Concatenating char buffer into a string

Question

In the following snippet, I receive the data until I have completely received all the data from the socket client. I keep on storing the data in a char buffer of size 300.

ssize_t b;
char buffer[300]
while((b = recv(socket_fd,buffer,sizeof(buffer))) > 0) {
   // keep on receiving the data

}

But the problem is on each iteration, buffer gets refilled and old values are lost. How do I concatenate the buffer values into one, so that at the end of the loop I get the message as one complete string?

Answer 1

Rather than passing buffer to recv() , pass the address of next element of buffer to be assigned and accumulate an index, adjusting the size argument of recv() to account for the data already written:

char buffer[300] ;
ssize_t buffer_index = 0 ;
ssize_t chunk = 0 ;

do 
{
   chunk = recv( socket_fd, 
                 &buffer[buffer_index], 
                 sizeof(buffer) - buffer_index - 1 ) ;

   if( chunk > 0 )
   {
       buffer_index += chunk ;
       buffer[buffer_index] = '\0' ;
   }

} while( buffer_index < sizeof(buffer) - 1 && 
         chunk > 0 ) ;

If rather then simply filling the buffer or timing out you need to terminate on a delimiter such as \n then you will need to read and inspect one character at a time:

char buffer[300] ;
ssize_t buffer_index = 0 ;
ssize_t status = 0 ;

do 
{
   status = recv( socket_fd, 
                  &buffer[buffer_index], 
                  1 ) ;

   if( status > 0 )
   {
       buffer_index++ ;
       buffer[buffer_index] = '\0' ;
   }

} while( ch != '\n' && 
         buffer_index < sizeof(buffer) - 1 && 
         status > 0 ) ;

A more complete example of a socket readline() function can be found at http://man7.org/tlpi/code/online/dist/sockets/read_line.c.html for example.

Answer 2

You can do this something like the following. I assume that buffer does not contain a string.

ssize_t b;
char buffer[300]
size_t n = 1;
char *s = calloc( 1, sizeof( char ) );

while((b = recv(socket_fd,buffer,sizeof(buffer))) > 0) {

    char *tmp = realloc( s, b + n );

    if ( !tmp )
    {
        // report an error
        break;
    }

    s = tmp;
    memcpy( s + n - 1, buffer, b );
    n += b;
    s[n - 1] = '\0';  
}