check space before tcp_get_buf call

 check space before tcp_get_buf call

I use the RTX TCP/IP stack with limited RAM. So the memory pool for the stack is very limited. To protect my application form running out of memory (a call to sys_error()) I will now the available size for my transmit buffer. It is also okey if i have a different function for tcp_get_buf() who return NULL or something like that when there is no space available. Anyone a suggestion how to solve this problem?