Thread: Pointer Help?

Is there any way to know how many bytes I allocated? Example:

Code:

char *mystring = (char *) malloc ( 100 * sizeof(char));
mystring = "1234512345";

Is there any way to get the "100" somehow?

sizeof(mystring) returns 4
strlen(mystring) returns the total number of bytes up to a null termination (10)

How do I get "100"?

You don't.

There is no standard function to obtain the allocated size. Most compilers on windows generally have a funcion named _msize from <malloc.h> that retrieves the size of the dynamic buffer. There are alternatives on other operating systems, but these are not portable.

Furthermore, just because you pass 100 to malloc doesn't mean that the CRT will necessarily allocate 100 bytes. In many cases the CRT will extend the size you passed (Usually to the closest power of 2)

Also note the above is a memory leak. You should use strcpy or some other appropriate derivative to add "1234512345" to the buffer you allocated (mystring), and at some point free() it.

Originally Posted by tabstop

You don't.

Hmm, ok. I guess I need to find another way to do this but I am having a lot of trouble.

I have two strings, string1 and string_total.

I am receiving data on string1 and then concatenating into string_total every time.

How do I know when I should realloc string_total?

Code:

while(something)
{
	string1 = read_from_file();
	
	if ( string_total is getting full ) // This is the condition I need help with
	{
		string_total = (char *) realloc ( string_total, new_total_size );
	}
	
	strncat( string_totall, string1, bytes_read );
}

if(strlen(string_total) + 1 > SOMETHING)
//Do stuff

Originally Posted by Paul22000

Hmm, ok. I guess I need to find another way to do this but I am having a lot of trouble.

I have two strings, string1 and string_total.

I am receiving data on string1 and then concatenating into string_total every time.

How do I know when I should realloc string_total?

Code:

while(something)
{
	string1 = read_from_file();
	
	if ( string_total is getting full ) // This is the condition I need help with
	{
		string_total = (char *) realloc ( string_total, new_total_size );
	}
	
	strncat( string_totall, string1, bytes_read );
}

In C, it's your job to manage memory. You must always know the size of the memory you have allocated, and how much of it you have used. You should not fear using variables to keep count with -- for sure, you'll need to know how many bytes you get back from read_from_file.

Originally Posted by Paul22000

Code:

if ( string_total is getting full ) // This is the condition I need help with

There is no straightforward way to do so other than as stated, using a counter to keep track of the memory.
The length of string1 is known completely after it has been read from the input file so in the first pass check if

Code:

100 >= strlen(string1)+1

and in subsequent passes check if

Code:

(100*counter - strlen(string_total)) >= strlen(string1)+1

If condition is true do the catenation otherwise realloc() string_total by a chunk of 100 bytes before doing the catenation.

Thanks guys. I made the counter as suggested. Just a global variable that holds the initial size I set (the "100") and checks against it after every read, and increment if necessary. Thanks again.