Mach-O Stitching #4: How C Variables Materialize in Segments & Sections

Code

// test.c

int global_elink = 1;          // External Linkage  => (__DATA,__data)         
static int global_ilink = 3;   // Internal Linkage  => (__DATA,__data)
const int constant_val = 7;    // Read-only         => (__TEXT,__const) 

void func() {
    static int count = 15;     // Static Storage    => (__DATA,__data)
    int local_val = 31;        // Automatic Storage => Stack only, no symbol
    
    global_ilink += 0;  // global_ilink might be wiped as a dead symbol even when `clang -O0`
                        // this statement keeps it alive
}

ᐅ clang -O0 -c test.c -o test.o

__TEXT Segment

ᐅ otool -tv test.o  # Display the contents of the (__TEXT,__text) section. 
                    # With the -v flag, this disassembles the text. 
                    # With the -V flag, it also symbolically disassembles the operands.
test.o:
(__TEXT,__text) section
_func:
0000000000000000	pushq	%rbp
0000000000000001	movq	%rsp, %rbp
0000000000000004	movl	$0x1f, -0x4(%rbp)  # int local_val = 31;
000000000000000b	movl	(%rip), %eax
0000000000000011	addl	$0x0, %eax
0000000000000014	movl	%eax, (%rip)
000000000000001a	popq	%rbp
000000000000001b	retq

ᐅ otool -v -s __TEXT __const test.o  # Display the contents of the (__TEXT,__const) section. 
test.o:
Contents of (__TEXT,__const) section
0000000000000018	07 00 00 00
                    #---# # const int constant_val = 7;

where 07 00 00 00 is little-endian 0x07 for const int constant_val = 7;.

__DATA Segment

ᐅ otool -dv test.o
test.o:
(__DATA,__data) section
0000000000000010	01 00 00 00 0f 00 00 00 03 00 00 00
                    #---------#                         # int global_elink = 1;
                                #---------#             # static int count = 15;
                                            #---------# # static int global_ilink = 3;

where:

• 01 00 00 00 is little-endian 0x01 for int global_ext = 1;
• 03 00 00 00 is little-endian 0x03 for static int global_ilink = 3;
• 0f 00 00 00 is little-endian 0x0f for local static int count = 15;