Memory of first element in dynamic char array

Question

My array is defined like this

int buffSize = 80;
char* buff = (char*) malloc(sizeof(char) * buffSize);

First, I thought &buff should be the same as &buff[0], but apparently, it isn't! Did I miss something here? This statement prints two different values for those two:

    printf("COMPARE: buff=%u, buff[0]=%u\n", &buff, &buff[0]);

Second, the reason I asked is because I'm trying to create a big buffer and "manually" divide it up to use with getline. Basically, I'd like to do something like this:

int byte_read, total_read = 0;
do
{
   read = getline(&buff[totalRead], buffSize, inFile); //where inFile is just a file handler
  totalRead += read;
}
while (byte_read > 0);

You probably wanted buffsize - total_read in the getline call, otherwise you can overflow. — Weak to Enuma Elish
– Weak to Enuma Elish, Commented Sep 14, 2015 at 2:23
You seem to be mentally confusing a pointer with the thing it is pointing to. Don't do this; they are two completely separate entities. — M.M
– M.M, Commented Sep 14, 2015 at 2:33
"I'm trying to create a big buffer and manually divide it up to use with getline." - that doesn't make any sense. How is that different to just reading the whole file into a buffer? — M.M
– M.M, Commented Sep 14, 2015 at 2:37

juanchopanza · Accepted Answer · 2015-09-14 02:19:47Z

4

buff is a pointer, and &buff is the address of that pointer. On the other hand, &buff[0] is the address of the location the pointer points to, and should have the same value as buff.

In summary, expect buff and &buff[0] to have the same value.

answered Sep 14, 2015 at 2:19

juanchopanza

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7 Comments

0x56794E Over a year ago

So because getline takes &buff, which is the address of the pointer to the char array, presumably, there's no clean way to do what I want as described above?

juanchopanza Over a year ago

@abcXYZ getline isn't taking &buff. I think you didn't understand my answer.

0x56794E Over a year ago

This page said getline takes the address of the pointer, i.e., &buff: crasseux.com/books/ctutorial/getline.html I must've misunderstood something?

M.M Over a year ago

@abcXYZ yes, it takes &buff, not &buff[totalRead]. This is because it may make the pointer point elsewhere (to a new memory allocation). It has no "append" functionality.

0x56794E Over a year ago

@M.M Right. It does take &buff, but is there anyway I can specify where it starts writing the value to in the buffer I pass?

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