The Nitty Gritty of “Hello World” on OS X

http://www.reinterpretcast.com/hello-world-mach-o

by joesavage • 12 years ago

48 7 12 years ago

userbinator2• 12 years ago

I don't want to explain each of the 8548 output bytes in detail here

I wonder how many people these days (including developers) would think "that's not so big for a Hello World program", and then change their mind after watching some 4k demos... 8KB might not sound like much in this era of gigabytes and terabytes, but eight thousand bytes is still, in absolute terms, quite a bit of data, and enough to do plenty more interesting things. Executable formats have become more complex with their headers, which are mostly unavoidable, but seeing empty space in the majority of the file is somewhat sad.

Here are some smaller Hello World programs to examine...

http://seriot.ch/hello_macho.php - OS X

http://timelessname.com/elfbin/ - Linux

...but they're still somewhat larger than the 20 bytes of the DOS version (95 ba 07 01 cd 21 c3 48 65 6c 6c 6f 20 57 6f 72 6c 64 21 24.)

maguirre• 12 years ago

truly curious here. what's the tradeoff. What did we gain with bigger/more sophisticated executable formats?

95014_refugee• 12 years ago

Page-aligning the segments makes it (much) cheaper to load pieces of the executable on demand (demand paging) which reduces the amount of physical memory required to launch the program. Note that most of that size is zeroes padding the file out. Also remember that most file allocations are quantized to 512 or 4096-byte blocks.

Dynamic linking makes it possible for executables to share a single copy of system frameworks; this saves physical memory, improves performance (due to reduced cache traffic), reduces the size of downloads, etc.

The UUID allows the debugger to reliably associate an executable with the dSYM bundle containing debug information.

Other features not discussed in the article allow a single program to run on multiple architectures (PPC, I386, X86_64, etc), allow the system to reliably determine whether a program has changed, to decide what capabilities the program has been granted, etc.

It's worth putting these size number into perspective; yes, EIGHT THOUSAND BYTES sounds big and scary next to twenty, but the overhead scales linearly with the number of executables and their functional complexity, not geometrically. Also, you have on the order of ONE TRILLION BYTES of storage for these headers, rather than three hundred and sixty thousand...

snuxoll• 12 years ago

I'm curious how much space would get saved if the binary was stripped, a step most people forget to run if they're trying to see what a bare minimum binary looks like. Of course, I'm not very familiar with the Mach-O format, but for ELF binaries it can take a 4K hello world from GCC down to a couple hundred bytes.

The Nitty Gritty of “Hello World” on OS X

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