nasm - Can't link object file with ld on macOS Mojave

Question

I'm trying to assemble a simple Hello World, which worked fine in the previous macOS version:

        global   start
        section  .text
start:  mov      rax, 0x02000004
        mov      rdi, 1
        mov      rsi, msg
        mov      rdx, 13
        syscall
        mov      rax, 0x02000001
        xor      rdi, rdi
        syscall

        section  .data
msg:    db       "Hello world!", 10

Then I use nasm and ld as I did before:

$ nasm -f macho64 hello.asm
$ ld hello.o -o hello

But ld gives me the following error:

ld: warning: No version-min specified on command line
Undefined symbols for architecture x86_64:
  "_main", referenced from:
     implicit entry/start for main executable
ld: symbol(s) not found for inferred architecture x86_64

I tried switching start to _main, but got the following:

ld: warning: No version-min specified on command line
ld: dynamic main executables must link with libSystem.dylib for inferred architecture x86_64

Don't even know what that might mean.

Answer 1

ld needs -lSystem flag to prevent it from throwing this error. Also it needs -macosx_version_min to remove the warning. The correct way of using ld would be: ld hello.o -o hello -macosx_version_min 10.13 -lSystem.

Updated on macOS 11 and above, you need to pass -L/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib as well so that it locates the -lSystem library correctly. You can use -L$(xcode-select -p)/SDKs/MacOSX.sdk/usr/lib to evaluate the right path dynamically if required.

Answer 2

In addition to the @Verloren answer above (https://stackoverflow.com/a/52830915/1189569)

I had an issue with macOS Big Sur (macOS 11.1), where flag -lSystem could not locate libSystem.dylib, with the error

ld: library not found for -lSystem

I found out for macOS Big Sur, quoted from the link: https://developer.apple.com/documentation/macos-release-notes/macos-big-sur-11_0_1-release-notes

New in macOS Big Sur 11.0.1, the system ships with a built-in dynamic linker cache of all system-provided libraries. As part of this change, copies of dynamic libraries are no longer present on the filesystem. Code that attempts to check for dynamic library presence by looking for a file at a path or enumerating a directory will fail...

that all copies of dynamic libraries are not located in usr/lib/ and similar, so flag -lSystem could not found libSystem.dylib by default.

The solution to this was to update/install the latest version of Command Line Tools, if not already, and to set flag -L of the ld command to /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib.

So full command would look like this:

ld hello.o -o hello -macosx_version_min 11.0 -L /Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/lib -lSystem

Answer 3

Simpler answer. ld is defaulting to dynamic linking and tries to load crt1 which is looking for main. So specify static linking.

% ld -e start -static hello.o -o hello
% ./hello
Hello world!