Thread: Quick question on malloc and strings

What is strlcpy()??? Don't you mean strncpy()?

Originally Posted by officedog

cpjust - here's a link http://en.wikipedia.org/wiki/Strlcpy

I read that this would be a 'safer' function than strcpy. Apparently it adds a '\0' to the end of the string as a bonus.

Quite. It is a very good function to use.

Originally Posted by officedog

b) Don't free pointer within function

Careful. Depends.
If it is only going to be used inside the function, then you must free it.
But if you free it inside the function, its memory will be unaccessible outside and any data it contains will be destroyed. So therefore, if you plan to keep the data inside some allocated memory after the function's end, you must not free it.

Elysia - thanks for the extra explanation. I assume then that because I pass the address of the struct into the function, this is essentially acting as you suggest - a buffer/place to store the char*. I have taken the last 2 lines out as well (free(word) and word = NULL).

Indeed it is.
But I meant you to be careful with pointers you allocate. You need to take into account where the data should be used and thusly, whose responsibility it is to free the data later.

Very last question (I'll have to delete the word "quick" in the title now). My last function is one that will replace the word previously stored. Here's what I wrote - the only difference between this and the initialisation function is in blue, where I free the memory (or at least I think I free the memory) associated with the previous word.

Code:

void setWord(Word *wordStruct, const char *newWord)
{
    free(wordStruct->word);
    
     int isAllocated;
    char* word;
    int length;
    
    length = strlen(newWord) +1;
    word = malloc(length); //sizeof(char) = 1 byte
    if (word != NULL) {
        isAllocated = 1;
        strlcpy(word, newWord, length); //strlcpy adds '\0'
        
        wordStruct->word = word;
        wordStruct->length = length;
        wordStruct->isAllocated = isAllocated;
    }
    else {
        isAllocated = 0;
    }
}

Indeed, if you replace the word, you have to get rid of the old memory if you need to allocate more space.
However, you should add a check to see if there is sufficient space already in there, and if not, allocate new space and free the old.