Thread: Realloc Fail

Hi everyone,
i have been trying to make a function that receives a string if the form of characters and dynamically allocates memory for every subsequent character, the function itself works fine, as long as i try to do it for only one variable (char * type), when i try to use twice(for two variables) it corrupts them both and gives a memory error

using visual studio 2008 or 2010

Code:

#include <stdio.h>
#include <conio.h>
#include <stdlib.h>

void string_realloc(char* str);
int main()
{
	/*dynalloc_req1*/char *str;
	                 char *str2;
    /*dynalloc_req2*/
					 
	str=(char*)malloc(2);
        str2=(char*)malloc(2);
        if(str==NULL)
		{printf("strfail");getch();return 0;}
	if(str2==NULL)
	       {printf("str2fail");getch();return 0;}				 
	printf("Enter a string to compress: \n");
	string_realloc(str);
        printf("Enter a string to compress: \n");
	string_realloc(str2);
	printf("%s",str);
	printf("%s",str2);
        free(str);
	free(str2);
	getch();

	return 0;
}

void string_realloc(char* str)
{
	int i;  //string index counter
	char c; //temp storage for input char

	for(i=0;(c=getchar())!='\n';i++)   
	//stop condition is user "enter"
	{
		str[i]=c;
		if ((str=(char*)realloc(str,i+2))==NULL)  
		  {//alloc fail msg 
		   printf("\n\nallocation failed, not enough memory");
		   free(str);
			 return;
		  }         
	}
	str[i]='\0';//close the string
}

You're going to get a problem when realloc MOVES the memory to another area of memory, because the current one cannot be expanded.

When this happens, you need to return the modified pointer back to main.

Look at realloc, it is
newpointer = realloc( oldpointer, newsize );

Now look at your function.
void string_realloc(char* str)

This needs to follow the same style, that is, return a pointer to the (possibly moved) block of memory containing your string.

From the code it seems that your realloc the pointer str inside the function. But to the str_realloc function you just send the value of str. But that wouldn't work, as your're changing the pointer 'str' inside the str_realloc function to point to a newly allocated block. You will have to look into the concept called pointer-to-pointer. Where you do something similar to follow

Code:

int main( void )
{
    char *str = malloc( sizeof *str * 10 );
    .
    .
    str_realloc( &str ); free( str );
    return 0;

}

void str_realloc( char **str  )
{
     char *temp; 
     
     if( ( temp = realloc( *str,  strlen( *str ) * 10 ) ) == NULL )
     {
         printf("realloc fail");
         return;
     }
     *str = temp;
}

Thats is just a sample code on how to use pointer-to-pointer. And also you can check how I use the realloc function. In your case, the new allocated memeory block is never been initialied to the original pointer str in main. As de-referening str in main after str_realloc is undefined.

PS: I've skipped out few error checking which i shall leave it to you to do so. And also your code indentation is very poor. You will have to start indenting your code properly, or your bound to get no much help.

ssharish

Originally Posted by ssharish2005

From the code it seems that your realloc the pointer str inside the function. But to the str_realloc function you just send the value of str. But that wouldn't work, as your're changing the pointer 'str' inside the str_realloc function to point to a newly allocated block. You will have to look into the concept called pointer-to-pointer. Where you do something similar to follow

Code:

int main( void )
{
    char *str = malloc( sizeof *str * 10 );
    .
    .
    str_realloc( &str ); free( str );
    return 0;

}

void str_realloc( char **str  )
{
     char *temp; 
     
     if( ( temp = realloc( *str,  strlen( *str ) * 10 ) ) == NULL )
     {
         print "realloc fail";
         return;
     }
     *str = temp;
}

Thats is just a sample code on how to use pointer-to-pointer. And also you can check how I use the realloc function. In your case, the new allocated memeory block is never been initialied to the original pointer str in main. As de-referening str in main after str_realloc is undefined.

PS: I've skipped out few error checking which i shall leave it to you to do so. And also your code indentation is very poor. You will start indenting your properly, or you gte no much help.

ssharish

Love your print function ssharish , that should really be part of the standard no pun intended.

>Love your print function ssharish , that should really be part of the standard no pun intended.
haha my typo. Yeah we could perhaps macro it!!!

ssharish