How do you iterate through a pointer?

Question

For example:

int *start;
start = (int*)malloc(40);

If I wanted to iterate through all 40 bytes, how would I do so? I tried doing something like:

while(start != NULL){
     start++;
}

but that iterates through a massive number of values, which is much greater than 40. Thus, how do you ensure that you iterate through all 40 bytes.

Thanks for all the help.

Answer 1

There are two issues here.

A single ptr++ skips as many bytes as the type of element it points to.

Here the type is int, so it would skip 4 bytes each time (assuming a 32 bit machine since integer is 4 bytes (32 bits) there).

If you want to iterate through all 40 bytes (one byte at a time), iterate using say a char data type (or type cast your int* to char* and then increment)

The other problem is your loop termination.

There is no one putting a NULL at the end here, so your loop would keep running (and pointer advancing forward) until it runs into may be a null or goes out of your allotted memory area and crashes. The behavior is undefined.

If you allocated 40 bytes, you have to terminate at 40 bytes yourself.

Update:

Based upon a comment cum down vote to the original question, it is worth mentioning that type casting the result of malloc is not a good idea in C. The primary reason is that it could potentially tamper a failed allocation. It is a requirement in C++ though. The details can be found in the exact same question on SO. Search "casting return value of malloc"

Answer 2

First of all, you should be allocating ints correctly:

int* start = malloc( sizeof( int )*40 ) ;

Then you can use array subscripting:

for( size_t i = 0 ; i < 40 ; i++ )
{
    start[i] = 0 ;
}

or a pointer to the end of the allocated memory:

int* end = start+40 ;
int* iter = start ;

while( iter < end )
{
    *iter= 0 ;
    iter++ ;
}