Am I Doomed ?
System administrators, diagnosticians and trouble-shooters will find it invaluable for solving problems with programs for which the source is not readily available since they do not need to be recompiled in order to trace them. Students, hackers and the overly-curious will find that a great deal can be learned about a system and its system calls by tracing even ordinary programs. And programmers will find that since system calls and signals are events that happen at the user/kernel interface, a close examination of this boundary is very useful for bug isolation, sanity checking and attempting to capture race conditions.A system call is the most common way programs communicate with the kernel. System calls include reading and writing data, opening and closing files and all kinds of network communication. Under Linux, a system call is done by calling a special interrupt with the number of the system call and its parameters stored in the CPU's registers.
strace program-nameFor example let us trace uname command.
$strace uname
execve("/bin/uname", ["uname"], [/* 23 vars */]) = 0
brk(0) = 0x871d000
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7f30000
access("/etc/ld.so.preload", R_OK) = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=59428, ...}) = 0
mmap2(NULL, 59428, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb7f21000
close(3) = 0
open("/lib/libc.so.6", O_RDONLY) = 3
read(3, "\177ELF\1\1\1\0\0\0\0\0\0\0\0\0\3\0\3\0\1\0\0\0p\220\272"..., 512) = 512
fstat64(3, {st_mode=S_IFREG|0755, st_size=1577052, ...}) = 0
mmap2(0x45b93000, 1295780, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x45b93000
mmap2(0x45cca000, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x137) = 0x45cca000
mmap2(0x45ccd000, 9636, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x45ccd000
close(3) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7f20000
set_thread_area({entry_number:-1 -> 6, base_addr:0xb7f206c0, limit:1048575, seg_32bit:1, contents:0, read_exec_only:0, limit_in_pages:1, seg_not_present:0, useable:1}) = 0
mprotect(0x45cca000, 8192, PROT_READ) = 0
mprotect(0x451dd000, 4096, PROT_READ) = 0
munmap(0xb7f21000, 59428) = 0
brk(0) = 0x871d000
brk(0x873e000) = 0x873e000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_LARGEFILE) = 3
fstat64(3, {st_mode=S_IFREG|0644, st_size=55511552, ...}) = 0
mmap2(NULL, 2097152, PROT_READ, MAP_PRIVATE, 3, 0) = 0xb7d20000
close(3) = 0
uname({sys="Linux", node="localhost.localdomain", ...}) = 0
fstat64(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 1), ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0xb7f2f000
write(1, "Linux\n", 6Linux
) = 6
close(1) = 0
munmap(0xb7f2f000, 4096) = 0
exit_group(0) = ?
Process 7477 detached
Even though output from strace looks very complicated, but this is only due to many system calls made when loading shared libraries. However, once we have found which system calls are the important ones (mainly open, read, write and the like), the results will look fairly intuitive to us.#include <stdio.h>
#include <unistd.h>
int main()
{
sleep(20);
return 0;
}
We will compile the above code and run it as a background process. Then using its process id and -p flag we will run strace on it.$ gcc main.c $ ./a.out & [1] 7609 $ strace -p 7609 Process 7609 attached - interrupt to quit restart_syscall(<... resuming interrupted call ...>) = 0 exit_group(0) = ? Process 7609 detached [1]+ Done ./a.outUsing -p flag we even debug daemon processes. In contrast to a debugger, strace does not need a program's source code to produce human-readable output.
$ strace -o uname_output unameIf you use the -t option, then strace will prefix each line of the trace with the time of day. We can even specify the system call functions to trace using the -e option. For example, to trace only open() and close() function system calls use the following command:
$ strace -o uname_output -e trace=open,close uname
double mean(double a, double b)
{
return (a+b) / 2;
}
calc_mean.hdouble mean(double, double);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;
}
Creating the static librarygcc -c calc_mean.c -o calc_mean.oThen, using archiver (ar) we produce a static library (named libmean.a) out of the object file calc_mean.o.
ar rcs libmean.a calc_mean.oNOTE: A static library must start with the three letters 'lib' and have the suffix '.a'.
gcc -static main.c -L. -lmean -o mainNow run the executable program 'main'
$./main
ctags *.c *.hWhen 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,./*/tagsThis finds a tags file in the same directory as the current file, one directory level higher and in all subdirectories.
$ cd /proj-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.
$ ctags -R *
:set tags=~/proj/tags3. Start using ctags with VIM editor
vi -t function_nameThis will find the correct file or list of files having function_name definition.
:tag function_nameThis will jump automatically from the current file to the file containing the function_name definition.
or
:ta function_name
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 + tThis command will jump back to previous location.
ctrl + iTo travel forward through the tag history.
ctrl + oTo travel backward through the tag history.
:tagsShows tag stack (history).
:stag function_nameCursor command for above feature is
ctrl + w + ]Auto Complete
:tag start-of-tag-name_TABJump to a tag name found by a search.
:tag /search-stringWhen 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-nameJumps to next matching tag.
:tnextJump to previous matching tag.
or
:tn
:tpreviousJump to first matching tag.
or
:tp
:tfirstJump to last matching tag.
or
:tf
or
:trewindor:tr
:tlastFinding global identifiers
or
:tl
[ + shiftkey + iVim will list the matching lines it can find. Not only in the current file,
The advantage over using tags or the preview window is that included files are
structs.h
1: 29 unsigned column; /* column number */
export LD_LIBRARY_PATH=/home/cf/libIf 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_PATHNow recompile the main program
gcc -o test main.c -lcalc_mean -L/home/cf/slibNow 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
$./testLD_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.
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.
#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;
}
gcc -c -fPIC calc_mean.c -o calc_mean.oAbove we are compiling calc_mean.c with -fPIC option and generating calc_mean.o object file.
gcc -shared -o libcalc_mean.so calc_mean.oAbove command on successful produces a shared library named "libcalc_mean.so".
$ gcc -o test main.c -lcalc_mean /usr/bin/ld: cannot find -lcalc_mean collect2: ld returned 1 exit statusThe linker doesn’t know where to find libcalc_mean. But why ?
gcc -o test main.c -lcalc_mean -L/home/cf/slib
mv libcalc_mean.so /home/cf/slibNow compile main.c. It would be successful and creates an executable named "test".
$ 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.
$ find / -name 'filename' 2>/dev/null/ - Start searching from the root directory (i.e / directory)
$ find / -name 'filename' 2>errors.txt
$ 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.
$find . -name 'temp'Where d - directory, p - named pipe (FIFO), f - regular file and so on
./keepout/temp
./temp
$find . -name 'temp' -type d
./temp
$ 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.
$ find /home/songs -name '*.mp3' -size -5000kFirst command finds within a directory called /home/songs, only those mp3 files that have a size less than 5000 Kilobytes.
$ find / -size +10000k
$ find /home/temp -amin -10 -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.
$ find /home/temp -atime -2 -name '*.c'
$ find /home/temp -mmin -10 -name '*.c'
$ find /home/temp -mtime -2 -name '*.c'
$ 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 +10000kBoolean operators such as AND, OR and NOT make find an extremely useful tool.
$ find /home/songs -size +10000k ! -name "Metallica*"
$ find /home/songs -name 'Metallica*' -or -size +10000k
$ find . -name '*.sh' -exec chmod o+r '{}' \; -printThis 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.$ find . -exec grep "hello" '{}' \; -printPrints all files that contain the string 'hello' will have their path printed to standard output.find . -exec grep -q "hello" '{}' \; -printFinding Empty files (-empty)$find . -emptyTo delete empty files in the current directory:
$ find . -empty -maxdepth 1 -exec rm '{}' \;For more examples try out
#include <stdio.h>
#include <string.h>
#define BUF_SIZE 6
#define STRING_SIZE 4
/*
* void getStringStdin(char *, int , int );
*
* 1: BUF :Pointer to the array of characters where input string
is to be stored.
* 2: BUF_LEN :Is the length of the array of characters where the
string is stored.buffer where we save the string.
* 3: STRING_LEN :Is the length of the string.
*
* NOTE: STRING_LEN < BUF_LEN
*
*/
getStringStdin(char *buf, int buf_len, int str_len)
{
int ch, len;
char *s;
if(str_len>=buf_len)
len=buf_len-1;
else
len=str_len;
printf ("\nEnter string of length %d(Remaining is ignored): ",len);
if( (fgets(buf, len+1, stdin)) != NULL )
{
s=my_strchr(buf,'\n');
if(s!=NULL)
{
*s='\0';
}
else
{
while ((ch = getchar()) != '\n' && ch != EOF);
}
}
}
int main(void)
{
int i=0;
char buf[BUF_SIZE];
do
{
getString(buf, BUF_SIZE, STRING_SIZE);
printf ("\nString : %s\n", buf);
i++;
}while(i<2);
return 0;
}
cat /etc/passwd
cat /etc/passwd | grep "/home"
cat /etc/passwd | grep "/home" |cut -d: -f1Now what we have done is that we have piped the output of previous command to another variable "cut"
cat /etc/passwd | grep "/home" |cut -d: -f1This works until all your users have their home share in /home. If you have defined their home share to some other destination. Modify the above command accordingly.
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<pre name="code" class="cpp"> ...Your html-escaped code goes here... </pre>
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<b:else/>
<title><data:blog.pageName/> ~ <data:blog.title/></title>
</b:if>
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<b:widget id='Label10' locked='false' title='' type='Label'>
<b:includable id='main'>
<div id='tabsF'>
<ul>
<li><a expr:href='data:blog.homepageUrl'><span>Home</span></a></li>
<b:loop values='data:labels' var='label'>
<li><a expr:href='data:label.url'><span><data:label.name/></span></a></li>
</b:loop>
</ul>
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</div>
</b:includable>
</b:widget>
<b:widget id='Label10' locked='false' title='' type='Label'>
<b:includable id='main'>
<div id='tabsF'>
<ul>
<li><a expr:href='data:blog.homepageUrl'><span>Home</span></a></li>
<b:loop values='data:labels' var='label'>
<!--I am adding the label - Linux in this code -->
<b:if cond='data:label.name == "Linux"'>
<a expr:href='data:label.url'>
<span>
<data:label.name/>
</span>
</a>
</b:if>
<!--I am adding the label - Opensource in this code -->
<b:if cond='data:label.name == "Open Source"'>
<a expr:href='data:label.url'>
<span>
<data:label.name/>
</span>
</a>
</b:if>
</b:loop>
</ul>
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</b:includable>
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float:left;
display:block;
background:url("http://www.site.com/tabright.gif") no-repeat right top;
padding:5px 15px 4px 6px;
color:#666;
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color:#FF9834;
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background-position:0% -42px;
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/*- Menu Tabs B--------------------------- */
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/* Commented Backslash Hack hides rule from IE5-Mac \*/
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color:#000;
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#tabsB a:hover {
background-position:0% -42px;
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background-position:100% -42px;
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/*- Menu Tabs C--------------------------- */
#tabsC {
float:left;
width:100%;
background:#EDF7E7;
font-size:93%;
line-height:normal;
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/* Commented Backslash Hack hides rule from IE5-Mac \*/
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#tabsC a:hover span {
color:#FFF;
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#tabsC a:hover {
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/*- Menu Tabs D--------------------------- */
#tabsD {
float:left;
width:100%;
background:#FCF3F8;
font-size:93%;
line-height:normal;
border-bottom:1px solid #F4B7D6;
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#tabsF a span {float:none;}
/* End IE5-Mac hack */
#tabsF a:hover span {
color:#FFF;
}
#tabsF a:hover {
background-position:0% -42px;
}
#tabsF a:hover span {
background-position:100% -42px;
}
/*- Menu Tabs G--------------------------- */
#tabsG {
float:left;
width:100%;
background:#666;
font-size:93%;
line-height:normal;
}
#tabsG ul {
margin:0;
padding:10px 10px 0 50px;
list-style:none;
}
#tabsG li {
display:inline;
margin:0;
padding:0;
}
#tabsG a {
float:left;
background:url("http://www.site.com/tableftG.gif") no-repeat left top;
margin:0;
padding:0 0 0 4px;
text-decoration:none;
}
#tabsG a span {
float:left;
display:block;
background:url("http://www.site.com/tabrightG.gif") no-repeat right top;
padding:5px 15px 4px 6px;
color:#FFF;
}
/* Commented Backslash Hack hides rule from IE5-Mac \*/
#tabsG a span {float:none;}
/* End IE5-Mac hack */
#tabsG a:hover span {
color:#FFF;
}
#tabsG a:hover {
background-position:0% -42px;
}
#tabsG a:hover span {
background-position:100% -42px;
}
/*- Menu Tabs H--------------------------- */
#tabsH {
float:left;
width:100%;
background:#000;
font-size:93%;
line-height:normal;
}
#tabsH ul {
margin:0;
padding:10px 10px 0 50px;
list-style:none;
}
#tabsH li {
display:inline;
margin:0;
padding:0;
}
#tabsH a {
float:left;
background:url("http://www.site.com/tableftH.gif") no-repeat left top;
margin:0;
padding:0 0 0 4px;
text-decoration:none;
}
#tabsH a span {
float:left;
display:block;
background:url("http://www.site.com/tabrightH.gif") no-repeat right top;
padding:5px 15px 4px 6px;
color:#FFF;
}
/* Commented Backslash Hack hides rule from IE5-Mac \*/
#tabsH a span {float:none;}
/* End IE5-Mac hack */
#tabsH a:hover span {
color:#FFF;
}
#tabsH a:hover {
background-position:0% -42px;
}
#tabsH a:hover span {
background-position:100% -42px;
}
/*- Menu Tabs I--------------------------- */
#tabsI {
float:left;
width:100%;
background:#EFF4FA;
font-size:93%;
line-height:normal;
border-bottom:1px solid #DD740B;
}
#tabsI ul {
margin:0;
padding:10px 10px 0 50px;
list-style:none;
}
#tabsI li {
display:inline;
margin:0;
padding:0;
}
#tabsI a {
float:left;
background:url("http://www.site.com/tableftI.gif") no-repeat left top;
margin:0;
padding:0 0 0 5px;
text-decoration:none;
}
#tabsI a span {
float:left;
display:block;
background:url("http://www.site.com/tabrightI.gif") no-repeat right top;
padding:5px 15px 4px 6px;
color:#FFF;
}
/* Commented Backslash Hack hides rule from IE5-Mac \*/
#tabsI a span {float:none;}
/* End IE5-Mac hack */
#tabsI a:hover span {
color:#FFF;
}
#tabsI a:hover {
background-position:0% -42px;
}
#tabsI a:hover span {
background-position:100% -42px;
}
/*- Menu Tabs J--------------------------- */
#tabsJ {
float:left;
width:100%;
background:#F4F4F4;
font-size:93%;
line-height:normal;
border-bottom:1px solid #24618E;
}
#tabsJ ul {
margin:0;
padding:10px 10px 0 50px;
list-style:none;
}
#tabsJ li {
display:inline;
margin:0;
padding:0;
}
#tabsJ a {
float:left;
background:url("http://www.site.com/tableftJ.gif") no-repeat left top;
margin:0;
padding:0 0 0 5px;
text-decoration:none;
}
#tabsJ a span {
float:left;
display:block;
background:url("http://www.site.com/tabrightJ.gif") no-repeat right top;
padding:5px 15px 4px 6px;
color:#24618E;
}
/* Commented Backslash Hack hides rule from IE5-Mac \*/
#tabsJ a span {float:none;}
/* End IE5-Mac hack */
#tabsJ a:hover span {
color:#FFF;
}
#tabsJ a:hover {
background-position:0% -42px;
}
#tabsJ a:hover span {
background-position:100% -42px;
}
/*- Menu Tabs K--------------------------- */
#tabsK {
float:left;
width:100%;
background:#E7E5E2;
font-size:93%;
line-height:normal;
border-bottom:1px solid #54545C;
}
#tabsK ul {
margin:0;
padding:10px 10px 0 50px;
list-style:none;
}
#tabsK li {
display:inline;
margin:0;
padding:0;
}
#tabsK a {
float:left;
background:url("http://www.site.com/tableftK.gif") no-repeat left top;
margin:0;
padding:0 0 0 4px;
text-decoration:none;
}
#tabsK a span {
float:left;
display:block;
background:url("http://www.site.com/tabrightK.gif") no-repeat right top;
padding:5px 15px 4px 6px;
color:#FFF;
}
/* Commented Backslash Hack hides rule from IE5-Mac \*/
#tabsK a span {float:none;}
/* End IE5-Mac hack */
#tabsK a:hover span {
color:#FFF;
background-position:100% -42px;
}
#tabsK a:hover {
background-position:0% -42px;
}
#tabsK a:hover span {
background-position:100% -42px;
}
Now you can see three tabs namely Posting, Settings, Layout. Depending upon your template you will be having different options under Layout tab and I got Page Elements, Fonts & colors, Edit HTML, Pick New Template.
Before you modify the HTML of the BLOGGER template remember that you have to backup the existing template code so that even if any problem occurs because of modification of HTML code we can revert back to previously working HTML code. Click the "Download Full Template" link to backup the existing template code. The backup is saved as a .xml file. If you have to revert back to previously existing code which is saved in your harddisk just upload it by clicking the upload button.
Once modifying the code present in the HTML editor we have to save the template or preview it using the appropriate button shown in above figure. Even if anything goes wrong we can revert back to the previous code as we already performed the backup of our previous code.