C Inline Assembly label issue

Question

I'm currently a total beginner with assembly and am learning how to use assembly inline with C for a class. That being said, I'm having a hard time with this particular error when I'm compiling my file:

/tmp/cckHnU89.s: Assembler messages:
/tmp/cckHnU89.s:550: Error: symbol `.L16' is already defined
/tmp/cckHnU89.s:571: Error: symbol `.L18' is already defined
/tmp/cckHnU89.s:576: Error: symbol `.L17' is already defined

I tried replacing the names of the labels with other names since I noticed from the .s file that the labels .L16, .L17, and .L18 are used in my main method as well as in one of my functions. However, when did that I just ended up with a segmentation fault from running the program. Is there a way to change the names of the labels or something else to fix which apparently seems to be a naming conflict?

As far as my CPU, I'm running an Intel Pentium T4500 processor and I'm compiling with gcc version 4.4.3. My code is 300+ lines for the inline assembly portion so I'll spare whoever reads this. Essentially, I'm just looking for a general answer on how one would normally fix the naming conflict that produces the error above. Anything insight would be greatly appreciated.

Can you show us the relevant code? Kinda hard to tell if we have nothing to go on. — Mysticial, Apr 09 '12 at 23:03
As well as the code, please tell us what CPU you're compiling for and what C compiler you're using. — user9876, Apr 09 '12 at 23:25
Don't think this deserves a downvote: (1) it's an honest attempt to provide the necessary information, (2) it explains what has already been attempted to address the problem, (3) it asks a specific question. — Robert Martin, Apr 09 '12 at 23:41

0xC0000022L · Accepted Answer · 2012-04-10T01:16:53.300

My hunch here is (and I just verified it with g++ -S and gcc -S) that your own labels are exactly mimicking the naming scheme (.L<num>) for labels automatically assigned to assembler code in GCC.

Do the following:

# for C:
gcc -S source.c
# for C++
g++ -S source.cpp

... and then cat (or less) the resulting .s file (same base name, .s suffix, e.g. source.s). You will find numerous labels of that scheme (.L<num>). Now, if you yourself create inline assembly containing the same names as already automatically created labels (from your C code), that would obviously lead to clashes.

So the gist: don't use .L<num> as your naming scheme for labels, because it will clash.

Generally names starting with .L seem to ask for trouble here.

Example (test.cpp), compile with g++ -S test.cpp:

#include <cstdio>

int main(int argc, char**argv)
{
        switch(argc)
        {
                case 0:
                        printf("test 0\n");
                        break;
                case 1:
                        printf("test %d\n", argc);
                        break;
                case 2:
                        printf("test %d -> %s\n", argc, argv[0]);
                        break;
                default:
                        printf("Default\n");
                        break;
        }
        return 0;
}

Compiled on x64 (contents of test.s):

        .file   "test.cpp"
        .section        .rodata
.LC0:
        .string "test 0"
.LC1:
        .string "test %d\n"
.LC2:
        .string "test %d -> %s\n"
.LC3:
        .string "Default"
        .text
.globl main
        .type   main, @function
main:
.LFB0:
        .cfi_startproc
        .cfi_personality 0x3,__gxx_personality_v0
        pushq   %rbp
        .cfi_def_cfa_offset 16
        movq    %rsp, %rbp
        .cfi_offset 6, -16
        .cfi_def_cfa_register 6
        subq    $16, %rsp
        movl    %edi, -4(%rbp)
        movq    %rsi, -16(%rbp)
        movl    -4(%rbp), %eax
        cmpl    $1, %eax
        je      .L4
        cmpl    $2, %eax
        je      .L5
        testl   %eax, %eax
        jne     .L8
.L3:
        movl    $.LC0, %edi
        call    puts
        jmp     .L6
.L4:
        movl    -4(%rbp), %eax
        movl    %eax, %esi
        movl    $.LC1, %edi
        movl    $0, %eax
        call    printf
        jmp     .L6
.L5:
        movq    -16(%rbp), %rax
        movq    (%rax), %rdx
        movl    -4(%rbp), %eax
        movl    %eax, %esi
        movl    $.LC2, %edi
        movl    $0, %eax
        call    printf
        jmp     .L6
.L8:
        movl    $.LC3, %edi
        call    puts
.L6:
        movl    $0, %eax
        leave
        ret
        .cfi_endproc
.LFE0:
        .size   main, .-main
        .ident  "GCC: (Debian 4.4.5-8) 4.4.5"
        .section        .note.GNU-stack,"",@progbits

Observe the names starting with .L in the resulting assembler file.

C Inline Assembly label issue

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