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From : http://eli.thegreenplace.net/2011/01/23/how-debuggers-work-part-1
1		From : http://eli.thegreenplace.net/2011/01/23/how-debuggers-work-part-1
2
3		#include<stdio.h>
4		#include<sys/reg.h>
5		#include<sys/types.h>
6		#include<sys/ptrace.h>
7		#include<sys/wait.h>
8		#include<unistd.h>
9		#include<sys/user.h>
10		#include<sys/syscall.h>
11
12		void run_debugger(pid_t child_pid);
13		void run_target(const char* programname);
14		int main(int argc, char** argv)
15		{
16		pid_t child_pid;
17
18		if (argc < 2) {
19		printf("Expected a program name as argument\n");
20		return -1;
21		}
22
23		child_pid = fork();
24		if (child_pid == 0)
25		run_target(argv[1]);
26		else if (child_pid > 0)
27		run_debugger(child_pid);
28		else {
29		printf("fork");
30		return -1;
31		}
32
33		return 0;
34		}
35		void run_target(const char* programname)
36		{
37		printf("target started.#include<stdio.h>
38		#include<sys/reg.h>
39		#include<sys/types.h>
40		#include<sys/ptrace.h>
41		#include<sys/wait.h>
42		#include<unistd.h>
43		#include<sys/user.h>
44		#include<sys/syscall.h>
45
46		void run_debugger(pid_t child_pid);
47		void run_target(const char* programname);
48		int main(int argc, char** argv)
49		{
50		pid_t child_pid;
51
52		if (argc < 2) {
53		printf("Expected a program name as argument\n");
54		return -1;
55		}
56
57		child_pid = fork();
58		if (child_pid == 0)
59		run_target(argv[1]);
60		else if (child_pid > 0)
61		run_debugger(child_pid);
62		else {
63		printf("fork");
64		return -1;
65		}
66
67		return 0;
68		}
69		void run_target(const char* programname)
70		{
71		printf("target started.#include<stdio.h>
72		#include<sys/reg.h>
73		#include<sys/types.h>
74		#include<sys/ptrace.h>
75		#include<sys/wait.h>
76		#include<unistd.h>
77		#include<sys/user.h>
78		#include<sys/syscall.h>
79
80		void run_debugger(pid_t child_pid);
81		void run_target(const char* programname);
82		int main(int argc, char** argv)
83		{
84		pid_t child_pid;
85
86		if (argc < 2) {
87		printf("Expected a program name as argument\n");
88		return -1;
89		}
90
91		child_pid = fork();
92		if (child_pid == 0)
93		run_target(argv[1]);
94		else if (child_pid > 0)
95		run_debugger(child_pid);
96		else {
97		printf("fork");
98		return -1;
99		}
100
101		return 0;
102		}
103		void run_target(const char* programname)
104		{
105		printf("target started.will run '%s'\n", programname);
106
107		/* Allow tracing of this process */
108		if (ptrace(PTRACE_TRACEME, 0, 0, 0) < 0) {
109		printf("ptrace");
110		return;
111		}
112		printf("replacing process image");
113		/* Replace this process's image with the given program */
114		execl(programname, programname, NULL);
115		}
116		void run_debugger(pid_t child_pid)
117		{
118		int wait_status;
119		unsigned icounter = 0;
120		printf("debugger started\n");
121
122		/* Wait for child to stop on its first instruction */
123		wait(&wait_status);
124
125		while (WIFSTOPPED(wait_status)) {
126		icounter++;
127		struct user_regs_struct regs;
128		ptrace(PTRACE_GETREGS, child_pid, 0, &regs);
129		unsigned instr = ptrace(PTRACE_PEEKTEXT, child_pid, regs.rip, 0);
130
131		/*printf("icounter = %u. EIP = 0x%08lld. instr = 0x%08x\n",
132		icounter, regs.rip, instr);*/
133
134		/* Make the child execute another instruction */
135		if (ptrace(PTRACE_SINGLESTEP, child_pid, 0, 0) < 0) {
136		printf("ptrace");
137		return;
138		}
139
140		/* Wait for child to stop on its next instruction */
141		wait(&wait_status);
142		}
143
144		printf("the child executed %u instructions\n", icounter);
145		}