How to call assembly code from C in Linux?

Robert W

Hi,

I am using Linaro Toolchain on Sitara SDK, Ubuntu 12.04, 32 bit. I would like to write C code and the C calls some assembly function. For below simple code snippet, it generates strange behavor.

    .align 4   //2       // Align the function code to a 4-byte (2^n) word boundary.
   .arm           // Use ARM instructions instead of Thumb.
//   .globl _$0       // Make the function globally accessible.
//   .no_dead_strip _$0   // Stop the optimizer from ignoring this function!
//   .private_extern _$0
   .globl _start
_start:
    mov r0, #2 /* Put a 2 inside the register r0 */
    bx lr      /* Return from main */

extern int _start(void);

int main(void) {
   printf("Hello World!\n");
   _start();
   printf("Hello Sitara World!\n");

   return 0;
}

The above code prints "Hello World" continuously in the terminal. Even I comment out two asm lines: mov r0,#2 and bx lr

The only way to stop continuous "Hello World" is that I comment out _start in the main() function.

Could you tell me how to make C calls asm function?

Thanks,

over 11 years ago

0 Steve Kipisz over 11 years ago

TI__Genius 15510 points

Instead of _start, can you rename your routine to something else? I just wrote and compiled a hello world application and disassembled it. It looks like the C runtime library already has a start function. This is from the disassembly without your _start routine:

Disassembly of section .text:

000082f4 <_start>:
    82f4:       f04f 0b00       mov.w   fp, #0
    82f8:       f04f 0e00       mov.w   lr, #0
    82fc:       f85d 1b04       ldr.w   r1, [sp], #4

Maybe rename yours from _start to _sitara.

Steve K.

0 Robert W over 11 years ago in reply to Steve Kipisz

Mastermind 7100 points

Thanks for the prompt answer. It has a new error now. The compiler complains that sitara():

Description Resource Path Location Type
undefined reference to `sitara' GNU_hello1.c /GNU_hello1 line 10 C/C++ Problem

Below is the main(). What is wrong?

extern int sitara(void);

int main(void) {
   printf("Hello World!\n");
   sitara();
   printf("Hello Sitara World!\n");

   return 0;
}

0 Robert W over 11 years ago in reply to Robert W

Mastermind 7100 points

Excuse me. I should attach the compiler and linker commands in last post.

09:22:53 **** Incremental Build of configuration Debug for project GNU_hello1 ****
make all
Building file: ../GNU_hello1.c
Invoking: Cross GCC Compiler
arm-linux-gnueabihf-gcc -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"GNU_hello1.d" -MT"GNU_hello1.d" -o "GNU_hello1.o" "../GNU_hello1.c"
Finished building: ../GNU_hello1.c

Building target: GNU_hello1
Invoking: Cross GCC Linker
arm-linux-gnueabihf-gcc -o "GNU_hello1" ./GNU_hello1.o
./GNU_hello1.o: In function `main':
/home/robert/workspace_v5_5/GNU_hello1/Debug/../GNU_hello1.c:10: undefined reference to `sitara'
collect2: error: ld returned 1 exit status
make: *** [GNU_hello1] Error 1

09:22:53 Build Finished (took 215ms)

In fact, I have noticed the problem from the above commands. It does not compile asm function sitara(). I am further puzzled about how to set it in makefile, or CCS v5.5.0? It is quite different from C6000 under CCS. Could you explain and help me?

Thanks again.

0 Robert W over 11 years ago in reply to Robert W

Mastermind 7100 points

Hi,

After I manage to set the file build settings, it compiles successfully. The new problem is that it complains

Segmentation fault

even there is a nop like command.

    .align 4   //2       // Align the function code to a 4-byte (2^n) word boundary.
   .arm           // Use ARM instructions instead of Thumb.
    .globl sitara
sitara:
mov r0,r0

What is wrong?

Thanks

0 Steve Kipisz over 11 years ago in reply to Robert W

TI__Genius 15510 points

Bear with me, I'm a command line guy, not CCS. Below is my simple C program, hello.c, that I compiled with arm-linux-gnueabihf-gcc -c hello.c just to look at the resultant object code

1 #include <stdio.h>
2
3 extern int sitara();
4
5 main()
6 {
7    int rc;
8
9    printf("Hello World!\n");
10    rc = sitara();
11    printf("rc=%d from sitara()\n", rc);
12 }

If I disassemble with arm-linux-gnueabihf-objdump -D hello.o I see

5 Disassembly of section .text:
6
7 00000000 <main>:
8    0:   b580            push    {r7, lr}
9    2:   b082            sub     sp, #8
10    4:   af00            add     r7, sp, #0
11    6:   f240 0000       movw    r0, #0
12    a:   f2c0 0000       movt    r0, #0
13    e:   f7ff fffe       bl      0 <puts>
14   12:   f7ff fffe       bl      0 <sitara>
15   16:   6078            str     r0, [r7, #4]
16   18:   f240 0000       movw    r0, #0
17   1c:   f2c0 0000       movt    r0, #0

Notice the half-word boundaries. This suggests to me that the main() code runs in thumb mode. So I changed the .arm to .thumb in my simple sitara.S file and I can build the code and it runs without the segfault. Below is my sitara.S file

1
2         .thumb
3         .text
4
5         .globl sitara
6 sitara:
7         mov     r0, #2
8         mov     pc, lr

I built with arm-linux-gnueabihf-gcc -o hello hello.c sitara.S

Steve K.

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How to call assembly code from C in Linux?