Doubt about freeing memory

This is a discussion on Doubt about freeing memory within the C Programming forums, part of the General Programming Boards category; Hello. I'm using this function to concatenate two strings, and return the resulting string. Code: char * concatenate (char* str1, ...

  1. #1
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    Doubt about freeing memory

    Hello. I'm using this function to concatenate two strings, and return the resulting string.

    Code:
    char * concatenate (char* str1, char* str2)
    {
    	char *result;
    	if ( str2 != NULL )
    	{
    		result = malloc(strlen(str1) + strlen(str2) + 1);
    		if ( result != NULL )
    		{
    			strcpy(result, str1);
    			strcat(result, str2);
    		}
    	}
    	return result;
    }
    My question, Is this function leaking memory?
    If it does, is there another way to do it? I just ran out of ideas.

    Thanks!!!

  2. #2
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    That function in itself doesn't leak memory, but if you don't eventually free the pointer that came back from the function, then you are leaking.

    [And in English you don't say "Doubt about ...", but "Question about ..."]

    --
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    Compilers can produce warnings - make the compiler programmers happy: Use them!
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    Thanks a lot. That's what I thought.

  4. #4
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    "I Doubt..." means "I don't believe..." which is a statement, not a question.

  5. #5
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    Quote Originally Posted by cpjust View Post
    "I Doubt..." means "I don't believe..." which is a statement, not a question.
    Not necessarily.

    "I am in doubt about how I would go about.. " is a perfectly valid sentence.

  6. #6
    Jack of many languages Dino's Avatar
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    Quote Originally Posted by cpjust View Post
    "I Doubt..." means "I don't believe..." which is a statement, not a question.
    No doubt!

    I've a question about matsp's remark... HE states that the pointer returned from concatenate() will be left on the stack. Should main, therefore, be coded such as:

    Code:
    int main(void) 
      char * newstring ; 
      newstring = concatenate("Hello ", "World") ; 
      printf("%s\b",newstring) ; 
      free(newstring) ;  
    }
    ??
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    Jack of many languages Dino's Avatar
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    More thoughts - that would free the new malloc'ed storage - but what about freeing the returned pointer?
    Last edited by Dino; 02-04-2008 at 05:23 PM.
    Mac and Windows cross platform programmer. Ruby lover.

    Quote of the Day
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    Quote Originally Posted by Todd Burch View Post
    More thoghts - that would free the new malloc'ed storage - but what about freeing the returned pointer?
    That's exactly what you should free. Obviously, if you do something like this:
    Code:
    char *a, *b, *c;
    a = malloc(10);
    strcpy(a, "Something");
    b = malloc(10);
    strcpy(b, "Otherstr");
    c = concatenate(a, b);
    ...
    free(c);
    would leave a and b allocated but not freed.

    Every malloc should have a matching free somewhere in the code - the number of malloc() and free() occurrences in the source code doesn't have to match, but the calls for malloc() should have a matching call to free() somewhere.

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    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

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    FWIW, a pointer to pointer might help you keep track of freeing memory where it is allocated, even if you previously allocated something.
    Code:
    #include <stdlib.h>
    #include <string.h>
    #include <stddef.h> /** for NULL and size_t **/
    
    /** Concatenate string a to string b and store the result **/
    char * concat ( char ** result, const char * a, const char * b )
    {
        char * more = NULL;
        if ( a && b )
        {
            size_t alen = strlen( a );
            size_t blen = strlen( b );
    
            more = realloc( *result, alen + blen + 1 );
            if ( more )
            {
                strcpy( more, a );
                strcat( more, b );
                *result = more;
            }
        }
        return more;
    }
    
    int main ( void )
    {
        char * p = NULL;
       
        if ( concat( &p, "Hello, ", "world." ) )
        {
            puts( p );
            free( p );
        }
        return 0;
    }

  10. #10
    Aia
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    Quote Originally Posted by matsp View Post
    [And in English you don't say "Doubt about ...", but "Question about ..."]

    --
    Mats
    Doubt is synonym of uncertainty.
    If it is grammatically correct to say: Uncertainty about freeing memory, as a tittle, it is correct to write doubt about freeing memory, even when most people confuse doubt with question.
    Last edited by Aia; 02-04-2008 at 05:32 PM.
    When the eagles are silent, the parrots begin to jabber. ~Winston Churchill

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