3.9 Calls to assembly language from C

You can call an assembly language routine from C by exporting the assembly function symbol and declaring the routine as extern in C.

Assembly language string copy subroutine

You must use the EXPORT directive to export the function symbol:

    AREA    SCopy, CODE
    EXPORT strcopy
strcopy               ; R0 points to destination string.
                      ; R1 points to source string.
    LDRB R2, [R1],#1  ; Load byte and update address.
    STRB R2, [R0],#1  ; Store byte and update address.
    CMP  R2, #0       ; Check for null terminator.
    BNE  strcopy      ; Keep going if not.
    BX   lr           ; Return.

Calling assembly language from C

To call this assembly language subroutine from C, declare it with extern:

#include <stdio.h>
extern void strcopy(char *d, const char *s);
int main()
{   const char *srcstr = "First string - source ";
    char dststr[] = "Second string - destination ";
/* dststr is an array since we’re going to change it */
    printf("Before copying:\n");
    printf("  %s\n  %s\n",srcstr,dststr);
    printf("After copying:\n");
    printf("  %s\n  %s\n",srcstr,dststr);
    return (0);
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