backlog - the socket backlog. If this value is less than or equal to zero, then a system default value is used.
what is corresponding valueof system default value?
It is operating system dependent. And for some operating systems, it is a setting that can be configured by the system administrators.
For example, man 2 listen
for Linux says:
Since Linux 5.4, the default in this file is 4096; in earlier kernels, the default value is 128. In kernels before 2.4.25, this limit was a hard coded value,
SOMAXCONN
, with the value 128.
By contrast, on Windows the description of the backlog
parameter says:
The maximum length of the queue of pending connections. If set to
SOMAXCONN
, the underlying service provider responsible for sockets will set the backlog to a maximum reasonable value.