Copy function to RAM and call it with function pointer

[EthanH]

Hello,

I am trying to copy the function to RAM and then call it using a function pointer like below:

It forced a system restart and it should have encountered an error.

Can you please help me deal with it? Thanks in advance!

uint8_t ram_func(uint8_t data_in)
{
  data_in++;
  return data_in;
}
/*---------------------------------------------------------------*/
int main() {
    typedef uint8_t (*func_ptr)(uint8_t);
    uint8_t func_buffer[256];
    memset(func_buffer, 0, 256);
    // Copy the machine code of func into func_buffer
    memcpy(&func_buffer, (uint8_t*)ram_func, 255);

    // Create a function pointer and cast the buffer address to it
    func_ptr fp = (func_ptr)func_buffer;

    // Call the function through the function pointer
    uint8_t data_out = fp(3);

    printf("Result = %d\n", data_out);
    return 0;
}

[hmolesworth]

I put a breakpoint on the call to fp() on line 31 then clicked the ASM window and single-stepped through the assembler using "into" button and it works ok with data_out2 set to 4 on return. This is on a nRF52832 with no other code such as bootloader or softdevice. Maybe try using the debugger in the same way; if I get time I'll try this on an nRF52840.

Edit: Works fine single-stepping through asm window but running to a breakpoint placed after the fp() call generates a hard fault, not sure what the difference is. I'll try to find time to look at this

[hmolesworth]

Ah, I forgot to use the lsbit Thumb Mode on the return; here's the fixed code which runs even without stepping:

uint8_t ram_func(uint8_t data_in)
{
  data_in++;
  return data_in;
}
uint8_t data_out1 = 0;
uint8_t data_out2 = 0;
uint32_t func_buffer[64];

static void RamFuncTest(void) {
    typedef uint8_t (*func_ptr)(uint8_t);
    // Copy the machine code of func into func_buffer
    uint32_t *pRAM=func_buffer;
    uint32_t *pCode=(uint32_t *)((uint32_t)&ram_func & 0xFFFFFFFEUL);
    for (uint32_t i=0UL; i<sizeof(func_buffer)/sizeof(func_buffer[0]); i++)
    {
      *pRAM++ = *pCode++;
    }
    // Create a function pointer and cast the buffer address to it, remember Thumb Mode
    uint32_t *pfunc_buffer = (uint32_t*)(((uint32_t)func_buffer) | 0x00000001UL);
    func_ptr fp = (func_ptr)(pfunc_buffer);
    // Test code function to verify that works
    data_out1 = ram_func(2);
    // Call the function through the function pointer
    data_out2 = fp(3);
    printf("Result = %d\n", data_out2);
    return 0;
}

[EthanH]

Really appreciate your code and support!
Same reboot for the code below:

#include <stdint.h>
#include <stddef.h>
#include <string.h>

uint8_t ram_func(uint8_t data_in)
{
  data_in++;
  return data_in;
}
uint8_t data_out1 = 0;
uint8_t data_out2 = 0;
uint32_t func_buffer[64];
uint32_t *pfunc_buffer = func_buffer;

static void RamFuncTest(void) {
    typedef uint8_t (*func_ptr)(uint8_t);
    // Copy the machine code of func into func_buffer
    uint32_t *pRAM=func_buffer;
    uint32_t *pCode=(uint32_t *)((uint32_t)&ram_func & 0xFFFFFFFEUL);
    for (uint32_t i=0UL; i<sizeof(func_buffer)/sizeof(func_buffer[0]); i++)
    {
      *pRAM++ = *pCode++;
    }
    // Create a function pointer and cast the buffer address to it, remember Thumb Mode
    pfunc_buffer = (uint32_t*)(((uint32_t)pfunc_buffer) | 0x00000001UL);
    func_ptr fp = (func_ptr)(pfunc_buffer);
    // Test code function to verify that works
    data_out1 = ram_func(2);
    // Call the function through the function pointer
    data_out2 = fp(3);
    printf("Result = %d\n", data_out2);
}

/*---------------------------------------------------------------*/
int main() {
    printf("start\n\n");
    RamFuncTest();
    return 0;
}

Did I miss something?

[hmolesworth]

Just loaded your code, works ok here but I comment out the printf(). I moved some of the definitions back into the function, edited above, but it works with or without that. Try a breakpoint on the printf() and see if you can get there with data_out2 correct

This is what runs ok:

uint8_t ram_func(uint8_t data_in)
{
  data_in++;
  return data_in;
}
uint8_t data_out1 = 0;
uint8_t data_out2 = 0;
uint32_t func_buffer[64];
static void RamFuncTestSub(void) {
    typedef uint8_t (*func_ptr)(uint8_t);
    // Copy the machine code of func into func_buffer
    uint32_t *pRAM=func_buffer;
    uint32_t *pCode=(uint32_t *)((uint32_t)&ram_func & 0xFFFFFFFEUL);
    for (uint32_t i=0UL; i<sizeof(func_buffer)/sizeof(func_buffer[0]); i++)
    {
      *pRAM++ = *pCode++;
    }
    // Create a function pointer and cast the buffer address to it, remember Thumb Mode
    uint32_t *pfunc_buffer = (uint32_t*)(((uint32_t)func_buffer) | 0x00000001UL);
    func_ptr fp = (func_ptr)pfunc_buffer;
    // Test code function to verify that works
    data_out1 = ram_func(2);
    // Call the function through the function pointer
    data_out2 = fp(3);
//    while(1) ;
//    printf("Result = %d\n", data_out2);
//    return 0;
}
static void RamFuncTest(void) {
    //printf("start\n\n");
    RamFuncTestSub();
    while(1) ;
}

[hmolesworth]

Same code, but also returning a value on stack in case something was amiss there; seems ok:

uint8_t ram_func(uint8_t data_in)
{
  data_in++;
  return data_in;
}
static uint8_t RamFuncTestSub(void) {
    typedef uint8_t (*func_ptr)(uint8_t);
    uint32_t func_buffer[64];
    // Copy the machine code of func into func_buffer
    uint32_t *pRAM=func_buffer;
    uint32_t *pCode=(uint32_t *)((uint32_t)&ram_func & 0xFFFFFFFEUL);
    for (uint32_t i=0UL; i<sizeof(func_buffer)/sizeof(func_buffer[0]); i++)
    {
      *pRAM++ = *pCode++;
    }
    // Create a function pointer and cast the buffer address to it, remember Thumb Mode
    uint32_t *pfunc_buffer = (uint32_t*)(((uint32_t)func_buffer) | 0x00000001UL);
    func_ptr fp = (func_ptr)pfunc_buffer;
    // Test code function to verify that works
    uint8_t data_out1 = ram_func(2);
    // Call the function through the function pointer
    uint8_t data_out2 = fp(3);
    return data_out2;
}
static void RamFuncTest(void) {
    //printf("start\n\n");
    uint8_t result = RamFuncTestSub();
    //printf("Result = %d\n", result);
    while(1) ;
}

I even tested with buffer on stack; all ok.