Continuously monitor linux console logs using C/C++ application - linux

Question

I have one third party library application which runs continuously and generates console print when some event occurs.
I want to take some action when some specific event occurs so I need to monitor console prints continuously to trigget my action.
Is it possible to write application which can continuously monitor string dumper on console(stdout) and do processing when one line is detected.
I have tried to use 'popen' function but it keeps waiting until library application stops execution.
Here is my sample code using open
#include <stdio.h>
int main()
{
FILE *fd = NULL;
char buf[512] = {0};
fd = popen ("./monitor","r");
while (fgets (buf, 512, fd) != NULL)
{
printf ("__FILE__ : message : %s\n",buf);
}
printf ("EOF detected!\n");
return 0;
}
Can anyone please let me know proper way of monitoring console logs and take action.
Thanks in advance.
Pratik

Answer 1

Here is an example piece o code I 've written recently that reads from stdin and prints to stdout .
void echo(int bufferSize) {
// Disable output buffering.
setbuf(stdout, NULL);
char buffer[bufferSize];
while (fgets(buffer, sizeof(buffer), stdin)) {
printf("%s", buffer);
}
}
As I understand you have a similar issue as I had initially getting delayed output because I didn't use:
setbuf(stdout, NULL);
You can also read from stdin(that's what my example code does ) just pipe your command to your c code or if you just want to filter output pipe it to grep. If it's a standardized syslog log you could also use tail on the log file:
tail -f <logfile>| <your c prgoramme>
or
for just filering
tail -f <logfile>|grep "<your string here>"
or if without log file pipe stdout logs this way:
<your app>|<your c prgoramme>
or
<your app>| grep "<your string here>"

Answer 2

3rd party program simulated by a shell script that writes to stdout
#!/bin/bash
while true; do
echo "something"
sleep 2
done
You want to write something like this to capture the output from the 3rd party program and then act on the information:
#!/bin/bash
while read line; do
if [[ $line == "something" ]]; then
echo "do action here"
fi
done
Then combine them with a pipe operator:
./dosomething.sh | act.sh