function parameters

It has been a while since I have been working with details on specifically appropriately passing parameters into a function. Basically, I have a snippet of code as shown below:

Code:

---


#define BUFFER_SIZE 5

void main(){
      unsigned int temp_res;
      char adc[BUFFER_SIZE];

      void usart_str_write(char* string);
      char Usart_write(char data);        // given by built-in library

      /* some register assignment commands that's not relevant to the problem */

    while(1) {
          temp_res = Adc_Read(0);
          n = sprintf(adc, "%u", temp_res);
          if (n < 0 || n >= BUFFER_SIZE)
                break;
          usart_str_write(adc);
    }
}

void usart_str_write(char* string)
{
        while (null character hasn't been reached)
        {
                Usart_write(string);
                string++;
        }
}

---

I'm not sure if I should have my usart_str_write as "void usart_str_write(char* string)" to pass adc[] by reference or by "void usart_str_write(char string[]". Does anyone have an insight on this?

Main should return int. See FAQ.
Don't put declarations inside your functions; they should go into a header file.
It's up to you, really, how to pass them. If you do char string[], the entire string is duplicated and passed to the function, but if you use (const) char* string, it will pass the address, which is faster (but also allows it to modify your string). In any case, char* and const char* is standard for all (AFAIK) C library functions.

My code runs on microcontrollers, which is correct to use void main(). I don't want to pass char string[] b/c I don't have the luxury of having the duplication take up more memory. If I pass the address, does the function call automatically dereference string to the address of adc[]?

No. If you pass a pointer, you pass a pointer, end of story. You're going to have to derference it yourself.

So are you suggesting I have something like:

Code:

---

#define BUFFER_SIZE 5

void usart_str_write(char* string);
char Usart_write(char data);        // given by built-in library

void main(){
      unsigned int temp_res;
      char adc[BUFFER_SIZE];
      char* adc_str;

      /* some register assignment commands that's not relevant to the problem */

    while(1) {
          temp_res = Adc_Read(0);
          n = sprintf(adc, "%u", temp_res);
          if (n < 0 || n >= BUFFER_SIZE)
                break;
          adc_str = &adc;
          usart_str_write(adc_str);
    }
}

void usart_str_write(char* string)
{
        while (null character hasn't been reached)
        {
                Usart_write(string);
                string++;
        }
}

---

Quote:

---

Originally Posted by stanlvw

So are you suggesting I have something like:

Code:

---

#define BUFFER_SIZE 5

void usart_str_write(const char* string);
char Usart_write(char data);        // given by built-in library

void main(){
      unsigned int temp_res;
      char adc[BUFFER_SIZE];
      char* adc_str; /* Remove */

      /* some register assignment commands that's not relevant to the problem */

    while(1) {
          temp_res = Adc_Read(0);
          n = sprintf(adc, "%u", temp_res);
          if (n < 0 || n >= BUFFER_SIZE)
                break;
          adc_str = &adc; /* Remove */
          usart_str_write(adc);
    }
}

void usart_str_write(const char* string)
{
        while (null character hasn't been reached)
        {
                Usart_write(string);
                string++;
        }
}

---

---

You don't need an extra var for getting the address.
Further, on (char) arrays, you don't need to do & to get the address. It's done automatically. So just pass adc.
Also add const to your function arguments unless you're going to modify the string (I assume you're not?).

I would not be modifying thstring in usart_str_write. Thanks for the heads up on the const for prevent unintended modifications.

No problem.

I have another problem with the compiler complaining about:

Code:

---

usart_str_write(adc[]);

---

I have tried both adc and adc[], but the compiler complained about the same message about it being an illegal expression and implicit cast of integral value to pointer. Any ideas on what's happening that's causing this complaint?

What's the code and what line does the error occour on?

you're doing pointer arithmetic with a variable that's not a pointer.

edit: ack: I thought you were doing this:

Code:

---

void usart_str_write(const char string[])

---

Code:

---

#define BUFFER_SIZE 5

void usart_str_write(const char* string);
char Usart_write(char data);        // given by built-in library

void main(){
      unsigned int temp_res;
      char adc[BUFFER_SIZE];
      char* adc_str; /* Remove */

      /* some register assignment commands that's not relevant to the problem */

    while(1) {
          temp_res = Adc_Read(0);
          n = sprintf(adc, "%u", temp_res);
          if (n < 0 || n >= BUFFER_SIZE)
                break;
          adc_str = &adc; /* Remove */
          usart_str_write(adc);
    }
}

void usart_str_write(const char* string)
{
        while (null character hasn't been reached)
        {
                Usart_write(string);
                string++;
        }
}

---

The error is on the line highlighted in red. The message is shown in http://imagebin.ca/view/89NYVWS.html.

I dunno, that should work, as it works for me.
Also,

Code:

---

Usart_write(string);

---

...should be...

Code:

---

Usart_write(*string);

---

Since it takes a char.

Elysia: It works on the compiler that you're using?

Yes, it does.
It compiles fine on VC++ 2005.

Aite. I'll post a mesage on the compiler's support forum to get insights there. Thank you.

Quote:

---

Originally Posted by Elysia

If you do char string[], the entire string is duplicated and passed to the function

---

Actually, it is also pass by reference.

Code:

---

$cat t.c
#include <stdio.h>

void f(char r[])
{
        r[4]='%';
        puts(r);

        return;
}

int main()
{
        char a[]="dlkfsgjflkgsd";

        f(a);
        puts(a);

        return 0;
}
$gcc t.c -Wall -Werror -ansi -pedantic
$./a.out
dlkf%gjflkgsd
dlkf%gjflkgsd
$

---

In C? Does references exist in C?

Quote:

---

Originally Posted by Elysia

In C? Does references exist in C?

---

I meant as a term, not an actual type. There are no references in C, but it's common to refer to pointer passing as passing "by reference". (as opposed to "by value")

Yes, I meant that you can actually pass data that can be modified w/o using pointers.
But it appears you can... Basically char[] and char* is the same, only char* needs dereference.

Quote:

---

Originally Posted by Elysia

only char* needs dereference.

---

How so? I don't think I understand this part.

Ah, nevermind. C just weirds me out.

Quote:

---

Originally Posted by Elysia

Ah, nevermind. C just weirds me out.

---

But there's no difference there between C and C++ - of course, if you pass a struct or class (such as string), rather than an array, then you would pass a COPY of the struct or class, which is why a std::string passed in C++ doesn't modify the original string unless you specify either a & or * in the function parameter [and relevant & if needed in the calling code].

--
Mats

I've learned that type[] and type* is basically the same thing, can be treated as the same thing when it comes to modifying strings, at least.
I never pass by char[], I pass by char* if such is needed, that's why it weirded me out.

I also remember that I was told that passing char[] is also by reference for arrays in my first college course on C.

Well, I suppose either is fine.

I have gotten a reply from mikroelectronika forums with a project that manages to compile properly (although I haven't tried it yet). Apparently it's a hardware problem. Passing char array as char* which is allocated in the RAM of the microcontroller through a parameter const char* which is allocated in the ROM is an illegal action in the hardware configuration.