How to execute a bash-skript from c without STDIN and STDOUT - linux

Question

I'm in the proccess of writing a deamon, which runs a shell-skript or any other executable upon a buzzer-press. Most of what I need works quite fine, but I'm facing a curious issue:
I do the call of the executable via execlp. And, when I do have STDIN and STDOUT open, this works just beautifully.
However, as this is supposed to be handled completely deamonized, thus in the background, I would like to run this without any console-interface. But, as soon as I close the standard interface identifiers, execlp does not work anymore.
I tried it with the snippet below: When it is run with a d as argument for debug, the identifiers will be kept open and it works. Otherwise, it does not. Thus, the question is: How to run an executable (by any means), without using STDIN and STDOUT.
Some remarks:
The call to execlp does not return. Which means, that the syslog-call is never done and which is as expected.
When leaving STDERR open, nothing comes up.
The script to be called returns with an error-code (consciously for debugging reasons).
When executing with file-handlers open, I do see the toggling of the pins and the wait-call returns the error-code of the sript.
When executing without the file-handlers, the wait call returns the exit-status success.
Note, that the c call to execlp does not return. Thus, inside this little programm, it looks like success.
Here is the c-code:
/** Global Includes: ****************************************************************/
#include <stdio.h>
#include <pthread.h>
#include <sys/stat.h>
#include <sys/un.h>
#include <unistd.h>
#include <syslog.h>
#include <sys/wait.h>
/** Main-Function: ******************************************************************/
int main (int argc, char **argv) {
/** Variables: */
pid_t pid, sid;
/** Set up the demon: ***********************************************************/
pid = fork();
if (pid < 0) {
return -2;
}
/** If we got a good PID, then we can exit the parent process: */
if (pid > 0) {
printf("Sturting Deamon as PID %i.\n", pid);
int iStatus;
pid_t pid;
pid = waitpid(-1, &iStatus, 0);
if ((pid == 0) || (pid == -1)) {
printf("Client exited unknown!\n");
}else{
if (WEXITSTATUS(iStatus) == 0) {
printf("Client %i exited success!\n", pid);
}else{
printf("Client %i exited with error!\n", pid);
}
}
return 0;
}
/** Close out the standard file descriptors: */
if ((argv[1][0]) != 'd') {
close(STDIN_FILENO);
close(STDOUT_FILENO);
//close(STDERR_FILENO);
}
/** Open syslog: */
openlog( "BuzzerD", LOG_PID | LOG_CONS | LOG_NDELAY, LOG_LOCAL0 );
/** Try to run: */
execlp("bash", "bash", "/home/pi/TexPrint", "/home/pi/SoLieb.tex", NULL);
syslog(LOG_ERR | LOG_DAEMON, "FAILURE running execlp!");
_exit(1);
}
Here is the sript to be called:
#SOURCE=$1
#TARGET=${SOURCE/tex/pdf}
#echo Compiling $SOURCE to $TARGET and print!
#echo pdflatex $SOURCE
#echo Trying to print $TARGET ...
#echo lp -d Brother_MFC_J5930DW $TARGET
#echo .
/home/pi/BuzzerD/buzzerd -l off
sleep 3
/home/pi/BuzzerD/buzzerd -l alive
exit 1