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

12.17.2009

ctags - vim with steroids

Basically C/C++ projects have thousands of lines of code divided into hundreds in some cases thousands of files. Inorder to access various function definitions within the sourcecode repository effectively using a VIM editor there is a great need for using addons like ctags which provides easy code go through.

Eventhough there are many effective GUI based code editors like eclipse e.t.c I prefer to use VIM editor as my primary code editor. I am not much into GUI funda so still prefer basic editor like VIM.

1. Installing ctags package
Almost all the linux flavours with 2.6.X kernel might have ctags installed by default. If not download the appropriate .deb or rpm file. Sorry I am not dealing with installing of ctags as I havent came across this stage as I am using FEDORA 9.

2. Generating ctags on your source code.
We have to generate a file named 'tags' for all the source code files, use the following command:
ctags *.c *.h
When we have many files in many directories then we have to create a tags file in each of the directories. But VIM will only be able to jump to tags within the same directory. To find more tags files, we have to set the 'tags' option in VIM to include all the relevant tags files. Just set the following command in ~/.vimrc file.
:set tags=./tags,./../tags,./*/tags
This finds a tags file in the same directory as the current file, one directory level higher and in all subdirectories.

But VIM searching many places for tags files is not really robust enough as it may get a bit slow. It's better to generate one big tags file offcourse it takes more time to create a single tags files for whole project. Just give the following command to recursively add all files in your project.
$ cd /proj 
$ ctags -R *
-R is to recursively go across all the directories, and a ‘tags’ file will be created including all the files in the sub-directories also.

Now we can simply include this single tags files as shown below
:set tags=~/proj/tags
3. Start using ctags with VIM editor
We have 3 different ways to use ctags in VIM editor.

From Shell:
We can invoke directly the file or the files containing the definition for a particular function from shell.
vi -t function_name
This will find the correct file or list of files having function_name definition.

OR

VIM command line:
We can invoke from VIM commandline (in command mode) the definition of a function.
:tag function_name
or
:ta function_name
This will jump automatically from the current file to the file containing the function_name definition.

OR

By cursor position:
This option is more user-friendly as we use a key combination instead of giving commands.
ctrl + ]
Place the cursor on the first character of the function name and press ctrl-]. This will jump to the file containing the definition of function_name.
ctrl + t
This command will jump back to previous location.
ctrl + i
To travel forward through the tag history.
ctrl + o
To travel backward through the tag history.

History
To display the list of tags that we have traversed in past give the following command.
:tags
Shows tag stack (history).

Divide and Conquer
As we saw already 'ctrl + ]' replaces the file in the current window with the one containing the new function. But suppose if we want to see not only the old function but also the new one?

We can split the window using the ":split" command followed by the ":tag" command.
:stag function_name
Cursor command for above feature is
ctrl + w + ]
Auto Complete
VIM supports tag name completion. Start typing the tag name (i.e. function name) and then hit TAB key and name completion will be done automatically if there is a tag name.
:tag start-of-tag-name_TAB
Jump to a tag name found by a search.
:tag /search-string
When multiple entries exist in the tags file, such as a function declaration in a header file and a function definition (the function itself), the operator can choose by issuing this command. The user will be presented with all the references to the function and the user will be prompted to enter the number associated with the appropriate one.
:tselect function-name
Jumps to next matching tag.
:tnext
or
:tn
Jump to previous matching tag.
:tprevious
or
:tp
Jump to first matching tag.
:tfirst
or
:tf
or
:trewindor:tr
Jump to last matching tag.
:tlast
or
:tl
Finding global identifiers
You are editing a C program and wonder if a variable is declared as "int" or "unsigned". A quick way to find this is with the "[I" command. Suppose the cursor is on the word "column" then type
[ + shiftkey + i
Vim will list the matching lines it can find. Not only in the current file,
but also in all included files (and files included in them, etc.). The result
looks like this:

structs.h
1: 29 unsigned column; /* column number */
The advantage over using tags or the preview window is that included files are
searched. In most cases this results in the right declaration to be found.
Also when the tags file is out of date. Also when you don't have tags for the
included files.

However, a few things must be right for "[I" to do its work. First of all,
the 'include' option must specify how a file is included. The default value
works for C and C++. For other languages you will have to change it.

Reference:
ctags documentation

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