Programming (and related) – my random thoughts

fun fact about TCP

2020-03-18 2021-01-20 omik

There’s one less-than-expected thing about TCP: even if you provide a small buffer to recv() and the other side sends exactly the same amount of data in the send() call, there’s no guarantee that the data is received in one call.

Take for example Python:

# sock is a socket.socket(), already connected
data = socket.recv(4)
length = struct.unpack('=l', data)[0]  # this might fail!

There are two ways to mitigate it:

really portable way: receive in a loop, until all data are received – but beware of recv() returning b'' – this means there’s an error in the communication
Linux/Unix way: use MSG_WAITALL flag to force recv() to block until whole buffer is filled

convert glob-style pattern to regular expression in python

2017-10-03 2021-01-20 omik

Python’s fnmatch library makes it easy to convert human-readable glob pattern to re.match()-compatible regexps:

% python3
>>> import fnmatch
>>> fnmatch.translate('dupa.[ch]pp.*')
'dupa\.[ch]pp\..*\Z(?ms)'
>>>

split string in python

2017-02-23 2021-01-20 omik

Python splits string via str.split() and str.rsplit() as well, the latter of which starts splitting from the end of the string.
This can be useful when using the second parameter, maxsplit.

For example:

>>> "string with multiple spaces".split(" ", maxsplit=1)
['string', 'with multiple spaces']

This parameter can be used e.g. to split command and arguments apart.

find DNS servers in nmcli

2017-02-22 2021-01-20 omik

NetworkManager usually puts “127.0.1.1” in /etc/resolv.conf. To get currently used DNS servers, type:

% nmcli d show | grep DNS
IP4.DNS[1]:                             10.1.2.3
IP4.DNS[2]:                             10.2.3.4

print strace summary

2016-04-29 2021-01-20 omik

To summarise syscalls by time or count, use strace -c. To include child processed, add -f switch. For example:

λ strace -cf git status
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ----------------
100.00    0.000008           0       247        24 lstat
  0.00    0.000000           0        48           read
  0.00    0.000000           0         3           write
                     ... list goes on ...

Platform