How to get number of processes and threads in a C program? - c

Question

I would like to get the number of ALL running processes and ALL running threads of my Linux system. I need this information IN a C application. From the terminal, I can get his info with
$ ps -A --no-headers | wc -l
for processes and
$ ps -AL --no-headers | wc -l
for processes including tasks.
I didn't find this info in /proc.
Any suggestions?

Answer 1

The ps examples in your question don't really give you the information you're asking about: the first lists all the processes on your system, not just those spawned by a particular program, and similarly the second lists the number of threads in every process.
If you want to find information about threads spawned by a particular process, you can look in /proc under /proc/<pid>/task. For example, here is a process with a single thread:
bash-4.4$ ls /proc/15355/task/
15355
And here's one that has three threads (in addition to the main thread):
bash-4.4$ ls /proc/15295/task/
15295 15296 15297 15298
The corresponding ps -L output for that process looks like:
bash-4.4$ ps -L -p 15295
PID LWP TTY TIME CMD
15295 15295 pts/4 00:00:00 python
15295 15296 pts/4 00:00:00 python
15295 15297 pts/4 00:00:00 python
15295 15298 pts/4 00:00:00 python
Getting the number of running processes from /proc takes a little more work, since Linux only maintains information about a process' parent, rather than it's children. That means you would need to scan through /proc and find every process for which the parent is your target process...and then repeat recursively for each of those processes.
You can use something like pstree to get this information, of course, but that output isn't really designed to be machine parseable.

Answer 2

Every running process has a corresponding directory /proc/<pid>. You can use that to count the number of running processes (by counting all sub-directories of /proc that are numeric).
Inside each of these directories you can check /proc/<pid>/status to get information about the process. Specifically, the line Threads: <cnt> gives you the number of threads for that process.
Refer to man proc for more info on the /proc (pseudo) file system.

Answer 3

The simplest is to parse the output of commands using popen.
The following:
#include <assert.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void parse_output(char *buf, size_t bufsize, const char cmd[])
{
assert(buf != NULL);
assert(cmd != NULL);
FILE *fp;
// add dynamic allocation here
memset(buf, 0, bufsize);
if ((fp = popen(cmd, "r")) == NULL) {
printf("Error opening pipe!\n");
exit(-__LINE__);
}
// worst speed ever. And strlen is called twice...
while (fgets(&buf[strlen(buf)], bufsize - strlen(buf), fp) != NULL);
if(pclose(fp)) {
printf("Command not found or exited with error status\n");
exit(-__LINE__);
}
}
int main() {
char buf[256];
long num;
parse_output(buf, sizeof(buf), "ps -A --no-headers | wc -l");
if (sscanf(buf, "%ld", &num) != 1) {
exit(-__LINE__);
}
printf("Number of processes: %ld\n", num);
parse_output(buf, sizeof(buf), "ps -AL --no-headers | wc -l");
if (sscanf(buf, "%ld", &num) != 1) {
exit(-__LINE__);
}
printf("Number of processes including tasks: %ld\n", num);
}
will output on my system:
$ gcc 1.c && ./a.out
Number of processes: 241
Number of processes includeing tasks: 867