Generally speaking, GDB mainly completes the following four functions:
(1) Start your program and run the program as you wish according to your custom requirements.
(2) The debugged program can be stopped at the breakpoint of the debugging you specified. (Breakpoints can be conditional expressions)
(3) When the program is stopped, you can check what happens in your program at this time.
(4) Dynamically change the execution environment of your program.
1. Introduction
GDB is a powerful UNIX program debugging tool released by the GNU open source organization. If you are making software under the UNIX platform, you will find that GDB debugging tool has more powerful functions than VC and BCB graphical debuggers. At the same time, GDB also has a graphical debugging end such as ddd.
2. Debug C/C++ programs
Directly upload the code
#include<iostream>
using namespace std;
long factorial(int n);
int main()
{
int n(0);
cin>>n;
long val=factorial(n);
cout<<val<<endl;
();
return 0;
}
long factorial(int n)
{
long result(1);
while(n--)
{
result*=n;
}
return result;
}
Compilation
1
g++ -g -Wall -Werror -o main
Start debugging
[root@localhost code]# gdb ./main
GNU gdb (GDB) Red Hat Enterprise Linux (7.2-83.el6)
Copyright (C) 2010 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later </licenses/>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-redhat-linux-gnu".
For bug reporting instructions, please see:
</software/gdb/bugs/>...
Reading symbols from /code/main...done.
(gdb) l
warning: Source file is more recent than executable.
1 #include<iostream>
2 using namespace std;
3 long factorial(int n);
4
5 int main()
6 {
7 int n(0);
8 cin>>n;
9 long val=factorial(n);
10 cout<<val<<endl;
(gdb)
Set break point break linenumber
(gdb) b 9 Breakpoint 1 at 0x80486f9: file , line 9. (gdb) r Starting program: /code/main 4 Breakpoint 1, main () at :9 9 long val=factorial(n);
Setting the watch var
(gdb) s
factorial (n=4) at :17
17 long result(1);
(gdb) l
12 return 0;
13 }
14
15 long factorial(int n)
16 {
17 long result(1);
18 while(n--)
19 {
20 result*=n;
21 }
(gdb) watch n
Hardware watchpoint 2: n
(gdb) watch result
Hardware watchpoint 3: result
(gdb) c
Continuing.
Hardware watchpoint 3: result
Old value = 0
New value = 1
factorial (n=4) at :18
18 while(n--)
(gdb)
Continuing.
Hardware watchpoint 2: n
Old value = 4
New value = 3
0x08048764 in factorial (n=3) at :18
18 while(n--)
(gdb)
Continuing.
Hardware watchpoint 3: result
Old value = 1
New value = 3
factorial (n=3) at :18
18 while(n--)
(gdb)
Continuing.
Hardware watchpoint 2: n
Old value = 3
New value = 2
0x08048764 in factorial (n=2) at :18
18 while(n--)
(gdb)
Continuing.
Hardware watchpoint 3: result
Old value = 3
New value = 6
factorial (n=2) at :18
18 while(n--)
(gdb)
Continuing.
Hardware watchpoint 2: n
Old value = 2
New value = 1
0x08048764 in factorial (n=1) at :18
18 while(n--)
(gdb)
Continuing.
Hardware watchpoint 2: n
Old value = 1
New value = 0
0x08048764 in factorial (n=0) at :18
18 while(n--)
(gdb)
Continuing.
Watchpoint 2 deleted because the program has left the block in
which its expression is valid.
Watchpoint 3 deleted because the program has left the block in
which its expression is valid.
0x08048705 in main () at :9
9 long val=factorial(n);
(gdb) p val
$1 = 11476980
(gdb)
You can see that it was while there, causing n to cross the line, fix
while(n>0) //doesn't let n reach 0
{
result*=n;
n--; //decrements only after the evaluation
}
Some quick commands
l – list
p – print print {variable}
c – continue
s – step
b - break break line_number/break [file_name]:line_number/break [file_name]:func_name
r - run
set <var> = <value>
watch <var>
ENTER: pressing enter key would execute the previously executed command again.
Differences between c/n/s
•c or continue: Debugger will continue executing until the next break point.
•n or next: Debugger will execute the next line as single instruction.
•s or step: Same as next, but does not treats function as a single instruction, instead goes into the function and executes it line by line
3. Debug PHP programs
PHP Code
<?php.
for($i = 0; $i < 10; $i++){
echo $i."\n";
sleep(3);
if(in_array($i,[1,9,20])){
print_r($i*$i);
var_dump($i*$i);
print $i*$i;
}
}
Start debugging, add breakpoints
[root@localhost code]# gdb php GNU gdb (GDB) Red Hat Enterprise Linux (7.2-83.el6) Copyright (C) 2010 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later </licenses/> This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "i686-redhat-linux-gnu". For bug reporting instructions, please see: </software/gdb/bugs/>... Reading symbols from /usr/bin/php...done. (gdb) b zif_sleep Breakpoint 1 at 0x8435180: file /usr/local/src/php-5.5.23/ext/standard/basic_functions.c, line 4449. (gdb) b zif_in_array Breakpoint 2 at 0x8426923: file /usr/local/src/php-5.5.23/ext/standard/, line 1215. (gdb) b zif_print_r Breakpoint 3 at 0x8438273: file /usr/local/src/php-5.5.23/ext/standard/basic_functions.c, line 5553. (gdb) b zif_var_dump Breakpoint 4 at 0x847d296: file /usr/local/src/php-5.5.23/ext/standard/, line 178. (gdb) b zif_printf Function "zif_printf" not defined. Make breakpoint pending on future shared library load? (y or [n]) n (gdb) b zif_sprintf Function "zif_sprintf" not defined. Make breakpoint pending on future shared library load? (y or [n]) n (gdb) b printf Breakpoint 5 at 0x806a390 (gdb) b memcpy Breakpoint 6 at 0x8069390 (gdb) b zif_print Function "zif_print" not defined. Make breakpoint pending on future shared library load? (y or [n]) n (gdb) b zif_echo Function "zif_echo" not defined. Make breakpoint pending on future shared library load? (y or [n]) n (gdb) info b Num Type Disp Enb Address What 1 breakpoint keep y 0x08435180 in zif_sleep at /usr/local/src/php-5.5.23/ext/standard/basic_functions.c:4449 2 breakpoint keep y 0x08426923 in zif_in_array at /usr/local/src/php-5.5.23/ext/standard/:1215 3 breakpoint keep y 0x08438273 in zif_print_r at /usr/local/src/php-5.5.23/ext/standard/basic_functions.c:5553 4 breakpoint keep y 0x0847d296 in zif_var_dump at /usr/local/src/php-5.5.23/ext/standard/:178 5 breakpoint keep y 0x0806a390 <printf@plt> 6 breakpoint keep y 0x08069390 <memcpy@plt> (gdb)
Add a few breakpoints to test syntax: break [file_name]:func_name. Here you can roughly see that echo print and other functions are not functions.
Then start debugging
(gdb) p *return_value
$1 = {value = {lval = 1515870810, dval = 1.7838867517321418e+127, str = {val = 0x5a5a5a5a <Address 0x5a5a5a5a out of bounds>,
len = 1515870810}, ht = 0x5a5a5a5a, obj = {handle = 1515870810, handlers = 0x5a5a5a5a}}, refcount__gc = 1, type = 0 '\000', is_ref__gc = 0 '\000'}
(gdb) p return_value->value
$2 = {lval = 1515870810, dval = 1.7838867517321418e+127, str = {val = 0x5a5a5a5a <Address 0x5a5a5a5a out of bounds>,
len = 1515870810}, ht = 0x5a5a5a5a, obj = {handle = 1515870810, handlers = 0x5a5a5a5a}}
(gdb) p return_value->value->lval
$3 = 1515870810
We can also use the built-in gdbinit for debugging
(gdb) source /usr/local/src/php-5.5.23/.gdbinit (gdb) zbacktrace [0xb7fa1144] sleep(3) /code/:4
View the current stack, the execution process of the PHP kernel
(gdb) bt #0 zif_sleep (ht=1, return_value=0xb7fbd6f0, return_value_ptr=0x0, this_ptr=0x0, return_value_used=0) at /usr/local/src/php-5.5.23/ext/standard/basic_functions.c:4449 #1 0x085f6870 in execute_internal (execute_data_ptr=0xb7fa1144, fci=0x0, return_value_used=0) at /usr/local/src/php-5.5.23/Zend/zend_execute.c:1484 #2 0x085aea5f in dtrace_execute_internal (execute_data_ptr=0xb7fa1144, fci=0x0, return_value_used=0) at /usr/local/src/php-5.5.23/Zend/zend_dtrace.c:97 #3 0x00935c33 in pt_execute_core (internal=1, execute_data=0xb7fa1144, fci=0x0, rvu=0) at /usr/local/src/trace-0.3.0/extension/:941 #4 0x00935e49 in pt_execute_internal (execute_data=0xb7fa1144, fci=0x0, return_value_used=0) at /usr/local/src/trace-0.3.0/extension/:1005 #5 0x085f7523 in zend_do_fcall_common_helper_SPEC (execute_data=0xb7fa1144) at /usr/local/src/php-5.5.23/Zend/zend_vm_execute.h:552 #6 0x085fb2a9 in ZEND_DO_FCALL_SPEC_CONST_HANDLER (execute_data=0xb7fa1144) at /usr/local/src/php-5.5.23/Zend/zend_vm_execute.h:2332 #7 0x085f6deb in execute_ex (execute_data=0xb7fa1144) at /usr/local/src/php-5.5.23/Zend/zend_vm_execute.h:363 #8 0x085ae9dc in dtrace_execute_ex (execute_data=0xb7fa1144) at /usr/local/src/php-5.5.23/Zend/zend_dtrace.c:73 #9 0x00935c5e in pt_execute_core (internal=0, execute_data=0xb7fa1144, fci=0x0, rvu=0) at /usr/local/src/trace-0.3.0/extension/:946 #10 0x00935e10 in pt_execute_ex (execute_data=0xb7fa1144) at /usr/local/src/trace-0.3.0/extension/:1000 #11 0x085f6e4a in zend_execute (op_array=0xb7fbc7b4) at /usr/local/src/php-5.5.23/Zend/zend_vm_execute.h:388 #12 0x085c1cf2 in zend_execute_scripts (type=8, retval=0x0, file_count=3) at /usr/local/src/php-5.5.23/Zend/:1327 #13 0x085470f9 in php_execute_script (primary_file=0xbffff4a4) at /usr/local/src/php-5.5.23/main/:2525 #14 0x0865af46 in do_cli (argc=2, argv=0x8b9b908) at /usr/local/src/php-5.5.23/sapi/cli/php_cli.c:994 #15 0x0865bff3 in main (argc=2, argv=0x8b9b908) at /usr/local/src/php-5.5.23/sapi/cli/php_cli.c:1378
View code snippet
(gdb) l
4444 Delay for a given number of seconds */
4445 PHP_FUNCTION(sleep)
4446 {
4447 long num;
4448
4449 if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "l", &num) == FAILURE) {
4450 RETURN_FALSE;
4451 }
4452 if (num < 0) {
4453 php_error_docref(NULL TSRMLS_CC, E_WARNING, "Number of seconds must be greater than or equal to 0");
(gdb) l 4450
4445 PHP_FUNCTION(sleep)
4446 {
4447 long num;
4448
4449 if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "l", &num) == FAILURE) {
4450 RETURN_FALSE;
4451 }
4452 if (num < 0) {
4453 php_error_docref(NULL TSRMLS_CC, E_WARNING, "Number of seconds must be greater than or equal to 0");
4454 RETURN_FALSE;
(gdb) l zif_usleep
4463 /* }}} */
4464
4465 /* {{{ proto void usleep(int micro_seconds)
4466 Delay for a given number of micro seconds */
4467 PHP_FUNCTION(usleep)
4468 {
4469 #if HAVE_USLEEP
4470 long num;
4471
4472 if (zend_parse_parameters(ZEND_NUM_ARGS() TSRMLS_CC, "l", &num) == FAILURE) {
Continue to execute
(gdb) n
4452 if (num < 0) {
(gdb) p num
$6 = 3
(gdb) n
4457 RETURN_LONG(php_sleep(num));
(gdb) n
4462 }
(gdb) n
execute_internal (execute_data_ptr=0xb7fa1144, fci=0x0, return_value_used=0) at /usr/local/src/php-5.5.23/Zend/zend_execute.c:1488
1488 }
When you arrive at execute_internal, you can check a status of the current function
(gdb) p execute_data_ptr
$7 = (zend_execute_data *) 0xb7fa1144
(gdb) p *execute_data_ptr
$8 = {opline = 0xb7fbcacc, function_state = {function = 0x8bcf3e8, arguments = 0xb7fa119c}, op_array = 0xb7fbc7b4, object = 0x0,
symbol_table = 0x8b99cdc, prev_execute_data = 0x0, old_error_reporting = 0x0, nested = 0 '\000',
original_return_value = 0x38b4ac9, current_scope = 0x49, current_called_scope = 0x45, current_this = 0x0, fast_ret = 0x0,
call_slots = 0xb7fa1188, call = 0xb7fa1188}
(gdb) p *execute_data_ptr->function_state.function->common->function_name
$9 = 115 's'
(gdb) p execute_data_ptr->function_state.function->common->function_name
$10 = 0x8af03c9 "sleep"
(gdb) p execute_data_ptr->op_array->filename
$11 = 0xb7fbc8e8 "/code/"
View the current hashtable
(gdb) p *execute_data_ptr->symbol_table
$ = {nTableSize = , nTableMask = , nNumOfElements = , nNextFreeElement = , pInternalPointer = xbfbc,
pListHead = xbfbc, pListTail = xbfbd, arBuckets = xbfb, pDestructor = xbff <_zval_ptr_dtor_wrapper>,
persistent = '\', nApplyCount = '\', bApplyProtection = '\', inconsistent = }
After continuing to execute output c, press Enter, you can also see the execution information of in_array
(gdb) p *execute_data_ptr->function_state.function
$24 = {type = 1 '\001', common = {type = 1 '\001', function_name = 0x8af1841 "in_array", scope = 0x0, fn_flags = 256,
prototype = 0x0, num_args = 3, required_num_args = 2, arg_info = 0x8ae7554}, op_array = {type = 1 '\001',
function_name = 0x8af1841 "in_array", scope = 0x0, fn_flags = 256, prototype = 0x0, num_args = 3, required_num_args = 2,
arg_info = 0x8ae7554, refcount = 0x842691d, opcodes = 0x8bcf120, last = 0, vars = 0x0, last_var = 0, T = 1,
nested_calls = 3086618796, used_stack = 0, brk_cont_array = 0x0, last_brk_cont = 1, try_catch_array = 0xb7fa10dd,
last_try_catch = 96, has_finally_block = 160 '\240', static_variables = 0x0, this_var = 11482064,
filename = 0xaf1ff4 "|\035\257", line_start = 11482016, line_end = 146381272,
doc_comment = 0xbffff238 "x\362\377\277\244\aY\b\021", doc_comment_len = 10305959, early_binding = 11085989,
literals = 0x8b7a0a0, last_literal = 140062666, run_time_cache = 0xb7fa10d4, last_cache_slot = 90, reserved = {0x9, 0x8b5f7ac,
0x796, 0x0}}, internal_function = {type = 1 '\001', function_name = 0x8af1841 "in_array", scope = 0x0, fn_flags = 256,
prototype = 0x0, num_args = 3, required_num_args = 2, arg_info = 0x8ae7554, handler = 0x842691d <zif_in_array>,
module = 0x8bcf120}}
(gdb) p execute_data_ptr->function_state.function->common->function_name
$26 = 0x8af1841 "in_array"
(gdb) p execute_data_ptr->op_array->filename
$27 = 0xb7fbc8e8 "/code/"
You can also add monitoring watch, set some debug variables, etc.
Other debugging tools include strace viewing system calls, ltrace viewing class library calls, and vld viewing opcode.
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