/* Ajith - Syntax Higlighter - End ----------------------------------------------- */

12.17.2009

Creating Shared Libraries in Linux - Part 2

Check the PART1 of this article.

4. Making the library available at run-time

Using LD_LIBRARY_PATH
We have to create or set the environment variable "LD_LIBRARY_PATH" to the directory containing the shared libraries.
export LD_LIBRARY_PATH=/home/cf/lib
If in current directory you can give the following command
export LD_LIBRARY_PATH=.
If we have to append a new directory to the existing paths then add the directories separated by colons to environment variable "LD_LIBRARY_PATH".
export LD_LIBRARY_PATH=/opt/lib:$LD_LIBRARY_PATH
Now recompile the main program
gcc -o test main.c -lcalc_mean -L/home/cf/slib
Now check the ldd command output
$ ldd test
 linux-gate.so.1 =>  (0x007ad000)
 libcalc_mean.so => ./libcalc_mean.so (0x0081e000)
 libc.so.6 => /lib/tls/i686/cmov/libc.so.6 (0x005ff000)
 /lib/ld-linux.so.2 (0x00e19000)
It seems now linker is able to locate our shared library as we can see in above output. Now run the program
$./test
LD_LIBRARY_PATH is good for quick tests and for systems on which you don’t have admin privileges. As a downside, however, exporting the LD_LIBRARY_PATH environment variable  might screw up with other programs you run that also rely on LD_LIBRARY_PATH if you don’t reset it to its previous state when you’re done.

12.09.2009

Creating and using shared libraries in Linux

What is a Shared Library ??
A library that is loaded into physical memory only once and reused by multiple processes via virtual memory. 
Generally Shared libraries are .so (or in windows .dll) files.

Why shared libraries ??
  • They reduce memory consumption if used by more than one process, and they reduce the size of the executable.
  • They make developing applications easier: a small change in the implementation of a function in the library don't need the user to recompile and relink his application code every time. You need to only relink if you make incompatible changes, such as adding arguments to a call or changing the size of a struct.
NOTE: Debugging using a shared library is slightly more difficult when compared with static libraries, because the debugger usually used on Linux, gdb, has some problems with shared libraries.

Let us see how to create a shared library on Linux. We use following source code files for this post.
calc_mean.h
#ifndef calc_mean_h__
#define calc_mean_h__
double mean(double, double);
#endif  // calc_mean_h__
calc_mean.c
double mean(double a, double b)
{
return (a+b) / 2;
}
main.c - We are including our shared library in this application.
#include <stdio.h>
#include "calc_mean.h"

int main(int argc, char* argv[]) {

double v1, v2, m;
v1 = 5.2;
v2 = 7.9;

m  = mean(v1, v2);

printf("The mean of %3.2f and %3.2f is %3.2f\n", v1, v2, m);

return 0;
}

1. Creating Object File with Position Independent Code
All the code that goes into a shared library needs to be position independent. We can make gcc emit position-independent code by passing it one of the command-line switches -fpic or -fPIC (the former is preferred, unless the modules have grown so large that the relocatable code table is simply too small in which case the compiler will emit an error message, and you have to use -fPIC).

First we will create object files for all .c files that goes into a shared library.
gcc -c -fPIC calc_mean.c -o calc_mean.o
Above we are compiling calc_mean.c with -fPIC option and generating calc_mean.o object file.

2. Creating Shared library with the Object File
Every shared library has a prefix "lib", the name of the library, the phrase ".so", followed by a period and a version number that is incremented whenever the interface changes (as a special exception, the lowest-level C libraries don't start with "lib").
gcc -shared -o libcalc_mean.so calc_mean.o
Above command on successful produces a shared library named "libcalc_mean.so".
  • -shared: Produces a shared object which can then be linked with other objects to form an executable.

3. Using the Shared Library
Now let us link the created shared library with our application. Compile main.c as shown below
$ gcc -o test main.c -lcalc_mean
/usr/bin/ld: cannot find -lcalc_mean
collect2: ld returned 1 exit status
The linker doesn’t know where to find libcalc_mean. But why ?
GCC has a list of places to look by default for shared libraries, but our directory is not in that list. Bingo that's the reason compilation failed at linking level.

Now we need to tell GCC where to find libcalc_mean.so. We will do that with the -L option.
gcc -o test main.c -lcalc_mean -L/home/cf/slib
  •  -l option tells the compiler to look for a file named libsomething.so The something is specified by the argument immediately following the “-l”. i.e. -lmean
  • -L option tells the compiler where to find the library. The path to the directory containing the shared libraries is followed by "-L". If no “-L” is specified, the compiler will search the usual locations. "-L." means looking for the shared libraries in the current directory and "-L/home/cf/lib" means looking for the shared libraries at "/opt/lib" path. You can specify as many “-l” and “-L” options as you like.
NOTE: It would be a better idea to move all personal shared libraries in one directory rather in the current directory. For easy understanding I am moving "libcalc_mean.so" to "/home/cf/slib".
mv libcalc_mean.so /home/cf/slib
Now compile main.c. It would be successful and creates an executable named "test".
Let us check if the path to our shared library is included successfully into the executable by linker as shown below.

ldd executablename
$ ldd test 
 linux-gate.so.1 =>  (0x00332000)
 libcalc_mean.so => not found
 libc.so.6 => /lib/tls/i686/cmov/libc.so.6 (0x006aa000)
 /lib/ld-linux.so.2 (0x00db9000)
You can see that linker cannot find our shared library libcalc_mean.so.

Basically libraries are present in /usr/lib amongst other places. Static libraries (.a suffix) are incorporated into the binary at link time, whereas dynamic ones (.so suffix) are referenced by location.

Check the PART2 of this article to understand further

11.04.2009

Using 'find' command

'find' is one of the useful commands available in Unix/Linux systems. In this article I am trying to use 'find' in a effective way.

Basic syntax of find command as per man page is - find [path...] [expression]
codingfreak find
NOTE: There are many options available for find command which are available in man page for find. Only some of those options are shown in this tutorial.

Finding a particular file in your system
$ find / -name 'filename' 2>/dev/null
$ find / -name 'filename' 2>errors.txt
/ - Start searching from the root directory (i.e / directory)

-name - Given search text is the filename rather than any other attribute of a file 'filename'. Always enclose the filename in single quotes...

NOTE: 2>/dev/null is not related to find tool as such. 2 indicates the error stream in Linux, and /dev/null is the device where anything you send simply disappears. So 2>/dev/null in this case means that while finding for the files, in case any error messages pop up simply send them to /dev/null i.e. simply discard all error messages.

Alternatively you could use 2>error.txt where after the search is completed you would have a file named error.txt in the current directory with all the error messages in it.
$ find -name 'met*'
The above command would start searching for the files that begin with the letters 'met' within the current directory and the directories that are present within the current directory.

If no paths are given, the current directory is used. If no expression is given, the expression ‘-print’ is used.

Searching with respect to type of the file (-type)
$find . -name 'temp'
./keepout/temp
./temp

$find . -name 'temp' -type d
./temp
Where d - directory, p - named pipe (FIFO), f - regular file and so on

Ignoring case-sensitivity (-iname)
$ find /home/temp -iname 'index*'
This command searchs for a file starting with string 'index' without considering the case of the filename. So all files starting with any combination of letters in upper and lower case such as INDEX or indEX or index would be returned.

Searching for a file based on size and time
$ find /home/songs -name '*.mp3' -size -5000k

$ find / -size +10000k
First command finds within a directory called /home/songs, only those mp3 files that have a size less than 5000 Kilobytes.
$ find /home/temp -amin -10 -name '*.c'
$ find /home/temp -atime -2 -name '*.c'
$ find /home/temp -mmin -10 -name '*.c'
$ find /home/temp -mtime -2 -name '*.c'
The 1st command searches for those files that are present in the directory /home/temp and its subdirectories which end in .c and which have been accessed in the last 10 minutes.

The 2nd command does the same but searches for those files that have been accessed in the last 10 hours.

The 3rd and the 4th commands do the same as the 1st and 2nd commands but they search for modified files rather than accessed files. Only if the contents of the files have been modified, would their names be returned in the search results.
$ find / -mount -name 'win*'
This command searches for files starting with the letters 'win' in their filenames. The only difference is that the mounted filesystems would not be searched for this time. This is useful when you have your Windows partitions mounted by default. And a search for 'win' might return many files on those partitions, which you may not be really interested in. This is only one use of -mount parameter.
$ find /home/songs -name 'Metallica*' -and -size +10000k
$ find /home/songs -size +10000k ! -name "Metallica*"
$ find /home/songs -name 'Metallica*' -or -size +10000k
Boolean operators such as AND, OR and NOT make find an extremely useful tool.

The 1st command searches within the directory /songs for files that have their names beginning with 'Metallica' and whose size is greater than 10000 kilobytes (> 10 MB).

The 2nd command searches in the same directory as above case but only for files that are greater than 10MB, but they should not have 'Metallica' as the starting of their filenames.

The 3rd command searches in the same directory for files that begin with 'Metallica' in their names or all the files that are greater than 10 MB in size.

How to apply a unix command to a set of files (-exec) ?
$ find . -name '*.sh' -exec chmod o+r '{}' \; -print
This command will search in the current directory and all sub directories. All files ending with .sh extension will be processed by the chmod -o+r command. The argument '{}' inserts each found file into the chmod command line. The \; argument indicates the exec command line has ended.

The end results of this command is all .sh files have the other permissions set to read access (if the operator is the owner of the file).

Searching for a string in a selection of files (-exec grep ...).
$ find . -exec grep "hello" '{}' \; -print
Prints all files that contain the string 'hello' will have their path printed to standard output.

If you want to just find each file then pass it on for processing use the -q grep option. This finds the first occurrance of the search string. It then signals success to find and find continues searching for more files.
find . -exec grep -q "hello" '{}' \; -print
Finding Empty files (-empty)
$find . -empty
To delete empty files in the current directory:
$ find . -empty -maxdepth 1 -exec rm '{}' \;
For more examples try out

1. linux.ie
2. Devdaily
3. hccfl.edu

10.30.2009

Are you a Hacker ??

from ReDragon on IRC, handed to newbies...

Are You a Hacker?

Take a little quiz for me today. Tell me if you fit this description.
You got your net account several months ago. You have been surfing the
net, and you laugh at those media reports of the information superhighway.
You have a red box, you don't have to pay for phone calls. You have
crackerjack, and you have run it on the password file at a unix you got
an account on. Everyone at your school is impressed by your computer
knowledge, you are the one the teachers ask for help. Does this sound
like you? You are not a hacker.

There are thousands of you out there. You buy 2600 and you ask
questions. You read phrack and you ask questions. You join #hack and
you ask questions. You ask all of these questions, and you ask what is
wrong with that? After all, to be a hacker is to question things, is
it not? But, you do not want knowledge. You want answers. You do not
want to learn how things work. You want answers. You do not want to
explore. All you want to know is the answer to your damn questions.
You are not a hacker.

Hacking is not about answers. Hacking is about the path you take to
find the answers. If you want help, don't ask for answers, ask for
a pointer to the path you need to take to find out those answers for
yourself. Because it is not the people with the answers that are the
hackers, it is the people that are travelling along the path.

-ReDragon

10.05.2009

Signals in Linux - Blocking Signals

Blocking a signal means telling the operating system to hold it and deliver it later when it is unblocked. Between the time when it is generated and when it is delivered a signal is said to be pending.

Generally, a program does not block signals indefinitely - it might as well ignore them by setting their actions to SIG_IGN. But it is useful to block signals briefly, to prevent them from interrupting sensitive operations.

Is Blocking a signal similar to Ignoring a signal ?

No, blocking a signal is different from ignoring a signal. When a process blocks a signal, the operating system does not deliver the signal until the process unblocks the signal. A process blocks a signal by modifying its signal mask with sigprocmask. But when a process ignores a signal, the signal is delivered and the process handles it by throwing it away.

How Blocking Signals is Useful ?

Temporary blocking of signals with sigprocmask gives you a way to prevent interrupts during critical parts of your code. If signals arrive in that part of the program, they are delivered later, after you unblock them.

One example where this is useful is for sharing data between a signal handler and the rest of the program. If the type of the data is not sig_atomic_t, then the signal handler could run when the rest of the program has only half finished reading or writing the data. This would lead to confusing consequences.

To make the program reliable, you can prevent the signal handler from running while the rest of the program is examining or modifying that data - by blocking the appropriate signal around the parts of the program that touch the data. Blocking signals is also necessary when you want to perform a certain action only if a signal has not arrived.

All signal blocking functions use a data structure called a signal set to specify what signals are affected. Thus, every activity involves two stages: creating the signal set, and then passing it as an argument to a library function. These facilities are declared in the header file signal.h.

The sigset_t data type is used to represent a signal set. Internally, it may be implemented as either an integer or structure type. For portability, use only the functions described below to initialize, change, and retrieve information from sigset_t objects - don't try to manipulate them directly.

#include <signal.h>

int sigemptyset(sigset_t *set);

int sigfillset(sigset_t *set);

int sigaddset(sigset_t *set, int signum);

int sigdelset(sigset_t *set, int signum);

int sigismember(const sigset_t *set, int signum);

sigemptyset function initializes the signal set given by set to empty, with all signals excluded from the set.

sigfillset function initializes set to full, including all signals.

sigaddset and sigdelset functions add and delete respectively signal signum from set.

sigismember function tests whether signum is a member of set.

Objects of type sigset_t must be initialized by a call to either sigemptyset or sigfillset before being passed to the functions sigaddset, sigdelset and sigismember.

For more information checkout: man 3 sigsetops

The collection of signals that are currently blocked is called the signal mask. Each process has its own signal mask. When you create a new process, it inherits its parent's mask. You can block or unblock signals with total flexibility by modifying the signal mask.

#include <signal.h>

int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);

In a traditional single-threaded application, sigprocmask system call can be used to fetch and/or manipulate the signal mask of the calling thread.

how determines what operation to be performed on the signal mask.
If oldset is non-null, the previous value of the signal mask is stored in oldset.
set determines list of signals to be set in blocking state.

Signals, such as SIGSTOP and SIGKILL, cannot be blocked. If an attempt is made to block these signals, the system ignores the request without reporting an error.

NOTE: Do not use sigprocmask in multi-threaded processes, because each thread has its own signal mask and there is no single process signal mask. According to POSIX, the behavior of sigprocmask in a multi-threaded process is "unspecified". Instead, use pthread_sigmask.

For more information checkout: man 2 sigprocmask

In the below example we try to block and unblock the SIGINT signal continually in a loop. If a user enters Ctrl-C while SIGINT is blocked, then the program terminates only after it is unblocked. If a user types Ctrl-C while SIGINT is unblocked, the program terminates immediately.
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
int i;
sigset_t intmask;
int repeatfactor;
double y = 0.0;

if ((sigemptyset(&intmask) == -1) || (sigaddset(&intmask, SIGINT) == -1)){
perror("Failed to initialize the signal mask");
return 1;
}

for ( ; ; ) {
printf("Entering BLOCK state\n");
if (sigprocmask(SIG_BLOCK, &intmask, NULL) == -1)
break;
fprintf(stderr, "SIGINT signal blocked\n");
sleep(2);

printf("Leaving Blocking State & Entering UNBLOCK state\n");
if (sigprocmask(SIG_UNBLOCK, &intmask, NULL) == -1)
break;
fprintf(stderr, "SIGINT signal unblocked\n");
sleep(2);
}
perror("Failed to change signal mask");
return 1;
}


Output:

$ ./a.out
Entering BLOCK state
SIGINT signal blocked
Leaving Blocking State & Entering UNBLOCK state
SIGINT signal unblocked
^C

$ ./a.out
Entering BLOCK state
SIGINT signal blocked
^CLeaving Blocking State & Entering UNBLOCK state

$