I could not find any info on how to calculate checksum in my C
program, I am specifically interested in 'long' checksum, such as
output of sum -l file_name.

Presently I just have

if((pfp = popen("sum -l file_name_here", "r")) != (FILE *)NULL) etc...

but I would like to do it within my process, as over time I had some
instances of running out of processes.