Linking C
All about compiling, linking, and debugging C with GCC on macOx (Intel and Apple Silicon).
Notes
gcc
gcc the GNU Compiler Collection includes front ends for C, C++, Objective-C, Fortran, Ada, Go, and D, as well as libraries for these languages.
Original Examples Run on Intel Mac
gcc
Since I’m on macOS, gcc is actually linking to llvm’s clang:
$ gcc --version
Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
Apple LLVM version 10.0.0 (clang-1000.11.45.5)
Target: x86_64-apple-darwin17.7.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
Compiling and link a single file:
$ gcc example.c -o example -lc -g -Wall -O0 -std=gnu11
$ ./example
Hello, world.
-lcinstructs the linker to link the standard libc library. This is redundant and is assumed if not specified.-gadds symbols for debugging-std=gnu11is gcc-specific, and specifies that gcc should allow code conforming to the C11 and POSIX standards-O0indicates optimization level zero, effectively turning off all funny business.-Wallenables all compiler warnings
objdump
objdump displays object files on Unix-like operating systems.
Since I’m on macOS, objdump is actually linking to llvm’s objdump:
$ objdump --version
Apple LLVM version 10.0.0 (clang-1000.11.45.5)
Optimized build.
Default target: x86_64-apple-darwin17.7.0
Host CPU: haswell
Registered Targets:
aarch64 - AArch64 (little endian)
aarch64_be - AArch64 (big endian)
arm - ARM
arm64 - ARM64 (little endian)
armeb - ARM (big endian)
thumb - Thumb
thumbeb - Thumb (big endian)
x86 - 32-bit X86: Pentium-Pro and above
x86-64 - 64-bit X86: EM64T and AMD64
Using GCC to compile to object file:
$ gcc -c example.c
Disassemble all text sections:
$ objdump -d example.o
example.o: file format Mach-O 64-bit x86-64
Disassembly of section __TEXT,__text:
_status:
0: 55 pushq %rbp
1: 48 89 e5 movq %rsp, %rbp
4: 31 c0 xorl %eax, %eax
6: 5d popq %rbp
7: c3 retq
8: 0f 1f 84 00 00 00 00 00 nopl (%rax,%rax)
_main:
10: 55 pushq %rbp
11: 48 89 e5 movq %rsp, %rbp
14: 48 83 ec 10 subq $16, %rsp
18: 48 8d 3d 1c 00 00 00 leaq 28(%rip), %rdi
1f: c7 45 fc 00 00 00 00 movl $0, -4(%rbp)
26: b0 00 movb $0, %al
28: e8 00 00 00 00 callq 0 <_main+0x1d>
2d: 89 45 f8 movl %eax, -8(%rbp)
30: e8 00 00 00 00 callq 0 <_main+0x25>
35: 48 83 c4 10 addq $16, %rsp
39: 5d popq %rbp
3a: c3 retq
Linking from Object Files
$ gcc -c example.c
$ gcc example.o -lc -o example
$ ./example
Hello, world.
macOS with Apple Silicon Update
gcc
Since I’m on macOS, gcc is actually linking to llvm’s clang:
$ gcc --version
Apple clang version 15.0.0 (clang-1500.1.0.2.5)
Target: arm64-apple-darwin23.2.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin
Compiling and link a single file:
$ gcc example.c -o example -lc -g -Wall -O0 -std=gnu11
$ ./example
Hello, world.
objdump
objdump displays object files on Unix-like operating systems.
Since I’m on macOS, objdump is actually linking to llvm’s objdump:
$ objdump --version
Apple LLVM version 15.0.0
Optimized build.
Default target: arm64-apple-darwin23.2.0
Host CPU: apple-m1
Registered Targets:
aarch64 - AArch64 (little endian)
aarch64_32 - AArch64 (little endian ILP32)
aarch64_be - AArch64 (big endian)
arm - ARM
arm64 - ARM64 (little endian)
arm64_32 - ARM64 (little endian ILP32)
armeb - ARM (big endian)
thumb - Thumb
thumbeb - Thumb (big endian)
x86 - 32-bit X86: Pentium-Pro and above
x86-64 - 64-bit X86: EM64T and AMD64
Using GCC to compile to object file:
$ gcc -c example.c
Disassemble all text sections:
$ objdump -d example.o
example.o: file format mach-o arm64
Disassembly of section __TEXT,__text:
0000000000000000 <ltmp0>:
0: 52800000 mov w0, #0
4: d65f03c0 ret
0000000000000008 <_main>:
8: d10083ff sub sp, sp, #32
c: a9017bfd stp x29, x30, [sp, #16]
10: 910043fd add x29, sp, #16
14: b81fc3bf stur wzr, [x29, #-4]
18: 90000000 adrp x0, 0x0 <_main+0x10>
1c: 91000000 add x0, x0, #0
20: 94000000 bl 0x20 <_main+0x18>
24: 94000000 bl 0x24 <_main+0x1c>
28: a9417bfd ldp x29, x30, [sp, #16]
2c: 910083ff add sp, sp, #32
30: d65f03c0 ret
Linking from Object Files
$ gcc -c example.c
$ gcc -c example.c
$ gcc example.o -lc -o example
$ ./example
Hello, world.