read() function in socket programming in c

Question

I use this code for reading from socket :

int n ;
char buffer[256];
n = read(newsockfd, buffer, 255);
    if (n < 0)
    {
        error("ERROR reading from socket");
    }

if the data that must be read bigger than 255 byte (for example 1000) which change must be occured ?

I know change char buffer[1000], I need different solution .

Answer 1

Just read several times from the socket, until you got all the data you want to receive.

For example to receive 1000 bytes, it could look like this (on success read returns the number of bytes read):

int received = 0;
while (received < 1000) {
  n = read(newsockfd, buffer, 255);
  // error checking...
  // do something with the partial data in "buffer"...
  received += n;
}

Answer 2

Have you checked errno? http://linux.die.net/man/2/read

If you need more data, use a loop, no?

Answer 3

Use something like:

char buffer[256];
memset(buffer, 0, sizeof(buffer));
char* pBuf = buffer;
int bytesLeft = sizeof(buffer) - sizeof(char);

while(bytesLeft > 0)
{
    int n = read(newsockfd, pBuf, bytesLeft);
    if (n < 0)
    {
        error("ERROR reading from socket");
        break;
    }
    if(n == 0)
    {
        error("peer shutted down");
        break;
    }
    pBuf += n;
    bytesLeft -= n;
}

Answer 4

Does changing the third parameter of read to 1000 work or not? i.e.

char buffer[1001];
read(newsockfd, buffer, 1000);

If the read function does not accept a count that large, you may read it multiple times. In the loop, keep track of how much characters you have read so far and use read(newsockfd, buffer+m, 1000-m);

Answer 5

Adding to the solutions above, you can even read any number of bytes (assuming you don't have an idea beforehand how many Bytes you are expecting to be sent on the socket) by allocating some more memory to your buffer in which you are reading whenever you reach the maximum limit of the same.