From 5e33085491e2cfe660f35f87c4fa320813c352f8 Mon Sep 17 00:00:00 2001
From: Blau Araujo <blau@debxp.org>
Date: Mon, 28 Apr 2025 13:08:08 -0300
Subject: [PATCH] =?UTF-8?q?conte=C3=BAdo=20das=20partes=204=20e=205?=
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

---
 mods/04/README.org            |   5 +
 mods/05/README.org            | 231 ++++++++++++++++++++++++++++++++++
 mods/05/exemplos/.gdb_history |  72 +++++++++++
 mods/05/exemplos/iloop        | Bin 0 -> 17128 bytes
 mods/05/exemplos/iloop.c      |  12 ++
 mods/05/exemplos/notinit      | Bin 0 -> 17256 bytes
 mods/05/exemplos/notinit.c    |  13 ++
 mods/05/exemplos/segfault     | Bin 0 -> 17144 bytes
 mods/05/exemplos/segfault.c   |   8 ++
 9 files changed, 341 insertions(+)
 create mode 100644 mods/05/README.org
 create mode 100644 mods/05/exemplos/.gdb_history
 create mode 100755 mods/05/exemplos/iloop
 create mode 100644 mods/05/exemplos/iloop.c
 create mode 100755 mods/05/exemplos/notinit
 create mode 100644 mods/05/exemplos/notinit.c
 create mode 100755 mods/05/exemplos/segfault
 create mode 100644 mods/05/exemplos/segfault.c

diff --git a/mods/04/README.org b/mods/04/README.org
index b07a541..9a2668b 100644
--- a/mods/04/README.org
+++ b/mods/04/README.org
@@ -4,6 +4,11 @@
 
 * 4. Aplicações em baixo e alto nível
 
+** Objetivo
+
+Conhecer os recursos do GDB que podem auxiliar no desenvolvimento e
+depuração de programas em assembly e em C/C++. 
+
 ** O GDB e a programação em baixo nível
 
 O GDB permite observar e controlar a execução de programas no nível mais próximo
diff --git a/mods/05/README.org b/mods/05/README.org
new file mode 100644
index 0000000..4d4a1dd
--- /dev/null
+++ b/mods/05/README.org
@@ -0,0 +1,231 @@
+#+title: Curso prático de introdução ao GDB
+#+author: Blau Araujo
+#+email: blau@debxp.org
+
+* 5. Casos de uso
+
+** Objetivo
+
+Demonstrar o uso do GDB para localizar e solucionar bugs comuns, como:
+
+- Falha de segmentação;
+- Variável não inicializada;
+- Loop infinito;
+- Índice de vetor inválido;
+- Parâmetro incorreto na chamada de uma função;
+- Registradores incorretos em uma chamada de sistema;
+- Desalinhamento da pilha;
+- Erro de empilhamento e desempilhamento de registros;
+- Estouro de buffer;
+- Estouro de pilha;
+- Vazamento de memória.
+
+** Falha de segmentação
+
+Acesso inválido à memória causado por um ponteiro não inicializado.
+
+Código (=segfault.c=):
+
+#+begin_src c
+#include <stdio.h>
+
+int main() {
+    int *p = NULL;
+    *p = 42;  // ERRO: tentativa de escrever em um ponteiro nulo
+    printf("%d\n", *p);
+    return 0;
+}
+#+end_src
+
+Compilação:
+
+#+begin_example
+:~$ gcc -g -o segfault segfault.c
+$ gdb ./segfault
+Reading symbols from ./segfault...
+#+end_example
+
+*** Processo de depuração
+
+Verificar o que acontece ao executar:
+
+#+begin_example
+(gdb) r
+Starting program: /home/blau/git/gdb-pratico/mods/05/exemplos/segfault
+[Thread debugging using libthread_db enabled]
+Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
+
+Program received signal SIGSEGV, Segmentation fault.
+0x000055555555514d in main () at segfault.c:5
+5	    *p = 42;  // ERRO: tentativa de escrever em um ponteiro nulo
+#+end_example
+
+Matando o processo:
+
+#+begin_example
+(gdb) k
+[Inferior 1 (process 1034793) killed]
+#+end_example
+
+Definindo =main= como ponto de parada e executando:
+
+#+begin_example
+(gdb) b main
+Breakpoint 5 at 0x555555555141: file segfault.c, line 4.
+(gdb) r
+Starting program: /home/blau/git/gdb-pratico/mods/05/exemplos/segfault
+[Thread debugging using libthread_db enabled]
+Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
+
+Breakpoint 5, main () at segfault.c:4
+4	    int *p = NULL;
+#+end_example
+
+Listando variáveis locais antes e após a inicialização do ponteiro =p=:
+
+#+begin_example
+(gdb) i locals
+p = 0x7fffffffdf40
+(gdb) n
+5	    *p = 42;  // ERRO: tentativa de escrever em um ponteiro nulo
+(gdb) i locals
+p = 0x0
+#+end_example
+
+O endereço de memória no ponteiro é claramente nulo (=0x0=)!
+
+** Variável não inicializada
+
+Uso de uma variável antes da atribuição de um valor (seu valor é lixo de memória).
+
+Código do exemplo (=notinit.c=):
+
+#+begin_src c
+#include <stdio.h>
+
+int main(void) {
+
+    int a;  // PROBLEMA: Variável não inicializada!
+
+    printf("Digite um número: ");
+    scanf("%d", &a);  // Se scanf falhar, o valor de a será lixo!
+    
+    printf("Depois de %d vem %d\n", a, a + 1);
+
+    return 0;
+}
+#+end_src
+
+Compilação e teste:
+
+#+begin_example
+:~$ gcc -g -o notinit notinit.c
+:~$ ./notinit
+Digite um número: (teclar CTRL+D) Depois de 32765 vem 32766
+#+end_example
+
+*** Processo de depuração
+
+Basta executar o programa passo a passo examinando o valor da variável =a=:
+
+#+begin_example
+:~$ gdb ./notinit
+Reading symbols from ./notinit...
+(gdb) b main
+Breakpoint 1 at 0x1151: file notinit.c, line 7.
+(gdb) r
+Starting program: /home/blau/git/gdb-pratico/mods/05/exemplos/notinit
+[Thread debugging using libthread_db enabled]
+Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
+
+Breakpoint 1, main () at notinit.c:7
+7	    printf("Digite um número: ");
+(gdb) p a
+$1 = 32767
+(gdb) n
+8	    scanf("%d", &a);  // Se scanf falhar, o valor de a será lixo!
+(gdb) n
+Digite um número: 10	    printf("Depois de %d vem %d\n", a, a + 1);
+(gdb) p a
+$2 = 32767
+(gdb) n
+Depois de 32767 vem 32768
+12	    return 0;
+(gdb) c
+Continuing.
+[Inferior 1 (process 1039355) exited normally]
+#+end_example
+
+Com o erro na execução de =scanf= (simulado com o atalho =CTRL+D=), o valor
+da variável não inicializada continuou sendo lixo de memória (32767).
+
+** Loop infinito
+
+Uma condição de parada incorreta faz com que as repetições não tenham fim.
+
+Código de exemplo (=iloop.c=):
+
+#+begin_src c
+#include <stdio.h>
+
+int main() {
+    int i = 0;
+
+    // A condição de parada nunca será atendida!
+    while (i < 100) { 
+        printf("i = %d\n", i);
+        i *= 2;  // Sempre zero!
+    }
+    return 0;
+}
+#+end_src
+
+Compilação e teste:
+
+#+begin_example
+:~$ gcc -g -o iloop iloop.c
+:~$ ./iloop
+i = 0
+i = 0
+i = 0
+i = 0
+i = 0
+i = 0
+i = 0
+i = 0
+^C
+#+end_example
+
+*** Processo de depuração
+
+#+begin_example
+$ gdb ./iloop
+Reading symbols from ./iloop...
+(gdb) b main
+Breakpoint 1 at 0x1141: file iloop.c, line 4.
+(gdb) r
+Starting program: /home/blau/git/gdb-pratico/mods/05/exemplos/iloop
+[Thread debugging using libthread_db enabled]
+Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
+
+Breakpoint 1, main () at iloop.c:4
+4	    int i = 0;
+(gdb) n
+7	    while (i < 100) {
+(gdb) p i
+$1 = 0
+(gdb) n
+8	        printf("i = %d\n", i);
+(gdb) n
+i = 0
+9	        i *= 2;  // Sempre zero!
+(gdb) p i
+$2 = 0
+(gdb) n
+7	    while (i < 100) {
+(gdb) p i
+$3 = 0
+#+end_example
+
+Fica claro que o valor de nunca foi alterado, porque ~2 × 0 = 0~!
+
diff --git a/mods/05/exemplos/.gdb_history b/mods/05/exemplos/.gdb_history
new file mode 100644
index 0000000..1cf4b08
--- /dev/null
+++ b/mods/05/exemplos/.gdb_history
@@ -0,0 +1,72 @@
+r
+q
+r
+q
+r
+bt
+b main
+l
+b 5
+r
+info locals 
+k
+r
+info locals 
+c
+del
+l
+b main
+info locals 
+n
+info locals 
+c
+b main
+del breakpoints 
+i breakpoints 
+b main
+r
+bt
+i locals 
+n
+i locals 
+info registers 
+q
+b main
+p a
+r
+p a
+k
+q
+b main
+p a
+r
+p a
+n
+n
+p a
+n
+c
+k
+q
+b main
+r
+n
+n
+p i
+n
+p i
+c
+k
+del breakpoints 
+q
+b main
+r
+n
+p i
+n
+n
+p 1
+p i
+n
+p i
+q
diff --git a/mods/05/exemplos/iloop b/mods/05/exemplos/iloop
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diff --git a/mods/05/exemplos/iloop.c b/mods/05/exemplos/iloop.c
new file mode 100644
index 0000000..a80a563
--- /dev/null
+++ b/mods/05/exemplos/iloop.c
@@ -0,0 +1,12 @@
+#include <stdio.h>
+
+int main() {
+    int i = 0;
+
+    // A condição de parada nunca será atendida!
+    while (i < 100) { 
+        printf("i = %d\n", i);
+        i *= 2;  // Sempre zero!
+    }
+    return 0;
+}
diff --git a/mods/05/exemplos/notinit b/mods/05/exemplos/notinit
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HcmV?d00001

diff --git a/mods/05/exemplos/notinit.c b/mods/05/exemplos/notinit.c
new file mode 100644
index 0000000..f812b7e
--- /dev/null
+++ b/mods/05/exemplos/notinit.c
@@ -0,0 +1,13 @@
+#include <stdio.h>
+
+int main(void) {
+
+    int a;  // PROBLEMA: Variável não inicializada!
+
+    printf("Digite um número: ");
+    scanf("%d", &a);  // Se scanf falhar, o valor de a será lixo!
+    
+    printf("Depois de %d vem %d\n", a, a + 1);
+
+    return 0;
+}
diff --git a/mods/05/exemplos/segfault b/mods/05/exemplos/segfault
new file mode 100755
index 0000000000000000000000000000000000000000..812cb73f481f1f6ec0834465605702cfbdca4e01
GIT binary patch
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literal 0
HcmV?d00001

diff --git a/mods/05/exemplos/segfault.c b/mods/05/exemplos/segfault.c
new file mode 100644
index 0000000..e1adedc
--- /dev/null
+++ b/mods/05/exemplos/segfault.c
@@ -0,0 +1,8 @@
+#include <stdio.h>
+
+int main() {
+    int *p = NULL;  // NULL = (void *)0 - Ponteiro nulo
+    *p = 42;  // ERRO: tentativa de escrever em um ponteiro nulo
+    printf("%d\n", *p);
+    return 0;
+}