Linux – How to get PIDs waiting on a semaphore

ipclinuxredhatstrace

If I strace a stuck process and see output of the form:

[gaius@redhat64 bin]$ strace -p 18185
Process 18185 attached - interrupt to quit
[ Process PID=18185 runs in 32 bit mode. ]
semop(458760, 0xffa00af0, 1

How would I find out the PID that last incremented the semaphore/that I am waiting for? I know about the lpid column in ipcs -p but that only works for shared memory segments.

My OS is RHEL 5.4 (Tikanga) on x86_64. Thanks!

Best Answer

Quick and dirty:

#include <sys/sem.h>
#include <stdio.h>
#include <stdlib.h>

int main(int argc, char **argv)
{
    int pid;
    if (argc != 2) return 1;
    pid = semctl(atoi(argv[1]), 0, GETPID);
    printf("%d\n", pid);
    return 0;
}

(I'm guessing here.)

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As of GNU coreutils 7.5 released in August 2009, sort allows a -h parameter, which allows numeric suffixes of the kind produced by du -h:

du -hs * | sort -h

If you are using a sort that does not support -h, you can install GNU Coreutils. E.g. on an older Mac OS X:

brew install coreutils
du -hs * | gsort -h

From sort manual:

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Linux – How to run a server on port 80 as a normal user on Linux

Short answer: you can't. Ports below 1024 can be opened only by root. As per comment - well, you can, using CAP_NET_BIND_SERVICE, but that approach, applied to java bin will make any java program to be run with this setting, which is undesirable, if not a security risk.

The long answer: you can redirect connections on port 80 to some other port you can open as normal user.

Run as root:

# iptables -t nat -A PREROUTING -p tcp --dport 80 -j REDIRECT --to-port 8080

As loopback devices (like localhost) do not use the prerouting rules, if you need to use localhost, etc., add this rule as well (thanks @Francesco):

# iptables -t nat -I OUTPUT -p tcp -d 127.0.0.1 --dport 80 -j REDIRECT --to-ports 8080

NOTE: The above solution is not well suited for multi-user systems, as any user can open port 8080 (or any other high port you decide to use), thus intercepting the traffic. (Credits to CesarB).

EDIT: as per comment question - to delete the above rule:

# iptables -t nat --line-numbers -n -L

This will output something like:

Chain PREROUTING (policy ACCEPT)
num  target     prot opt source               destination         
1    REDIRECT   tcp  --  0.0.0.0/0            0.0.0.0/0           tcp dpt:8080 redir ports 8088
2    REDIRECT   tcp  --  0.0.0.0/0            0.0.0.0/0           tcp dpt:80 redir ports 8080

The rule you are interested in is nr. 2, so to delete it:

# iptables -t nat -D PREROUTING 2

Best Answer

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