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cpointersallocation

Address arithmetic using a static buffer

#define ALLOCSZE 1000  


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

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

Is size of allocbuf equal to size of char in the condition ( if(allocbuf+ALLOCSZE-allocp>=n))

Solution

  • sizeof(allocbuf) is equal to the size of the array which is 1000.

    When allocbuf is used in the expression allocbuf+ALLOCSZE-allocp>=n it will decay to a char pointer, thus the pointer arithmetic will be correct.

    When you run out of memory, your function will return 0.

    I suggest that you change the return value to NULL and the type of function argument to size_t. And don't forget that the memory you get from your function alloc() can only be used for type char.