In linux, is there a built-in C library function for getting the CPU load of the machine? Presumably I could write my own function for opening and parsing a file in /proc, but it seems like there ought to be a better way.
My understanding is that parsing the contains of /proc is the official interface for that kind of thing (there are a number of files there which are really meant to be parsed before presented to the user).
The preferred method of getting information about CPU load on linux is to read from /proc/stat, /proc/loadavg and /proc/uptime. All the normal linux utilities like top use this method.
from the proc (5) man page:
/proc/loadavg
The first three fields in this file are load average figures
giving the number of jobs in the run queue (state R) or waiting
for disk I/O (state D) averaged over 1, 5, and 15 minutes. They
are the same as the load average numbers given by uptime(1) and
other programs. The fourth field consists of two numbers sepaâ
rated by a slash (/). The first of these is the number of curâ
rently executing kernel scheduling entities (processes,
threads); this will be less than or equal to the number of CPUs.
The value after the slash is the number of kernel scheduling
entities that currently exist on the system. The fifth field is
the PID of the process that was most recently created on the
system.
If you really want a c interface use getloadavg(), which also works in unixes without /proc.
It has a man page with all the details.
"Load average" may not be very useful. We find it to be of limited use, as it doesn't actually tell you how much CPU is being used, only the average number of tasks "ready to run". "Ready to run" is somewhat subjective, but not very helpful as it often includes processes waiting for IO.
On busy systems, we see load average of 20+ on machines with only 8 cores, and still the CPUs are relatively idle.
If you want to see what CPU is in use, have a look at the various files in /proc