Thread: How to return a character array from a function?

what is the correct way of declaring a function that return an array of characters?

Also how do return array of characters from within a local function?
I tried use a pointer to the variable that was declared in the location function, sometimes, it gave incorrect result. maybe because of the lifetime of the local variable.

Code:

char *f ( char somearray[])
{
}

You can return an array with a pointer as long as it's not a local variable. If it is, you can make it static and it will be ok because the lifetime is like a global. You can also dynamically allocate an array and pass it back with a pointer, but then you'll have to free it later.

You don't.
Pass the char* buffer & size of the buffer to the function and write to those parameters.

Code:

char* func( char* buf, int size )
{
    char str[] = "Hello World";

    if ( size > strlen( str ) )
    {
        return strncpy( buf, str, (size - 1) );
    }

    return NULL; /* Error!  buf isn't big enough. */
}

Local arrays you create in a function disappear when the function returns. You'd have to allocate it with malloc() if you want a local variable to survive a function return, but then someone else has to delete it with free(). The method I showed above is the standard safer way to do it.
You can return a char* pointer like above, or better might be to return an int (0 for success, otherwise the buffer size required)...

Originally Posted by robwhit

You can return an array with a pointer as long as it's not a local variable. If it is, you can make it static and it will be ok because the lifetime is like a global. You can also dynamically allocate an array and pass it back with a pointer, but then you'll have to free it later.

Oops, forgot about the static option. But then of course your code wouldn't be multithread safe...

[QUOTE=cpjust;695015]You don't.
Pass the char* buffer & size of the buffer to the function and write to those parameters.

Code:

char* func( char* buf, int size )
{
    char str[] = "Hello World";

    if ( size > strlen( str ) )
    {
        return strncpy( buf, str, (size - 1) );
    }

    return NULL; /* Error!  buf isn't big enough. */
}

it is okay to do like this, but just what if we don want make changes to pointer which is passed as the function argument (buf).

I think using malloc() and add one more arqument to the function prototype.
so later on I can free the space claimed in the function.
what do you think? is it good? m not so sure at all.

Originally Posted by whichet

what if we don want make changes to pointer which is passed as the function argument (buf).

then make a copy of it.

Originally Posted by whichet

I think using malloc() and add one more arqument to the function prototype.
so later on I can free the space claimed in the function.
what do you think? is it good? m not so sure at all.

yeah, like that.

Originally Posted by whichet

I think using malloc() and add one more arqument to the function prototype.
so later on I can free the space claimed in the function.
what do you think? is it good? m not so sure at all.

That's like using a public toilet without flushing. You're forcing other people to clean up your mess.

If the person calling the function already has an array that they can use for the buffer, they can pass that to the function and then there's no call to malloc() and no need to call free() since the array is on the stack.
Or if they need the buffer to live longer, they can do the malloc() themselves and obviously they'd need to free it somewhere down the road too; but at least they have the option of using malloc() or using an array.