I have a bit of assembly in a hard fault handler.

The assembly is basically meant to pass the current stack pointer as a parameter (in R0). It looks like so...

__asm("    mov     r0, sp\n"  "    bl      SavePC\n"  "    bx      lr");

This works fine when SavePC is in the same c file.

However, when SavePC is placed in another c file I have no luck.

I have tried to IMPORT the function like so...

__asm("IMPORT SavePC\n"" mov r0, sp\n"" bl SavePC\n"" bx lr");

... but I must be doing something incorrect. The compiler reports the following...

Error[Og005]: Unknown symbol in inline assembly: "IMPORT" Error[Og005]: Unknown symbol in inline assembly: "SavePC" Error[Og006]: Syntax error in inline assembly: "Error[54]: Expression can not be forward"Error[Og005]: Unknown symbol in inline assembly: "SavePC" Error while running C/C++ Compiler

The c file with the assembly includes the header file with the SavePC prototype...

extern void SavePC(unsigned long);

Suggestions?

Your code won't work even with a correct call.

bl _SavePC             // LR be changedbx lr                  // while (1){}

What do you think will be the value in the LR register in the bx lr instruction?

The address of the instruction itself!

The bl instruction has put it there. This is effectively a while (1); 

with a bx instruction.

A nested function call looks more like this:

push {lr}bl _SavePCpop {pc}

To get the stack register one uses the corresponding CMSIS functions:

• __get_MSP() for the Main Stack Pointer (MSP)
• __get_PSP() for the Process Stack Pointer (PSP)

Using extern is a bad habit since it is prone to errors. C-99 standard provides an safe alternative for extern. You should write the function prototype in the header file without extern keyword. Then include the header file in both C files. The linker is then responsible for linking the function in different files.

Example:

File : custom_header.h

void SavePC(unsigned long);

File : source_c_file.c

#include "custom_header.h"void SavePC(unsigned long){      ....      ....      ....}

File : user_c_file.c

#include "custom_header.h"void someFunction(void){...__asm("mov     r0, sp\n"  "    push  {lr}  "    bl      SavePC\n"  "    pop    {pc}");...}