View Full Version : [SOLVED] Using the daemon() call in C to create daemons?

November 5th, 2008, 05:44 AM
Greetings -- In looking into createing simple daemons in C, I can find the basic info on it in a number of tutorials etc, however, I was wondering if anyone had any examples using the daemon() call that exists in some Linux versions (including Ubuntu) for creating simple daemons ?? I want to create a simple serial logging daemon that monitors the serial port (status from the hot tub) and logs it to a file for later analysis. Any simple examples or pointers would be appreciated !!


November 5th, 2008, 11:58 AM
There's nothing to using the daemon() call. Here's a simple example. Calling daemon() will detach your process from a controlling tty, close stdin/stdout, and will also fork & exit the parent so that your process becomes a child of init.

For debugging, I added a -f option. If you specify -f, the program won't call daemon() so that you'll get your printfs. If you don't specify -f, the application will call daemon() and you won't see any more printfs.

#include <ctype.h>
#include <stdbool.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char** argv)
bool run_in_foreground;
int loop_count;
int c;

opterr = 0;

while((c = getopt(argc, argv, "f")) != -1)
case 'f':
run_in_foreground = true;

case '?':
printf("Unrecognized option '%c'.\n", optopt);
return 1;


printf("This is my application...\n");

printf("This is the last message you'll see, since I'm going to call daemon()\n");
printf("I'm in the foreground, so you'll still see my printfs\n");

for(loop_count = 0; loop_count < 10; loop_count++)
printf("Loop count is %d\n", loop_count);

return 0;

Hope that helps a bit.

November 5th, 2008, 12:29 PM

#include <stdbool.h>

bool run_in_foreground;


Be careful of mixing C++ data types (e.g. bool) into a C program. The program may not be portable.

For further reference on the differences between C and C++: http://people.cs.uchicago.edu/~iancooke/osstuff/ccc.html

November 5th, 2008, 03:22 PM
thanks for the info people -- I will see if I can crash my system when I get home tonight. Looks like a really simple way to create the daemon (and the idea of the '-f' is good for debugging too !! ). thanks much !!


November 5th, 2008, 04:40 PM
That's not a c++ data type -- stdbool.h is part of the c99 standard... :confused:

November 5th, 2008, 07:03 PM
On my Ubuntu 8.04 system, stdbool.h is located in /usr/include/c++/4.2/tr1.

On my Fedora 9 system, stdbool.h is located in /usr/include/c++/4.3.0/tr1.

The thing I see in common between both systems is the "c++" in the path.

November 5th, 2008, 08:27 PM
Sorry, you're wrong.

$ locate stdbool.h

$gcc -v -c daemon.c
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v --enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr --enable-shared --with-system-zlib --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --enable-nls --with-gxx-include-dir=/usr/include/c++/4.2 --program-suffix=-4.2 --enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc --enable-mpfr --enable-targets=all --enable-checking=release --build=i486-linux-gnu --host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.2.4 (Ubuntu 4.2.4-1ubuntu3)
/usr/lib/gcc/i486-linux-gnu/4.2.4/cc1 -quiet -v daemon.c -quiet -dumpbase daemon.c -mtune=generic -auxbase daemon -version -fstack-protector -fstack-protector -o /tmp/ccsfMarj.s
ignoring nonexistent directory "/usr/local/include/i486-linux-gnu"
ignoring nonexistent directory "/usr/local/include"
ignoring nonexistent directory "/usr/lib/gcc/i486-linux-gnu/4.2.4/../../../../i486-linux-gnu/include"
ignoring nonexistent directory "/usr/include/i486-linux-gnu"
#include "..." search starts here:
#include <...> search starts here:
End of search list.
GNU C version 4.2.4 (Ubuntu 4.2.4-1ubuntu3) (i486-linux-gnu)
compiled by GNU C version 4.2.4 (Ubuntu 4.2.4-1ubuntu3).
GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072
Compiler executable checksum: e2971cf2189271aeeb10133511ecea3a
as --traditional-format -V -Qy -o daemon.o /tmp/ccsfMarj.s
GNU assembler version 2.18.0 (i486-linux-gnu) using BFD version (GNU Binutils for Ubuntu)

Notice the search path and the file location, highlighted in red.

Plus, see the c99 standard since you're confused: http://www.comeaucomputing.com/techtalk/c99/#bool

November 6th, 2008, 01:53 AM
I stand corrected.