Thread: passing variables among c functions

hello all
I have this code where I am trying to retrieve the contents of the variable dev1 and dev2. for some reason when i compile and run I am getting 0 and 0. What am i doing wrong?thanks

Code:

typedef struct 
{
    uint32_t x;
    uint32_t y;
} sample;


void get_sample(sample *one)
{
  sample var1,var2;
  uint32_t dev1,dev2;

//does something to dev1 and dev2


return dev1 and dev2;
}


int main()
{

  uint32_t dev1, dev2;
  sample *one;
  get_sample(&one);
  
  printf("%4x%4x\n",dev1,dev2);
  //I want to retrieve the value dev1 and dev2, print dev1 and dev2 and join them to form one string
}

You cannot return dev1 and dev2 because:

• You can only return one object from a function.
• The function's return type is void.

Also, in main you declared one to be a pointer to sample, then you passed &one as the argument to get_sample, but get_sample's parameter is a pointer to sample, not a pointer to a pointer to sample.

Perhaps, you should declare get_sample like this:

Code:

void get_sample(sample *one, uint32_t *dev1, uint32_t *dev2);

Now, you work with *dev1 and *dev2, and the changes to these variables will be reflected in the caller. The caller, in this case the main function, will presumably call get_sample like this:

Code:

sample one;
uint32_t dev1;
uint32_t dev2;
get_sample(&one, &dev1, &dev2);

What is your current code?

Ok My current code is

Code:

typedef struct
{
    uint32_t x;
    uint32_t y;
} sample;
 
 
  int get_sample(sample *one, uint32_t *dev1, uint32_t *dev2);
{
  sample var1,var2;
  // do something with dev1 and dev2 
 
  UART_printf(" %4x%4x \r\n ",dev1,dev2);//getting correct value

return dev1;
}
 
 
int main()
{
  sample one;
  uint32_t dev1, dev2; 

  get_sample(&one, &dev1, &dev2);
   UART_printf(" %4x \r\n ",dev1);
//here i am getting some random number
//I want dev1,dev2 in main to be same as in get_sample function. 
}

Also I am returning only dev1 while dev2 is left out.How to return both ?do i need to create a separate func to retrieve it??
thanks

That cannot possibly be your current code because it will not compile. Furthermore, dev1 and dev2 in get_sample are pointers, yet you are printing them without dereferencing them: there is no way that you could be "getting the correct value" from that UART_printf, except by some incredible coincidence.

Here's a variation of what you wrote:

Code:

#include <stdio.h>
#include <stdint.h>

typedef struct
{
    uint32_t x;
    uint32_t y;
} sample;

void get_sample(sample *one, uint32_t *dev1, uint32_t *dev2)
{
    *dev1 = 123;
    *dev2 = 456;
    printf("dev1=%4x; dev2=%4x\n", *dev1, *dev2);
}

int main(void)
{
    sample one;
    uint32_t dev1, dev2;
    get_sample(&one, &dev1, &dev2);
    printf("dev1=%4x; dev2=%4x\n", dev1, dev2);
    return 0;
}

It prints:

Code:

dev1=  7b; dev2= 1c8
dev1=  7b; dev2= 1c8

Hence dev1, dev2 in main has the same value as in the get_sample function.

Originally Posted by amaturequestion

Is there something I am doing wrong??

Yes, you did not dereference your pointers when you want to access what they point to. So this:

Code:

dev1 = *(__IO uint32_t*)(0x1FFFF7E8);
dev2 = *(__IO uint32_t*)(0x1FFFF7EC);
dev3 = *(__IO uint32_t*)(0x1FFFF7F0);
dev1 += dev2;

should have been:

Code:

*dev1 = *(__IO uint32_t*)(0x1FFFF7E8);
*dev2 = *(__IO uint32_t*)(0x1FFFF7EC);
*dev3 = *(__IO uint32_t*)(0x1FFFF7F0);
*dev1 += *dev2;

That said, I notice that you wrote:

Code:

one->x=dev1;
one->y=dev2;

This is also wrong because you need to dereference dev1 and dev2, but it looks like you don't need dev1 and dev2 to be parameters begin with. Rather, you could write:

Code:

void get_sample(sample *one)
{
    uint32_t dev1;
    uint32_t dev2;
    dev1 = *(__IO uint32_t*)(0x1FFFF7E8);
    dev2 = *(__IO uint32_t*)(0x1FFFF7EC);
    dev1 += dev2;

    one->x = dev1;
    one->y = dev2;
    UART_printf("%4x %4x\n", dev1, dev2);
}

Then in main:

Code:

get_sample(&one);
UART_printf("%4x %4x", one.dev1, one.dev2);

Originally Posted by amaturequestion

Also thank you for your constant efforts.AMAZING

You're welcome

Hello laserlight ..Thank u for the suggestion. It is working fine now..
Now I have to send contents of these two dev using a function

Code:

void Send_Packet(UINT8 type, const UINT8* pbuf, UINT8 len)

I want to combine the two hex numbers and fit it in UNIT8* pbuf.However wen I do a


uint32_t pbuf=(((uint32_t)one->y)<< 32) | (uint32_t(one->x));

and print pbuf it gives 30354131 instead of 27CA6159 30354131..Is there any other way of to achieve this?Is it a good idea to use memcopy instead??