Linux Daemon not working

Question

I have created a daemon for linux in c++, however, the child process does not seem to be doing anything. Once it reaches the if(pid > 0) statement, everything seems to stop. The code for the Daemon.Start() is as follows:

//Process ID and Session ID
pid_t pid,sid;

//Fork off the Parent Process
pid = fork();
if(pid < 0)
    exit(EXIT_FAILURE);
//If PID is good, then exit the Parent Process
if(pid > 0)
    exit(EXIT_SUCCESS);

//Change the file mode mask
umask(0);

//Create a new SID for the Child Process
sid = setsid();
if(sid < 0)
{
    exit(EXIT_FAILURE);
}

//Change the current working directory
if((chdir("/")) < 0)
{
    //Log the failure
    exit(EXIT_FAILURE);
}

//Close out the standard file descriptors
close(STDIN_FILENO);
close(STDOUT_FILENO);
close(STDERR_FILENO);

//The main loop.
Globals::LogError("Service started.");
while(true)
{
    //The Service task
    Globals::LogError("Service working.");
    if(!SystemConfiguration::IsFirstRun() && !SystemConfiguration::GetMediaUpdateReady())
    {
        SyncServer();
    }
    sleep(SystemConfiguration::GetServerConnectionFrequency()); //Wait 30 seconds

}

exit(EXIT_SUCCESS);

Any help would be great! :)

Use a library or a script to do this sort of thing, no need to reinvent this wheel. — John Zwinck
– John Zwinck, Commented Oct 10, 2014 at 9:44
Just put a fprintf on stderr to discover where the child process exits. — Claudio
– Claudio, Commented Oct 10, 2014 at 9:48

Géza Török · Accepted Answer · 2014-10-10 10:10:13Z

1

I'm pretty sure your child process dies within either the sid < 0 or the chdir("/") < 0 if statement. Write to stderr in these cases before exit to reveal what the problem is:

//Create a new SID for the Child Process
sid = setsid();
if(sid < 0)
{
    fprintf(stderr,"Failed to create SID: %s\n",strerror(errno));
    exit(EXIT_FAILURE);
}

//Change the current working directory
int chdir_rv = chdir("/");
if(chdir_rv < 0)
{
    fprintf(stderr,"Failed to chdir: %s\n",strerror(errno));
    exit(EXIT_FAILURE);
}

You need to include <errno.h> and <string.h> in order to have errno and strerror defined (respectively).

Regards

edited Oct 10, 2014 at 10:10

answered Oct 10, 2014 at 9:50

Géza Török

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3 Comments

Géza Török Over a year ago

scary story... what happens if you comment out everything related to fork (the call itself and the checks concerning pid variable)?

GenericMadman Over a year ago

"Failed to create SID: -1" is output.

Géza Török Over a year ago

I guess we found the problem. Check errno for a more detailed description of the error. If you wait a second I will extend my answer to cover detailed error reporting.

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