Testing C code by compiling with -s switch? [closed] - c

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I want to test my code (I know my code is still incomplete -- yes I am planning to complete it before I compile it) to see if it gives the correct assembly code by compiling with -s switch, how do I do this?
I am not very familiar with compiling. All I did so far was save my file. Now I need to compile it to be able to run it.
typedef enum {MODE_A, MODE_B, MODE_C, MODE_D, MODE_E} mode_t;
long switch3 (long *p1, long *p2, mode_t action) {
long result = 0;
switch(action){
case MODE_A:
case MODE_B:
case MODE_C:
case MODE_D:
case MODE_E:
default:; // don't forget the colon
}
return result;
}

Answer 1

Open an editor, Vi or Emacs for example
Type and save your code in a file, maybe main.c
Exit the editor
Type gcc -S main.c or clang -S main.c in the terminal. You can also add a -fverbose-asm flag to tell the complier to add more information in the output, or a -masm=intel flag to inspect the assembly output much nicer.
On success, a file named main.s will be generated under the current directory, containing the assembly code; on failure, error messages will be printed on the screen.
Also note that your C code will only be compiled when it's compilable, so you have to modify your code first. At least, change default; to default:;
Here is the assembly code produced by clang -S main.c on my machine:
.section __TEXT,__text,regular,pure_instructions
.macosx_version_min 10, 11
.globl _switch3
.align 4, 0x90
_switch3: ## #switch3
.cfi_startproc
## BB#0:
pushq %rbp
Ltmp0:
.cfi_def_cfa_offset 16
Ltmp1:
.cfi_offset %rbp, -16
movq %rsp, %rbp
Ltmp2:
.cfi_def_cfa_register %rbp
movq %rdi, -8(%rbp)
movq %rsi, -16(%rbp)
movl %edx, -20(%rbp)
movq $0, -32(%rbp)
movl -20(%rbp), %edx
subl $4, %edx
movl %edx, -36(%rbp) ## 4-byte Spill
ja LBB0_2
jmp LBB0_1
LBB0_1:
jmp LBB0_2
LBB0_2:
jmp LBB0_3
LBB0_3:
movq -32(%rbp), %rax
popq %rbp
retq
.cfi_endproc
.subsections_via_symbols

Answer 2

To compile without linking using GNU Compiler Collection (gcc) you can use the -S switch:
jan#jsn-dev:~/src/so> gcc -S main.c
main.c: In function ‘switch3’:
main.c:11:12: error: expected ‘:’ before ‘;’ token
default;
^
After correcting your code with the suggested fix, you get:
jan#jsn-dev:~/src/so> gcc -S main.c
jan#jsn-dev:~/src/so> cat main.s
.file "main.c"
.text
.globl switch3
.type switch3, #function
switch3:
.LFB0:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
movq %rdi, -24(%rbp)
movq %rsi, -32(%rbp)
movl %edx, -36(%rbp)
movq $0, -8(%rbp)
movq -8(%rbp), %rax
popq %rbp
.cfi_def_cfa 7, 8
ret
.cfi_endproc
.LFE0:
.size switch3, .-switch3
.ident "GCC: (SUSE Linux) 4.8.3 20140627 [gcc-4_8-branch revision 212064]"
.section .note.GNU-stack,"",#progbits