How to solve this series of string literal errors with embedded assembly?

Question

I was compiling some code that had this piece of code:

__asm__ __volatile__ ("
    pushw %%es
    movw %%ds, %%ax
    movw %%ax, %%es
    xorl %%eax, %%eax
    movl SurfBufD, %%edi
    xorl %%ebx, %%ebx
Blank2:
    movl SurfaceX, %%ecx
    rep
    stosw
    addl Temp1, %%edi
    subl SurfaceX, %%edi
    subl SurfaceX, %%edi
    addl $1, %%ebx
    cmpl SurfaceY, %%ebx
    jne Blank2
    popw %%es
" : : : "cc", "memory", "eax", "ebx", "ecx", "edi");

and when I tried to compile it, I got:

linux/sdllink.c:948:24: warning: missing terminating " character asm volatile (" ^ linux/sdllink.c:948:2: error: missing terminating " character asm volatile (" ^ linux/sdllink.c:949:3: error: expected string literal before ‘pushw’ pushw %%es ^ linux/sdllink.c:966:51: warning: missing terminating " character " : : : "cc", "memory", "eax", "ebx", "ecx", "edi"); ^ linux/sdllink.c:966:2: error: missing terminating " character " : : : "cc", "memory", "eax", "ebx", "ecx", "edi");

I tried to solve it changing this code to:

    __asm__ __volatile__ ( 
    "pushw %%es"
    "movw %%ds, %%ax"
    "movw %%ax, %%es"
    "xorl %%eax, %%eax"
    "movl SurfBufD, %%edi"
    "xorl %%ebx, %%ebx"
"Blank2:"
    "movl SurfaceX, %%ecx"
    "rep"
    "stosw"
    "addl Temp1, %%edi"
    "subl SurfaceX, %%edi"
    "subl SurfaceX, %%edi"
    "addl $1, %%ebx"
    "cmpl SurfaceY, %%ebx"
    "jne Blank2"
    "popw %%es"
" : : : cc, memory, eax, ebx, ecx, edi");

Basically, I assumed this function wanted every line to be a string literal, but it didn't change anything. So, what I have to do?

It did not change **anything**? You got **exactly** the same error message? — Eric Postpischil, Jan 02 '21 at 23:26
That must be some very old code. gcc [used to support multi-line string literals](https://gcc.gnu.org/onlinedocs/gcc-3.2.2/gcc/Multi-line-Strings.html) as an extension to standard C, but this feature was [removed in gcc 3.3](https://gcc.gnu.org/gcc-3.3/changes.html), circa May of 2003. — Nate Eldredge, Jan 02 '21 at 23:40
@NateEldredge Well, he's working with `es` and `ds`, so DOS? — David Wohlferd, Jan 03 '21 at 01:37
@DavidWohlferd: Well real-mode at least, could also be freestanding or depending only on BIOS, not DOS. (e.g. a bootloader or a 16-bit "OS" (toy or otherwise).) — Peter Cordes, Jan 03 '21 at 10:14

MikeCAT · Answer 1 · 2021-01-03T02:13:38.790

One string literal in C cannot written in multiple lines, but consecutive string literals are merged.
Also \ in the line end means that the line should be concatenated with the next line and treated as one line, so you can use this.
There should be newlines in the string. Newlines can be expressed as \n.

Consecutive string literals:

    __asm__ __volatile__ ( 
    "pushw %%es\n"
    "movw %%ds, %%ax\n"
    "movw %%ax, %%es\n"
    "xorl %%eax, %%eax\n"
    "movl SurfBufD, %%edi\n"
    "xorl %%ebx, %%ebx\n"
"Blank2:\n"
    "movl SurfaceX, %%ecx\n"
    "rep\n"
    "stosw\n"
    "addl Temp1, %%edi\n"
    "subl SurfaceX, %%edi\n"
    "subl SurfaceX, %%edi\n"
    "addl $1, %%ebx\n"
    "cmpl SurfaceY, %%ebx\n"
    "jne Blank2\n"
    "popw %%es\n"
: : : "cc", "memory", "eax", "ebx", "ecx", "edi");

Lines connected via \:

    __asm__ __volatile__ ( 
    "pushw %%es\n\
    movw %%ds, %%ax\n\
    movw %%ax, %%es\n\
    xorl %%eax, %%eax\n\
    movl SurfBufD, %%edi\n\
    xorl %%ebx, %%ebx\n\
Blank2:\n\
    movl SurfaceX, %%ecx\n\
    rep\n\
    stosw\n\
    addl Temp1, %%edi\n\
    subl SurfaceX, %%edi\n\
    subl SurfaceX, %%edi\n\
    addl $1, %%ebx\n\
    cmpl SurfaceY, %%ebx\n\
    jne Blank2\n\
    popw %%es\n"
: : : "cc", "memory", "eax", "ebx", "ecx", "edi");

If you're going to answer this instead of looking for a duplicate, worth pointing out that the resulting string literal gets fed to the assembler as-is (after substituting any `%[operand]` strings), which is why it has to be a string literal containing multiple lines for each instruction. (Or separated by `;`, the GAS "statement" separator for its x86 syntaxes.) — Peter Cordes, Jan 03 '21 at 10:13

How to solve this series of string literal errors with embedded assembly?

1 Answers1