Local variables in function stack: An Experience

When we were preparing for the interviews in College days, nothing was in mind except interviews. Also we were no thinking beyond the interview requirements. But when we face the same type of situations while working in any project that will make us to understand our present and past understanding capabilities and also make us to know more.

When I was in 6th sem of my engineering, I was attended an interview of RoboSoft, UDUPI. While preparing for that, I came across a C question as follows.


#include "stdio.h"
void main()
{
int a=10, b=20;
int *ptr = &a;

ptr--;
printf("%d", *ptr);
}
This will print as 20. Also this may depends on compilers. Assume GCC for this.

When local variables are declared, will be pushed into the stack in order of their declaration. Stack address will increment from TOP(decrement from BOTTOM). Here initially ptr will point to a's location and when we decrement it will point to b's location.

Lets come to point. I have following type of declarations.

struct abc
{
union data xyz;
int i;
};

In my program, I was written as follows.

uinon data a;
int temp = 10;
printf("%d", temp);
memset(a, 0, sizeof(struct abc));
printf("%d", temp);

While testing my code, I was getting the value of temp is 10 and 0 both. Initially I was wondering how temp value is changing without doing anything. After sometime, I understood that memset is making temp 0..!!!?? But how ?

Then, due to my interview question, I closely looked into the memset statement and its declaration and came to know how memset was making the temp variable 0. Because, structure is having sizeof union +1 and hence next immediate 32 bit is setting to 0.

After knowing this, I was very happy and was feeling like anything for that point of time.

This is what will make us our work enjoyable to some extent and thats what i expected from this industry.

Happy moment...!! gUd lUcK...

'find' Command Reference : Very Useful

When we use Linux/UNIX based systems, we need to know the usage of commands which will make our work easier and faster. Because, some of the operations can be done through commands itself..!!

Also, how much you know about Linux will be decided by your command knowledge and its usage. One of those is 'find'.

Please have a look once.

1. Find Files Using Name
This is a basic usage of the find command. This example finds all files with name — MyCProgram.c in the current directory and all it’s sub-directories.
# find -name "MyCProgram.c"
./backup/MyCProgram.c
./MyCProgram.c

2. Find Files Using Name and Ignoring Case
This is a basic usage of the find command. This example finds all files with name — MyCProgram.c (ignoring the case) in the current directory and all it’s sub-directories.
# find -iname "MyCProgram.c"
./mycprogram.c
./backup/mycprogram.c
./backup/MyCProgram.c
./MyCProgram.c

3. Limit Search To Specific Directory Level Using mindepth and maxdepth
Find the passwd file under all sub-directories starting from root directory.
# find / -name passwd
./usr/share/doc/nss_ldap-253/pam.d/passwd
./usr/bin/passwd
./etc/pam.d/passwd
./etc/passwd

Find the passwd file under root and one level down. (i.e root — level 1, and one sub-directory — level 2)
# find -maxdepth 2 -name passwd
./etc/passwd

Find the passwd file under root and two levels down. (i.e root — level 1, and two sub-directories — level 2 and 3 )
# find / -maxdepth 3 -name passwd
./usr/bin/passwd
./etc/pam.d/passwd
./etc/passwd

Find the password file between sub-directory level 2 and 4.
# find -mindepth 3 -maxdepth 5 -name passwd
./usr/bin/passwd
./etc/pam.d/passwd

4. Executing Commands on the Files Found by the Find Command.
In the example below, the find command calculates the md5sum of all the files with the name MyCProgram.c (ignoring case). {} is replaced by the current file name.
# find -iname "MyCProgram.c" -exec md5sum {} \;
d41d8cd98f00b204e9800998ecf8427e ./mycprogram.c
d41d8cd98f00b204e9800998ecf8427e ./backup/mycprogram.c
d41d8cd98f00b204e9800998ecf8427e ./backup/MyCProgram.c
d41d8cd98f00b204e9800998ecf8427e ./MyCProgram.c

5. Inverting the match.
Shows the files or directories whose name are not MyCProgram.c .Since the maxdepth is 1, this will look only under current directory.
# find -maxdepth 1 -not -iname "MyCProgram.c"
.
./MybashProgram.sh
./create_sample_files.sh
./backup
./Program.c

6. Finding Files by its inode Number.
Every file has an unique inode number, using that we can identify that file. Create two files with similar name. i.e one file with a space at the end.
# touch "test-file-name"

# touch "test-file-name "
[Note: There is a space at the end]

# ls -1 test*
test-file-name
test-file-name

From the ls output, you cannot identify which file has the space at the end. Using option -i, you can view the inode number of the file, which will be different for these two files.
# ls -i1 test*
16187429 test-file-name
16187430 test-file-name

You can specify inode number on a find command as shown below. In this example, find command renames a file using the inode number.
# find -inum 16187430 -exec mv {} new-test-file-name \;

# ls -i1 *test*
16187430 new-test-file-name
16187429 test-file-name

You can use this technique when you want to do some operation with the files which are named poorly as shown in the example below. For example, the file with name — file?.txt has a special character in it. If you try to execute “rm file?.txt”, all the following three files will get removed. So, follow the steps below to delete only the “file?.txt” file.
# ls
file1.txt file2.txt file?.txt

Find the inode numbers of each file.
# ls -i1
804178 file1.txt
804179 file2.txt
804180 file?.txt

Use the inode number to remove the file that had special character in it as shown below.
# find -inum 804180 -exec rm {} \;

# ls
file1.txt file2.txt
[Note: The file with name "file?.txt" is now removed]

7. Find file based on the File-Permissions
Following operations are possible.
Find files that match exact permission
Check whether the given permission matches, irrespective of other permission bits
Search by giving octal / symbolic representation

For this example, let us assume that the directory contains the following files. Please note that the file-permissions on these files are different.
# ls -l
total 0
-rwxrwxrwx 1 root root 0 2009-02-19 20:31 all_for_all
-rw-r--r-- 1 root root 0 2009-02-19 20:30 everybody_read
---------- 1 root root 0 2009-02-19 20:31 no_for_all
-rw------- 1 root root 0 2009-02-19 20:29 ordinary_file
-rw-r----- 1 root root 0 2009-02-19 20:27 others_can_also_read
----r----- 1 root root 0 2009-02-19 20:27 others_can_only_read

Find files which has read permission to group. Use the following command to find all files that are readable by the world in your home directory, irrespective of other permissions for that file.
# find . -perm -g=r -type f -exec ls -l {} \;
-rw-r--r-- 1 root root 0 2009-02-19 20:30 ./everybody_read
-rwxrwxrwx 1 root root 0 2009-02-19 20:31 ./all_for_all
----r----- 1 root root 0 2009-02-19 20:27 ./others_can_only_read
-rw-r----- 1 root root 0 2009-02-19 20:27 ./others_can_also_read

Find files which has read permission only to group.
# find . -perm g=r -type f -exec ls -l {} \;
----r----- 1 root root 0 2009-02-19 20:27 ./others_can_only_read

Find files which has read permission only to group [ search by octal ]
# find . -perm 040 -type f -exec ls -l {} \;
----r----- 1 root root 0 2009-02-19 20:27 ./others_can_only_read

8. Find all empty files (zero byte file) in your home directory and it’s subdirectory
Most files of the following command output will be lock-files and place holders created by other applications.
# find ~ -empty

List all the empty files only in your home directory.
# find . -maxdepth 1 -empty

List only the non-hidden empty files only in the current directory.
# find . -maxdepth 1 -empty -not -name ".*"

9. Finding the Top 5 Big Files
The following command will display the top 5 largest file in the current directory and it’s subdirectory. This may take a while to execute depending on the total number of files the command has to process.
# find . -type f -exec ls -s {} \; sort -n -r head -5

10. Finding the Top 5 Small Files
Technique is same as finding the bigger files, but the only difference the sort is ascending order.
# find . -type f -exec ls -s {} \; sort -n head -5

In the above command, most probably you will get to see only the ZERO byte files ( empty files ). So, you can use the following command to list the smaller files other than the ZERO byte files.
# find . -not -empty -type f -exec ls -s {} \; sort -n head -5


11. Find Files Based on file-type using option -type

Find only the socket files.
# find . -type s

Find all directories
# find . -type d

Find only the normal files
# find . -type f

Find all the hidden files
# find . -type f -name ".*"

Find all the hidden directories
# find -type d -name ".*"

12. Find files by comparing with the modification time of other file.
Show files which are modified after the specified file. The following find command displays all the files that are created/modified after ordinary_file.
# ls -lrt
total 0
-rw-r----- 1 root root 0 2009-02-19 20:27 others_can_also_read
----r----- 1 root root 0 2009-02-19 20:27 others_can_only_read
-rw------- 1 root root 0 2009-02-19 20:29 ordinary_file
-rw-r--r-- 1 root root 0 2009-02-19 20:30 everybody_read
-rwxrwxrwx 1 root root 0 2009-02-19 20:31 all_for_all
---------- 1 root root 0 2009-02-19 20:31 no_for_all

# find -newer ordinary_file
.
./everybody_read
./all_for_all
./no_for_all
13. Find Files by Size
Using the -size option you can find files by size.

Find files bigger than the given size
# find ~ -size +100M

Find files smaller than the given size
# find ~ -size -100M

Find files that matches the exact given size
# find ~ -size 100M

Note: – means less than the give size, + means more than the given size, and no symbol means exact given size.

14. Create Alias for Frequent Find Operations
If you find some thing as pretty useful, then you can make it as an alias. And execute it whenever you want.

Remove the files named a.out frequently.
# alias rmao="find . -iname a.out -exec rm {} \;"
# rmao

Remove the core files generated by c program.
# alias rmc="find . -iname core -exec rm {} \;"
# rmc


15. Remove big archive files using find command
The following command removes *.zip files that are over 100M.
# find / -type f -name *.zip -size +100M -exec rm -i {} \;"
Remove all *.tar file that are over 100M using the alias rm100m (Remove 100M). Use the similar concepts and create alias like rm1g, rm2g, rm5g to remove file size greater than 1G, 2G and 5G respectively.
# alias rm100m="find / -type f -name *.tar -size +100M -exec rm -i {} \;"
# alias rm1g="find / -type f -name *.tar -size +1G -exec rm -i {} \;"
# alias rm2g="find / -type f -name *.tar -size +2G -exec rm -i {} \;"
# alias rm5g="find / -type f -name *.tar -size +5G -exec rm -i {} \;"

# rm100m
# rm1g
# rm2g
# rm5g