[SOLVED] Understanding the function in c program . char * allocc(){} K&R page 101

[thannara123]

trying to understand the following function.
Can I get the correct working?

[COLOR=gray][FONT=Consolas]#define[/FONT][/COLOR][COLOR=black][FONT=Consolas] ALLOCSIZE [/FONT][/COLOR][COLOR=maroon][FONT=Consolas]1000[/FONT][/COLOR][/FONT][/COLOR]
[COLOR=black]
[/COLOR][COLOR=#00008B]static[/COLOR][COLOR=black] [/COLOR][COLOR=#00008B]char[/COLOR][COLOR=black] allocbuf[/COLOR][COLOR=black][[/COLOR][COLOR=black]ALLOCSIZE[/COLOR][COLOR=black]];[/COLOR][COLOR=black]
[/COLOR][COLOR=#00008B]static[/COLOR][COLOR=black] [/COLOR][COLOR=#00008B]char[/COLOR][COLOR=black] [/COLOR][COLOR=black]*[/COLOR][COLOR=black]allocp [/COLOR][COLOR=black]=[/COLOR][COLOR=black] allocbuf[/COLOR][COLOR=black];[/COLOR][COLOR=black]
[/COLOR][COLOR=#00008B]char[/COLOR][COLOR=black] [/COLOR][COLOR=black]*[/COLOR][COLOR=black]alloc[/COLOR][COLOR=black]([/COLOR][COLOR=#2B91AF]int[/COLOR][COLOR=black] n[/COLOR][COLOR=black])[/COLOR][COLOR=black]
[/COLOR][COLOR=black]{[/COLOR][COLOR=black] 
    [/COLOR][COLOR=#00008B]if[/COLOR][COLOR=black]([/COLOR][COLOR=black]allocbuf [/COLOR][COLOR=black]+[/COLOR][COLOR=black] ALLOCSIZE [/COLOR][COLOR=black]-[/COLOR][COLOR=black] allocp [/COLOR][COLOR=black]>=[/COLOR][COLOR=black] n[/COLOR][COLOR=black])[/COLOR][COLOR=black]
    [/COLOR][COLOR=black]{[/COLOR][COLOR=black]
       allocp [/COLOR][COLOR=black]+=[/COLOR][COLOR=black] n[/COLOR][COLOR=black];[/COLOR][COLOR=black]
       [/COLOR][COLOR=#00008B]return[/COLOR][COLOR=black] allocp [/COLOR][COLOR=black]-[/COLOR][COLOR=black] n[/COLOR][COLOR=black];[/COLOR][COLOR=black]
    [/COLOR][COLOR=black]}[/COLOR][COLOR=black]
    [/COLOR][COLOR=#00008B]else[/COLOR][COLOR=black]
      [/COLOR][COLOR=#00008B]return[/COLOR][COLOR=black] [/COLOR][COLOR=maroon]0[/COLOR][COLOR=black];[/COLOR][COLOR=black] [/COLOR][COLOR=#222426][FONT=Arial][COLOR=black][FONT=Consolas]}[/FONT][/COLOR]

For more https://stackoverflow.com/questions...ram-char-allocc-kr-page-101/36245596#36245596

May i get any simple example program in microcontroler section by calloc() function .

 

[FvM]

The code is mutilated and hard to read, e.g. missing spaces. Please correct.

 

[bigdogguru]

The referenced code from The C Programming Language, 2nd edition, page 101, simply allocates n number of bytes from a predefined character array, allocbuf[] which has been initially allocated 1000 bytes of char for storage.

Essentially the routine:

char * alloc(int n)

Packs one of more smaller character arrays into the predefined:

static char allocbuf[ALLOCSIZE];

With each of the smaller character arrays accessible by the pointer returned by the routine.

Each successive call of:

char * alloc(int n)

First checks the current position of the pointer:

static char * allocp

In relation to the end of the allocbuf, to ensure there is sufficient space to allocate n bytes/char:

if(allocbuf + ALLOCSIZE - allocp >= n)

If there is sufficient space, the pointer allocp is update to the new position and a pointer is returned for the beginning of the allocated section:

       allocp += n;
       return allocp - n;

If there is NOT sufficient space to allocate n bytes/char a NULL is returned instead:

return 0;

When utilizing such a technique with a microcontroller, some thought should be given to the:

#define ALLOCSIZE 1000

A 1000 bytes of RAM maybe too large is many cases, depending of the microcontroller in question, to dedicate solely to a character array.


BigDog

 

[thannara123]

this is the full code

#define ALLOCSIZE 1000 /* size of available space */

static char allocbuf[ALLOCSIZE]; /* storage for alloc */
static char *allocp = allocbuf; /* next free position */

char *alloc(int);
void afree(char *);

int main(void)
{
    char *p;
    printf("%p\n",allocp);
    
    p=alloc(100);
    printf("%p\n",allocp);

    afree(p);
    printf("%p\n",allocp);

    return 0;
}
    
    
char *alloc(int n) /* return pointer to n characters */
{
    if( allocbuf + ALLOCSIZE - allocp >= n)
    {
        allocp += n;
        return allocp -n; /* old p */
    }
    else
        return 0;
}

void afree(char *p) /* free storage pointed to by p */
{
    if(p >= allocbuf && p < allocbuf + ALLOCSIZE)
        allocp = p;
}

And i tried to get the variable values at each time and understood the result as seen the picture below