/* Mike Chirico mmc mchirico@users.sourceforge.net
CopyRight GPL 2004
You don't want a zombie process (a child process
that's kept running until after the parent has
exited). And, you don't want to wait in a
loop taking up valuable cpu resources.
The program below, waits for a signal telling
the parent that each child has terminated..ah
but using a signal is a bad idea, because
only realtime signals will queue...read on.
sigsuspend(...) blocks and it's run for each
forked process, or NUM_PROC of them. Ok, that's
fine for the first signal.
*/
#include <signal.h>
#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
#define NUM_PROC 4
sig_atomic_t sigusr1_count = 0;
/* according to recent literature
this function call allows signals to
be pending, or queued provided real-time
signals are being used. Currently, I have
2 versions of Linux
(works)
2.4.20-28.9 #1 Thu Dec 18 13:45:22 EST 2003
(does not work)
2.4.20-28.8 #1 Thu Dec 18 12:53:39 EST 2003
*/
/* handler may only execute once
for the first process only, the other
processes are dropped when they are
queued...so it's a race. If the first
one finishes you're ok */
void handler (int signal_number)
{
++sigusr1_count;
fprintf(stderr,"in handler \n");
}
int main(void)
{
struct sigaction sa;
sigset_t blocked_sigs,no_sigs,all_sigs;
pid_t p[NUM_PROC],pid;
int i;
sigfillset(&all_sigs);
sigemptyset(&no_sigs);
sigemptyset(&blocked_sigs);
memset(&sa, 0, sizeof(sa));
sa.sa_handler = &handler;
if( sigaction(SIGCHLD,&sa, NULL) == -1) {
fprintf(stderr,"sigaction error \n");
exit(-1);
}
for(i=0; i < NUM_PROC; ++i)
{
if ( (p[i] = fork()) == 0 ) {
/* take out sleep and
put your main code here */
sleep(2);
exit(0);
}
}
int status;
while(sigusr1_count < NUM_PROC)
{
printf("before: sigsuspend %d\n",sigusr1_count);
sigsuspend(&no_sigs);
printf("after sigsuspend: %d \n",sigusr1_count);
/* this is our circuit breaker for safety.
if the other processors finish at the same
time, they may not queue. sigusr1_count will
never go above one! You'll get a zombie
process...this protects us, until Linux
support realtime signals. Only realtime
signals queue. Ref man 7 signal
*/
for(i=0; i < NUM_PROC; ++i)
{
pid=waitpid(p[i],&status,0);
fprintf(stderr,"pid=%d\n",pid);
if ((sigusr1_count < NUM_PROC) && (i == (NUM_PROC -1))) {
fprintf(stderr,"Signal didn't queue\n");
exit(-2);
}
}
}
return 0;
}
Linux System Admin Tips: There are over 200 Linux tips and tricks in this article. That is over 100 pages covering everything from NTP, setting up 2 IP address on one NIC, sharing directories among several users, putting running jobs in the background, find out who is doing what on your system by examining open sockets and the ps command, how to watch a file, how to prevent even root from deleting a file, tape commands, setting up cron jobs, using rsync, using screen conveniently with emacs, how to kill every process for a user, security tips and a lot more. These tip grow weekly. The above link will download the text version for easy grep searching. There is also an html version here.
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MySQL Tips and Tricks: Find out who is doing what in MySQL and how to kill the process, create binary log files, connect, create and select with Perl and Java, remove duplicates in a table with the index command, rollback and how to apply, merging several tables into one, updating foreign keys, monitor port 3306 with the tcpdump command, creating a C API, complex selects, and much more.
Create a Live Linux CD - BusyBox and OpenSSH Included: These steps will show you how to create a functioning Linux system, with the latest 2.6 kernel compiled from source, and how to integrate the BusyBox utilities including the installation of DHCP. Plus, how to compile in the OpenSSH package on this CD based system. On system boot-up a filesystem will be created and the contents from the CD will be uncompressed and completely loaded into RAM -- the CD could be removed at this point for boot-up on a second computer. The remaining functioning system will have full ssh capabilities. You can take over any PC assuming, of course, you have configured the kernel with the appropriate drivers and the PC can boot from a CD. This tutorial steps you through the whole processes.
SQLite Tutorial : This article explores the power and simplicity of sqlite3, first by starting with common commands and triggers, then the attach statement with the union operation is introduced in a way that allows multiple tables, in separate databases, to be combined as one virtual table, without the overhead of copying or moving data. Next, the simple sign function and the amazingly powerful trick of using this function in SQL select statements to solve complex queries with a single pass through the data is demonstrated, after making a brief mathematical case for how the sign function defines the absolute value and IF conditions.
The Lemon Parser Tutorial: This article explains how to build grammars and programs using the lemon parser, which is faster than yacc. And, unlike yacc, it is thread safe.
How to Compile the 2.6 kernel for Red Hat 9 and 8.0 and get Fedora Updates: This is a step by step tutorial on how to compile the 2.6 kernel from source.
Virtual Filesystem: Building A Linux Filesystem From An Ordinary File. You can take a disk file, format it as ext2, ext3, or reiser filesystem and then mount it, just like a physical drive. Yes, it then possible to read and write files to this newly mounted device. You can also copy the complete filesystem, since it is just a file, to another computer. If security is an issue, read on. This article will show you how to encrypt the filesystem, and mount it with ACL (Access Control Lists), which give you rights beyond the traditional read (r) write (w) and execute (x) for the 3 user groups file, owner and other.
Working With Time: What? There are 61 seconds in a minute? We can go back in time? We still tell time by the sun?
Mike Chirico, a father of triplets (all girls) lives outside of
Philadelphia, PA, USA. He has worked with Linux since 1996, has a Masters
in Computer Science and Mathematics from Villanova University, and has
worked in computer-related jobs from Wall Street to the University of
Pennsylvania. His hero is Paul Erdos, a brilliant number theorist who was
known for his open collaboration with others.
Mike's notes page is souptonuts. For
open source consulting needs, please send an email to
mchirico@gmail.com. All consulting work must include a donation to
SourceForge.net.
