CODEWITHSUNDEEP
cmemory-managementmemory-leaks
Bag Dev
Bag Dev

Reputation: 185

Free memory outside function

Is this correct, is it gonna leak memory in C? 

unsigned char * prep( int length,int args, ... )
{
    unsigned char *message = (unsigned char *) malloc(length );

    va_list listp;
    va_start( listp, args );

    int i = 0;
    int len = 0;
    unsigned char *source_message ; 
    int step = 0;
    for( i = 0 ; i < args; i++ )
    {

    source_message = va_arg( listp, unsigned char *);
    len = va_arg( listp, long);
    memcpy(message+step, source_message, (long) len);
    step+=len;

    } 
    va_end( listp );
 return message;
}

than call it and free the pointer outside

unsigned char *mess = prepare_packet_to_send(some vars here);
free(*mess);

Upvotes: 0

Views: 4065

Answers (4)

pizza
pizza

Reputation: 7630

It is fine, sometimes you have to do that, the C function strdup() does the same, you just have to follow the convention to free it after use. But I saw a bigger issue in you code, you actually allow more data to be written than the size you allocated. This is an attempt to correct this:

unsigned char * prep( int length,int args, ... )
{
 int i = 0;
 int len = 0;
 unsigned char *source_message ;
 int step = 0;
 unsigned char *message = (unsigned char *) malloc(length);
 va_list listp;
 va_start( listp, args );
 for(i = 0 ; i < args && step <= length; i++ ) {
  source_message = va_arg( listp, unsigned char *);
  len = va_arg( listp, long);
  memcpy(message+step,source_message,(step+len >length)?length-step:len);
  step+=len;
 }
 va_end( listp );
 return message;
}

use it like. 

char * p = prep(size,2,"message 1",9,"message 2",9);
if (p) {
 .... work with p....
  free(p);
}

Upvotes: 0

ShinTakezou
ShinTakezou

Reputation: 9661

You will use your own function like this:

unsigned char *p = prep(100, 1, "hello", 3);
// ...
free(p);

and then you'll have no memory leaks.

Upvotes: 1

Michael
Michael

Reputation: 55405

This is perfectly legal. Functions may return memory that is intended to be freed elsewhere. In fact, the malloc function you use has this exact contractual behavior.

I haven't carefully stepped through your code to verify there are no other issues, but returning malloc'd memory is definitely not an issue.

Upvotes: 2

Richard J. Ross III
Richard J. Ross III

Reputation: 55543

That's correct, it will leak memory. As long as you remember to free the return value of that function, you should be fine, however.

Upvotes: 2

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