CODEWITHSUNDEEP
cpointers
BugRepairMan
BugRepairMan

Reputation: 221

Why does the argument use an address of a pointer here?

I found an string buffer implemented in C here (https://sites.google.com/site/rickcreamer/Home/cc/c-implementation-of-stringbuffer-functionality). There is a function called dispose(), one of whose argument is an address of a pointer:

void dispose( SB **sb, int freeStrings ) {
    if ( !sb || !*sb || !(*sb)->ptrList )
        return; /* TODO: Decide if should abort here or take other action */
    if ( freeStrings ) {
        for ( int i = 0; i < (*sb)->count; ++i )
            free( (*sb)->ptrList[ i ] );
    }
    free( (*sb)->ptrList );
    free( *sb );
    *sb = 0; /* Set value of pointer to zero */
}

I cannot understand why its first argument is the address of the pointer, rather than the pointer. What I would convert the function to is: 

void dispose( SB *sb, int freeStrings ) {
    if ( !sb || !sb->ptrList )
        return; /* TODO: Decide if should abort here or take other action */
    if ( freeStrings ) {
        for ( int i = 0; i < sb->count; ++i )
            free( sb->ptrList[ i ] );
    }
    free( sb->ptrList );
    free( sb );
    sb = 0; /* Set value of pointer to zero */
}

Do I miss something here?

Upvotes: 1

Views: 62

Answers (3)

Jova
Jova

Reputation: 1

Pointer is variable type just like int, float, etc... So, if you want to change it's value in a function, you have to pass a reference to it as an argument. That is why you have **sb as parameter instead of *sb.

Upvotes: 0

Kirill Bulygin
Kirill Bulygin

Reputation: 3836

In your example, sb = 0; /* Set value of pointer to zero */ actually sets the value of the local variable sb to zero. That's the reason.

Upvotes: 0

Iharob Al Asimi
Iharob Al Asimi

Reputation: 53046

Because this prevents dangling pointers. Although it should be

*sb = NULL;

instead.

This way, after calling dispose(&sb) you can check if sb == NULL prior to dereferencing it and prevent dereferencing a pointer that has been freed.

And in your case, sb = 0 has no effect because you never use sb after that.

Also, being freeStrings a paramter I would argue that this is a flaw design. If the author wanted to have dual free() behavior which is already bad, they should have had a freeStrings member in the structure and then there would be a lot of control over whether the strings were owned by the structure or not.

Upvotes: 2

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