CODEWITHSUNDEEP
cmallocfree
Mathias
Mathias

Reputation: 41

double free or corruption (top) in IF section of function

I've written the follow function: 

/*!
 * @brief Checks if a string contains a certain keyword
 * @param char *chkstring - String to search in
 * @param char *keyword - String to search for
 * @return int - 1 if found, 0 if not found
*/

int check_string(char *chkstring, char *keyword)
{
    char *buffer;
    char *buffer2;
    buffer = malloc(256);
    buffer2 = malloc(256);
    strcpy(buffer2,chkstring);

    if((buffer = strstr(buffer2,keyword)) != NULL)      // Check for first appearance of keyword in chkstring
    {
        //free(buffer);   // <- Problem sits here                                   
        //free(buffer2);  //    and/or here 
        return 1;                                       // if something is found, return 1
    }
    else
    {
        free(buffer);                                   // else return 0   
        free(buffer2);
        return 0;
    }
}

If I run it with the problematic part uncommented I get an

double free or corruption (top)

error. Why is this? Shouldn't the memory be freed in the part the function returns? Or did I messed up the if parameter and both instructions are used - which I wouldn't hope, because the code works like intended otherwise.

Thanks for your help to understand this matter!

Upvotes: 0

Views: 117

Answers (1)

r-sniper
r-sniper

Reputation: 1493

Check strstr's return value.

Return Value

A pointer to the first occurrence in str1 of the entire sequence of characters specified in str2, or a null pointer if the sequence is not present in str1

This means that if str2 is found in str1 than it will return the address of the that location.

And when you free(buffer) you are actually freeing str1 instead of the memory allocated earlier.

Also you don't need buffer = malloc(256);

EDIT: As pointed out by others, you dont need any buffers

int check_string(char *chkstring, char *keyword)
{
    //No need for buffers. Simply check in original string and return accordingly

    if(strstr(chkstring,keyword) != NULL)      // Check for first appearance of keyword in chkstring
    {
        return 1;                                       // if something is found, return 1
    }
    else
    {
        return 0;
    }
}

Similar thing can be achieved by just 1 line

return (strstr(chkstring,keyword) != NULL)
//But then why do you need function if you only want if string exists or not.

Upvotes: 3

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