CODEWITHSUNDEEP
clinuxping
lppier
lppier

Reputation: 1987

Calling ping using popen in C

I have the following code in C code in CentOS linux. This code is called every 5 seconds in a thread. The code works fine if I disable pclose(fp), but I know I am supposed to do it. Can someone explain why it hangs at pclose(fp) when I enable it? Many thanks. 

FILE* fp;
int status;
char path[255];

fp = popen("ping -c 3 -W 3 200.0.0.51", "r");
sleep(3); 
int i=0; 
while (i < 255) {
    path[i] = fgetc(fp); 
    i++;
}

char* str;
str = strstr(path, "100% packet loss");

if (str!=NULL) {
    //pclose(fp); // <--- hangs here if I enable
    OS_Reboot(); 
}

Upvotes: 3

Views: 2359

Answers (3)

Volker Stolz
Volker Stolz

Reputation: 7402

The behaviour you see should only happen if ping does not terminate, which it should in your case after sending three packets because of the -c 3.

You should maybe add -n to the invocation to avoid getting stuck on DNS lookups in case your connectivity goes down?

Upvotes: 2

Matt
Matt

Reputation: 744

The posted code works fine for me with pclose enabled (I also removed the branch to ensure it would be called). Note that "The pclose() function waits for the associated process to terminate and returns the exit status of the command as returned by wait4(2)" (from the docs) so it could be that "ping -c 3 -W 3 200.0.0.51" is actually hanging on your machine (not sure why it would though).

Upvotes: 2

Maurice
Maurice

Reputation: 610

The problem seems to be that pclose() only returns when the process created from popen is terminated, but ping keeps looping forever.

In any case, pclose() shall not return before the child process created by popen() has terminated

A solution would be to fork the process yourself and create pipes for the stdin and stdout that are returned to your process. A possible implementation of this shown here by another stackoverflow member.

Upvotes: 2

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