0.0)
	/usr/lib/system/libsystem_c.dylib (compatibility version 1.0.0, current version 1158.50.2)
	/usr/lib/system/libsystem_configuration.dylib (compatibility version 1.0.0, current version 888.60.2)
	/usr/lib/system/libsystem_coreservices.dylib (compatibility version 1.0.0, current version 41.4.0)

An important dylib in here is /usr/lib/system/libsystem_c.dylib. It defines a bunch of functions used by C programs. For example, this dylib defines the function fprintf. We can see this using a tool nm (“name”), which shows the name/symbol table of a Mach-O file.

$ nm -g /usr/lib/system/libsystem_c.dylib | grep fprintf
000000000003ed45 T _fprintf
000000000003ee18 T _fprintf_l
0000000000046355 T _vfprintf
0000000000046308 T _vfprintf_l

Notice that the symbol is not fprintf, but _fprintf. This is because “The name of a symbol representing a function that conforms to standard C calling conventions is the name of the function with an underscore prefix”, according to Apple.

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