You cannot create a process in linux that cannot be killed. The root user (uid=0) can send a signal to a process, and there are two signals which cannot be caught, SIGKILL=9, SIGSTOP=19. And other signals (when uncaught) can also result in process termination.
You may want a more general daemonize function, where you can specify a name for your program/daemon, and a path to run your program (perhaps "/" or "/tmp"). You may also want to provide file(s) for stderr and stdout (and possibly a control path using stdin).
Here are the necessary includes:
#include <stdio.h> //printf(3)
#include <stdlib.h> //exit(3)
#include <unistd.h> //fork(3), chdir(3), sysconf(3)
#include <signal.h> //signal(3)
#include <sys/stat.h> //umask(3)
#include <syslog.h> //syslog(3), openlog(3), closelog(3)
And here is a more general function,
int
daemonize(char* name, char* path, char* outfile, char* errfile, char* infile )
{
if(!path) { path="/"; }
if(!name) { name="medaemon"; }
if(!infile) { infile="/dev/null"; }
if(!outfile) { outfile="/dev/null"; }
if(!errfile) { errfile="/dev/null"; }
//printf("%s %s %s %s\n",name,path,outfile,infile);
pid_t child;
//fork, detach from process group leader
if( (child=fork())<0 ) { //failed fork
fprintf(stderr,"error: failed fork\n");
exit(EXIT_FAILURE);
}
if (child>0) { //parent
exit(EXIT_SUCCESS);
}
if( setsid()<0 ) { //failed to become session leader
fprintf(stderr,"error: failed setsid\n");
exit(EXIT_FAILURE);
}
//catch/ignore signals
signal(SIGCHLD,SIG_IGN);
signal(SIGHUP,SIG_IGN);
//fork second time
if ( (child=fork())<0) { //failed fork
fprintf(stderr,"error: failed fork\n");
exit(EXIT_FAILURE);
}
if( child>0 ) { //parent
exit(EXIT_SUCCESS);
}
//new file permissions
umask(0);
//change to path directory
chdir(path);
//Close all open file descriptors
int fd;
for( fd=sysconf(_SC_OPEN_MAX); fd>0; --fd )
{
close(fd);
}
//reopen stdin, stdout, stderr
stdin=fopen(infile,"r"); //fd=0
stdout=fopen(outfile,"w+"); //fd=1
stderr=fopen(errfile,"w+"); //fd=2
//open syslog
openlog(name,LOG_PID,LOG_DAEMON);
return(0);
}
Here is a sample program, which becomes a daemon, hangs around, and then leaves.
int
main()
{
int res;
int ttl=120;
int delay=5;
if( (res=daemonize("mydaemon","/tmp",NULL,NULL,NULL)) != 0 ) {
fprintf(stderr,"error: daemonize failed\n");
exit(EXIT_FAILURE);
}
while( ttl>0 ) {
//daemon code here
syslog(LOG_NOTICE,"daemon ttl %d",ttl);
sleep(delay);
ttl-=delay;
}
syslog(LOG_NOTICE,"daemon ttl expired");
closelog();
return(EXIT_SUCCESS);
}
Note that SIG_IGN indicates to catch and ignore the signal. You could build a signal handler that can log signal receipt, and set flags (such as a flag to indicate graceful shutdown).
