Neo

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Posts by Neo

Identify processes that have open sockets

Jul 26th

Posted by Neo in Linux

1 comment

The lsof can be used to find the list of processes that have active sockets open.

Let us see the list of current TCP connections using netstat command.

[liz@techpulp ~]# netstat -nt
Active Internet connections (w/o servers)
Proto Recv-Q Send-Q Local Address               Foreign Address             State
tcp        0      0 192.168.20.24:40816         74.125.19.83:80             ESTABLISHED
tcp        0      0 192.168.20.24:43059         74.125.19.83:80             ESTABLISHED
[liz@techpulp ~]#

Now let us find which process has these TCP sockets open using lsof command.

[liz@techpulp ~]# /usr/sbin/lsof -i
COMMAND    PID   USER   FD   TYPE DEVICE SIZE NODE NAME
firefox-b 3288    liz   13u  IPv4 157814       TCP
                          liz.techpulp.com:43058->cf-in-f83.google.com:http (ESTABLISHED)
firefox-b 3288    liz   40u  IPv4 158205       TCP
                          liz.techpulp.com:43059->cf-in-f83.google.com:http (ESTABLISHED)
[liz@techpulp ~]#

So Firefox More >

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Linux file compression tools/utilities

Jul 26th

Posted by Neo in Linux

1 comment

Linux has various types of compression and decompression tools like tar, gzip, bzip2, zip and compress. This article describes practical usage of these utilities.

tar and untar

tar command operates on one or more files or directories and creates an archive. An archive created by tar command is called a tarball. Generally such archives will have .tar as file extension. tar is more of a archiving utility than a compression utility. Typically it is used along with file compression utilities like gzip and bzip2. See various ways of creating a tarball using -cvf option in the following examples.

[liz@techpulp ~]# tar -cvf mydir.tar mydir/
[liz@techpulp ~]# tar -cvf myfiles.tar file1.txt More >
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Print first N characters of a string using printf

Jul 26th

Posted by Neo in C/C++

No comments

#include <stdio.h>

int main(int argc, char *argv[])
{
        char *p = "My Name is Neo";

        printf("First 5 bytes of string is '%.5s'\n", p);
        return 0;
}

Result

[neo@techpulp ex]# gcc example.c -o example
[neo@techpulp ex]# ./example
First 5 bytes of string is 'My Na'
[neo@techpulp ex]#

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Strip unused/dead functions from Linux application

Jul 26th

Posted by Neo in C/C++

1 comment

ELF executable can be opimized for size by removing unused or dead functions. The options -ffunction-sections -Wl,–gc-sections can be used to do that.

Let us practically verify it using the following C file which has a function named dead_one which is not used anywhere in the code.

bash# cat test.c

#include <stdio.h>

int dead_one()
{
	int i;
	i = 10*30*40;
	return i;
}

int main(int argc, char *argv[])
{
	printf("Wake up, Neo\n");
}

Now let us compile the above C file with and without -ffunction-sections -Wl,–gc-sections options. Let us produce binary named test.1 without the options and a binary named test.2 with the options.

bash# cc test.c -o test.1 More >
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Stack of a Running Process in Unix/Linux

Jul 26th

Posted by Neo in C/C++

1 comment

The command pstack can be used to view the stack trace of a running process.

Let us select a running process.

[neo@techpulp ~]# ps -e | grep konqueror
22937 pts/5 00:00:17 konqueror

Run pstack command with PID of the process.

[neo@techpulp ~]# pstack 22937
22937: konqueror
(No symbols found)
0x001fb402: ???? (89c6ed8, 2, bfa39b08, 3812df0, 89c6ed8, 3c21362)
0x0325074b: ???? (89c6ed8, 8a00908)
0x03250656: ???? (89c6ed8, 3c372ec)
0x03237a59: ???? (bfa39c20, bfa39d48, bfa39de8, bfa39d1c, 0, bfa39fb4) + 4c0
0x03bb1d0d: ???? (1, bfa3a094, bfa3a09c, 0, bfa3a094, 1) + 40
0x00248de6: ???? (80485ec, 1, bfa3a094, 8048600, 8048650, 21ff2d) + 405c5f78
[neo@techpulp ~]#

There is an alternate way to use gdb and attach More >

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