Programmatically read all the processes status from /proc - c

Question

I want to save all of the running processes' status from the /proc folder in a file. After reading some questions and answers here I think I should use the pstatus struct to determine which fields I want to save (correct me if I'm wrong?), but I don't know how I can efficiently loop through all of the running processes.

Answer 1

In Linux process status is saved in /proc/PID/status pseudo-file and represented in textual form (other OS have completely different structure of their procfs):
$ grep State /proc/self/status
State: R (running)
So you need a "parser" for that file:
void print_status(long tgid) {
char path[40], line[100], *p;
FILE* statusf;
snprintf(path, 40, "/proc/%ld/status", tgid);
statusf = fopen(path, "r");
if(!statusf)
return;
while(fgets(line, 100, statusf)) {
if(strncmp(line, "State:", 6) != 0)
continue;
// Ignore "State:" and whitespace
p = line + 7;
while(isspace(*p)) ++p;
printf("%6d %s", tgid, p);
break;
}
fclose(statusf);
}
To read all processes you have to use opendir()/readdir()/closedir() and open only directories that have numerical characters (other are sysctl variables, etc.):
DIR* proc = opendir("/proc");
struct dirent* ent;
long tgid;
if(proc == NULL) {
perror("opendir(/proc)");
return 1;
}
while(ent = readdir(proc)) {
if(!isdigit(*ent->d_name))
continue;
tgid = strtol(ent->d_name, NULL, 10);
print_status(tgid);
}
closedir(proc);
Alternatively, you may use procps tools which already implemented it.

Answer 2

This snippet below invokes two C programs that do just that:
find /proc -maxdepth 2 -wholename '/proc/[0-9]*/status' | xargs cat

Answer 3

It's an old question, but still definitely a relevant one.
If you don't want to mess with parsing procfs yourself, you should definitely check out pfs. It a library for parsing most of procfs written in C++. (Disclaimer: I'm the author of the library)
On a Linux system, every task (process or thread) has an entry under the /procfs directory.
When enumerating the directories under /procfs you'll get a directory for each running processes.
Under each directory, you can find two files: stat and status, that include the current process status.
The possible statuses are described (almost correctly) under man proc(5):
(3) state %c
One of the following characters, indicating process state:
R Running
S Sleeping in an interruptible wait
D Waiting in uninterruptible disk sleep
Z Zombie
T Stopped (on a signal) or (before Linux 2.6.33) trace stopped
t Tracing stop (Linux 2.6.33 onward)
W Paging (only before Linux 2.6.0)
X Dead (from Linux 2.6.0 onward)
x Dead (Linux 2.6.33 to 3.13 only)
K Wakekill (Linux 2.6.33 to 3.13 only)
W Waking (Linux 2.6.33 to 3.13 only)
P Parked (Linux 3.9 to 3.13 only)
This is almost correct, because there's an additional possible value that is missing from this list:
I Idle
If you want to get the value from stat, the simplest way to go is probably use as fopen & fscanf. The format is intimately described under man proc(5) as well. Note: Just be carefull with the comm value format. They say it's %s, but it's actually much more complicated, since it might include spaces or any other character, it might screw up your parser (one more reason to use a mature parsing library).
If you want to get the value from status, you should probably open the file using std::ifstream or something like that and use std::getline till the lines starts with Status:, and then extract the value you want.