C linux daemon does not write into file after opening FIFO - c

Question

I have following program in C, which should run as a deamon and whenever is something written into FIFO, it should write it into a file.
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
#include <signal.h>
#include <syslog.h>
#define BUF_LENGTH 255
volatile int signal_flag = 1;
void signal_handler(int sig)
{
signal_flag = 1;
}
char *getTimeString()
{
time_t rawtime;
struct tm * timeinfo;
time ( &rawtime );
timeinfo = localtime ( &rawtime );
char *timeStr = asctime (timeinfo);
timeStr[strlen(timeStr) - 1] = 0;
return timeStr;
}
void printUsage()
{
printf("Usage: syslog_daemon PATH INTERVAL\n");
}
int main(int argc, char *argv[])
{
/* print usage */
if(argc != 3)
{
printUsage();
exit(EXIT_SUCCESS);
}
/* process arguments */
char *logFilePath = argv[1];
int interval = atoi(argv[2]);
/* establish the signal handler */
struct sigaction action;
sigemptyset(&action.sa_mask);
action.sa_flags = 0;
action.sa_handler = signal_handler;
sigaction(SIGALRM, &action, NULL);
/* initialize variables */
int fd;
/*char buf[BUF_LENGTH];
int length;*/
int msgs = 0;
/* Create FIFO if not created */
if (mkfifo("/tmp/pb173_syslog", 0766) == -1 && errno != EEXIST)
{
fprintf(stderr, "Making FIFO failed with error %d\n", errno);
exit(EXIT_FAILURE);
}
/* Run */
daemon(1, 1);
while(1)
{
/* Open FIFO */
fd = open("/tmp/pb173_syslog", O_RDONLY);
close(fd);
/* Open and write into file */
FILE *f = fopen(logFilePath, "a");
fprintf(f, "Daemon write: %d\n", msgs);
fclose(f);
/* Process SIGALRM and write syslog */
if(signal_flag)
{
openlog("syslog_daemon v2", LOG_CONS, LOG_DAEMON);
syslog(LOG_INFO, "Messages written: %d\n", msgs);
closelog();
msgs++;
signal_flag = 0;
alarm(interval);
}
}
return 0;
}
But this program does not write anything into the file. It seems, that when the FIFO is open, it cannot write anywhere. But if I don't open the FIFO, the program writes into the file without any problems. Does anyone know what is the problem? Thanks for any help.

Answer 1

It hangs on open trying to open a FIFO which does not have the second endpoint (the writer) connected.
You may want to use O_NONBLOCK.
Here is a quote from strace output that shows where it hangs:
$ strace -p 23114
Process 23114 attached - interrupt to quit
open("/tmp/pb173_syslog", O_RDONLY
If you write something to the FIFO (e.g. echo test > /tmp/pb173_syslog) it unblocks and starts working.