Beginning C From Beginner to Pro by Unknown

Author:Unknown
Language: eng
Format: epub

■ Tip To see all the output for large numbers of primes under Microsoft Windows, you will need to increase the buffer size for the output window.

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CHAPTER 7 ■ PoinTERs

How It Works

With this example, you can enter the upper limit for the prime numbers you want the program to generate. The pPrimes pointer variable refers to a memory area that will be used to store the prime numbers as they’re calculated. Memory for capacity primes is allocated initially in the program: size_t capacity = 10;

pPrimes = calloc(capacity, sizeof(unsigned long long));

The second statement allocates space for ten values of type unsigned long long. The amount of memory will be extended as necessary. There’s the standard check for a NULL return value from calloc() following these two statements.

The process for finding primes is essentially the same as in Program 7.11, the only difference being the while loop control that continues the loop as long as the trial value is less than or equal to the limit value that was entered.

When a prime is found, it’s necessary to check whether there is sufficient memory to accommodate it: if(count == capacity)

{ // We need more memory

capacity += CAP_INCR;

pTemp = realloc(pPrimes, capacity*sizeof(unsigned long long));

if(!pTemp)

{

printf("Unfortunately memory reallocation failed.
");

free(pPrimes);

return 2;

}

pPrimes = pTemp;

}

When count, the number of primes found, reaches capacity, the number of primes for which there is memory allocated, more memory must be acquired. The capacity is increased by the amount specified by the value of the symbol CAP_INCR, so each time all the currently allocated memory is occupied, there is provision for an additional ten primes. i chose the increment value as a low number to ensure that extending the memory would occur relatively frequently to illustrate the mechanism, but it is generally inefficient to allocate memory in such small amounts. The realloc() function makes the new memory available and preserves the original contents of the old memory in the new memory.

The pointer that is returned by realloc() is stored in pTemp. This avoids overwriting the address in pPrimes when the allocation fails. in general, you are likely to want to access the original memory that was allocated when realloc() fails, possibly to make use of the data it contains and also to release the memory. The rest of the program proceeds as the previous example, so i won’t discuss it further.

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