Can't Access Global Variable Declared In C From Arm Assembly Function

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Introduction

In the world of embedded systems and Android app development, it's not uncommon to encounter situations where you need to interact between C code and ARM assembly functions. However, when trying to access a global variable declared in C from an ARM assembly function, you may encounter issues. In this article, we'll explore the possible reasons behind this problem and provide a step-by-step guide to help you overcome it.

Understanding Global Variables in C

Before diving into the assembly code, let's quickly review how global variables work in C. A global variable is a variable that is declared outside any function and is accessible from any part of the program. In C, global variables are typically declared at the top of the file, outside any function.

// global_variable.c
int global_var = 10;  // global variable declaration

ARM Assembly and Global Variables

Now, let's move to the ARM assembly code. When you try to access a global variable from an ARM assembly function, you'll need to use the ldr (load register) instruction to load the value of the global variable into a register.

// global_variable.s
    .global global_var
    .data
    .align 4
global_var:
    .word 10

.text
.global main



main:
ldr r0, =global_var  // load global variable into r0
mov r1, r0
bx lr

However, when you try to compile and run this code, you may encounter issues. The problem lies in the way the ARM assembly code interacts with the C code.

The Issue: Register Allocation and Global Variables

The issue arises from the way the ARM assembly code allocates registers and interacts with the C code. In ARM assembly, registers are allocated based on the instruction set architecture (ISA) of the processor. When you try to access a global variable from an ARM assembly function, the register allocation mechanism may not be able to allocate a register that is compatible with the C code.

Clang Manual and Global Register Variables

As mentioned in the Clang manual, Clang only supports global register variables when the register specified is non-volatile. This means that if you try to declare a global register variable with a volatile register, Clang will not support it.

// clang_manual.c
volatile int global_var = 10;  // global variable declaration with volatile register

In this case, the ARM assembly code will not be able to access the global variable.

Solution: Use a Non-Volatile Register

To overcome this issue, you can use a non-volatile register to declare the global variable. In ARM assembly, the non-volatile registers are R0-R3, R12, and SP.

// solution.s
    .global global_var
    .data
    .align 4
global_var:
    .word 10

.text
.global main



main:
ldr r0, =global_var  // load global variable into r0
mov r1, r0
bx lr

In this case, the assembly code can access the global variable without any issues.

Conclusion

In conclusion, when trying to access a global variable declared in C from an ARM assembly function, you may encounter issues due to register allocation and global variable declaration. By using a non-volatile register to declare the global variable, you can overcome this issue and access the global variable from the ARM assembly function.

Best Practices

To avoid this issue in the future, follow these best practices:

  • Use non-volatile registers to declare global variables.
  • Avoid using volatile registers to declare global variables.
  • Use the ldr instruction to load global variables into registers.
  • Use the mov instruction to move values between registers.

By following these best practices, you can ensure that your ARM assembly code interacts correctly with the C code and access global variables without any issues.

Additional Resources

For more information on ARM assembly and global variables, refer to the following resources:

  • ARM Assembly Language Tutorial
  • Clang Manual
  • ARM Architecture Reference Manual

Q: What is the main issue when trying to access a global variable declared in C from an ARM assembly function?

A: The main issue is due to register allocation and global variable declaration. When you try to access a global variable from an ARM assembly function, the register allocation mechanism may not be able to allocate a register that is compatible with the C code.

Q: What is the difference between volatile and non-volatile registers in ARM assembly?

A: In ARM assembly, volatile registers are registers that can be modified by the operating system or other parts of the program. Non-volatile registers, on the other hand, are registers that are not modified by the operating system or other parts of the program. When declaring a global variable, it's recommended to use a non-volatile register to avoid issues with register allocation.

Q: How can I declare a global variable in C that can be accessed from an ARM assembly function?

A: To declare a global variable in C that can be accessed from an ARM assembly function, you can use a non-volatile register to declare the global variable. For example:

// global_variable.c
int global_var = 10;  // global variable declaration

Q: What is the ldr instruction in ARM assembly, and how is it used to access global variables?

A: The ldr instruction in ARM assembly is used to load a value from memory into a register. To access a global variable, you can use the ldr instruction to load the value of the global variable into a register. For example:

// global_variable.s
    .global global_var
    .data
    .align 4
global_var:
    .word 10

.text
.global main



main:
ldr r0, =global_var  // load global variable into r0
mov r1, r0
bx lr

Q: What is the mov instruction in ARM assembly, and how is it used to move values between registers?

A: The mov instruction in ARM assembly is used to move a value from one register to another. To move a value between registers, you can use the mov instruction. For example:

// global_variable.s
    .global global_var
    .data
    .align 4
global_var:
    .word 10

.text
.global main



main:
ldr r0, =global_var  // load global variable into r0
mov r1, r0  // move value from r0 to r1
bx lr

Q: What are some best practices to follow when working with global variables in ARM assembly?

A: Some best practices to follow when working with global variables in ARM assembly include:

  • Use non-volatile registers to declare global variables.
  • Avoid using volatile registers to declare global variables.
  • Use the ldr instruction to load global variables into registers.
  • Use the mov to move values between registers.

Q: What are some common mistakes to avoid when working with global variables in ARM assembly?

A: Some common mistakes to avoid when working with global variables in ARM assembly include:

  • Using volatile registers to declare global variables.
  • Not using the ldr instruction to load global variables into registers.
  • Not using the mov instruction to move values between registers.

Q: Where can I find more information on ARM assembly and global variables?

A: For more information on ARM assembly and global variables, refer to the following resources:

  • ARM Assembly Language Tutorial
  • Clang Manual
  • ARM Architecture Reference Manual

By following these best practices and avoiding common mistakes, you can ensure that your ARM assembly code interacts correctly with the C code and access global variables without any issues.