## Exercise 17
### 原始
```c
#include <stdio.h>
#include <assert.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#define MAX_DATA 512
#define MAX_ROWS 100
struct Address {
int id;
int set;
char name[MAX_DATA];
char email[MAX_DATA];
};
struct Database {
struct Address rows[MAX_ROWS];
};
struct Connection {
FILE *file;
struct Database *db;
};
void die(const char *message)
{
if (errno) {
perror(message);
} else {
printf("ERROR: %s\n", message);
}
exit(1);
}
void Address_print(struct Address *addr)
{
printf("%d %s %s\n", addr->id, addr->name, addr->email);
}
void Database_load(struct Connection *conn)
{
int rc = fread(conn->db, sizeof(struct Database), 1, conn->file);
if (rc != 1)
die("Failed to load database.");
}
struct Connection *Database_open(const char *filename, char mode)
{
struct Connection *conn = malloc(sizeof(struct Connection));
if (!conn)
die("Memory error");
conn->db = malloc(sizeof(struct Database));
if (!conn->db)
die("Memory error");
if (mode == 'c') {
conn->file = fopen(filename, "w");
} else {
conn->file = fopen(filename, "r+");
if (conn->file) {
Database_load(conn);
}
}
if (!conn->file)
die("Failed to open the file");
return conn;
}
void Database_close(struct Connection *conn)
{
if (conn) {
if (conn->file)
fclose(conn->file);
if (conn->db)
free(conn->db);
free(conn);
}
}
void Database_write(struct Connection *conn)
{
rewind(conn->file);
int rc = fwrite(conn->db, sizeof(struct Database), 1, conn->file);
if (rc != 1)
die("Failed to write database.");
rc = fflush(conn->file);
if (rc == -1)
die("Cannot flush database.");
}
void Database_create(struct Connection *conn)
{
int i = 0;
for (i = 0; i < MAX_ROWS; i++) {
// make a prototype to initialize it
struct Address addr = {.id = i, .set = 0};
// then just assign it
conn->db->rows[i] = addr;
}
}
void Database_set(struct Connection *conn, int id, const char *name,
const char *email)
{
struct Address *addr = &conn->db->rows[id];
if (addr->set)
die("Already set, delete it first");
addr->set = 1;
// WARNING: bug, read the "How To Break It" and fix this
char *res = strncpy(addr->name, name, MAX_DATA);
// demonstrate the strncpy bug
if (!res)
die("Name copy failed");
res = strncpy(addr->email, email, MAX_DATA);
if (!res)
die("Email copy failed");
}
void Database_get(struct Connection *conn, int id)
{
struct Address *addr = &conn->db->rows[id];
if (addr->set) {
Address_print(addr);
} else {
die("ID is not set");
}
}
void Database_delete(struct Connection *conn, int id)
{
struct Address addr = {.id = id, .set = 0};
conn->db->rows[id] = addr;
}
void Database_list(struct Connection *conn)
{
int i = 0;
struct Database *db = conn->db;
for (i = 0; i < MAX_ROWS; i++) {
struct Address *cur = &db->rows[i];
if (cur->set) {
Address_print(cur);
}
}
}
int main(int argc, char *argv[])
{
if (argc < 3)
die("USAGE: ex17 <dbfile> <action> [action params]");
char *filename = argv[1];
char action = argv[2][0];
struct Connection *conn = Database_open(filename, action);
int id = 0;
if (argc > 3) id = atoi(argv[3]);
if (id >= MAX_ROWS) die("There's not that many records.");
switch (action) {
case 'c':
Database_create(conn);
Database_write(conn);
break;
case 'g':
if (argc != 4)
die("Need an id to get");
Database_get(conn, id);
break;
case 's':
if (argc != 6)
die("Need id, name, email to set");
Database_set(conn, id, argv[4], argv[5]);
Database_write(conn);
break;
case 'd':
if (argc != 4)
die("Need id to delete");
Database_delete(conn, id);
Database_write(conn);
break;
case 'l':
Database_list(conn);
break;
default:
die("Invalid action: c=create, g=get, s=set, d=del, l=list");
}
Database_close(conn);
return 0;
}
```
這個程式的功能在建立一個基本聯絡資訊的資料庫 `struct Database`， 這個資料庫包含一個 `element` ： `struct Address rows[MAX_ROWS]`
`struct Address` 包含 `int id` 、 `int set` 、 `char name[MAX_DATA]` 、 `char email[MAX_DATA]` ， 其中 `MAX_ROWS` = 1000, `MAX_DATA` = 512 。
程式使用方式:
```
./ex17 <dbfile> <action> [action params]
<dbfile> : 資料庫檔名
<action> : 執行動作
c : 建立資料庫
g <id> : 取得某筆 id 的資料並印出
s <id> <name> <email> ： 設定第 <id> 筆資料的內容
d <id> : 刪除第 <id> 筆資料，這裡實作把 <set> 改為 0
l : 列出所有建立的 Address
[action params] ： 其他內容
<action> 之後接的參數
```
### Extra Credit
- The die function needs to be augmented to let you pass the conn
variable, so it can close it and clean up.
---> 完成
> ex17_ext1
```c=
#include <stdio.h>
#include <assert.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#define MAX_DATA 512
#define MAX_ROWS 100
struct Address {
int id;
int set;
char name[MAX_DATA];
char email[MAX_DATA];
};
struct Database {
struct Address rows[MAX_ROWS];
};
struct Connection {
FILE *file;
struct Database *db;
};
void Database_close(struct Connection *conn);
void die(const char *message, struct Connection *conn)
{
if (errno) {
if (conn) {
Database_close(conn);
}
perror(message);
} else {
printf("ERROR: %s\n", message);
if (conn) {
Database_close(conn);
}
}
exit(1);
}
void Address_print(struct Address *addr)
{
printf("%d %s %s\n", addr->id, addr->name, addr->email);
}
void Database_load(struct Connection *conn)
{
int rc = fread(conn->db, sizeof(struct Database), 1, conn->file);
if (rc != 1)
die("Failed to load database.", conn);
}
struct Connection *Database_open(const char *filename, char mode)
{
struct Connection *conn = malloc(sizeof(struct Connection));
if (!conn)
die("Memory error", conn);
conn->db = malloc(sizeof(struct Database));
if (!conn->db)
die("Memory error", conn);
if (mode == 'c') {
conn->file = fopen(filename, "w");
} else {
conn->file = fopen(filename, "r+");
if (conn->file) {
Database_load(conn);
}
}
if (!conn->file)
die("Failed to open the file", conn);
return conn;
}
void Database_close(struct Connection *conn)
{
if (conn) {
if (conn->file)
fclose(conn->file);
if (conn->db)
free(conn->db);
free(conn);
}
}
void Database_write(struct Connection *conn)
{
rewind(conn->file);
int rc = fwrite(conn->db, sizeof(struct Database), 1, conn->file);
if (rc != 1)
die("Failed to write database.", conn);
rc = fflush(conn->file);
if (rc == -1)
die("Cannot flush database.", conn);
}
void Database_create(struct Connection *conn)
{
int i = 0;
for (i = 0; i < MAX_ROWS; i++) {
// make a prototype to initialize it
struct Address addr = {.id = i, .set = 0};
// then just assign it
conn->db->rows[i] = addr;
}
}
void Database_set(struct Connection *conn, int id, const char *name,
const char *email)
{
struct Address *addr = &conn->db->rows[id];
if (addr->set)
die("Already set, delete it first", conn);
addr->set = 1;
// WARNING: bug, read the "How To Break It" and fix this
char *res = strncpy(addr->name, name, MAX_DATA);
// bug fixed
addr->name[MAX_DATA - 1] = '\0';
// demonstrate the strncpy bug
if (!res)
die("Name copy failed", conn);
res = strncpy(addr->email, email, MAX_DATA);
if (!res)
die("Email copy failed", conn);
// bug fixed
addr->email[MAX_DATA - 1] = '\0';
}
void Database_get(struct Connection *conn, int id)
{
struct Address *addr = &conn->db->rows[id];
if (addr->set) {
Address_print(addr);
} else {
die("ID is not set", conn);
}
}
void Database_delete(struct Connection *conn, int id)
{
struct Address addr = {.id = id, .set = 0};
conn->db->rows[id] = addr;
}
void Database_list(struct Connection *conn)
{
int i = 0;
struct Database *db = conn->db;
for (i = 0; i < MAX_ROWS; i++) {
struct Address *cur = &db->rows[i];
if (cur->set) {
Address_print(cur);
}
}
}
int main(int argc, char *argv[])
{
if (argc < 3)
die("USAGE: ex17 <dbfile> <action> [action params]", NULL);
char *filename = argv[1];
char action = argv[2][0];
struct Connection *conn = Database_open(filename, action);
int id = 0;
if (argc > 3) id = atoi(argv[3]);
if (id >= MAX_ROWS) die("There's not that many record.", conn);
switch(action) {
case 'c':
Database_create(conn);
Database_write(conn);
break;
case 'g':
if (argc != 4)
die("Need an id to get", conn);
Database_get(conn, id);
break;
case 's':
if (argc != 6)
die("Need id, name, email to set", conn);
Database_set(conn, id, argv[4], argv[5]);
Database_write(conn);
break;
case 'd':
if (argc != 4)
die("Need id to delete", conn);
Database_delete(conn, id);
Database_write(conn);
break;
case 'l':
Database_list(conn);
break;
default:
die("Invalid action: c=create, g=get, s=set, d=del, l=list", conn);
}
Database_close(conn);
return 0;
}
```
- Change the code to accept parameters for `MAX_DATA` and `MAX_ROWS`,
store them in the Database `struct`, and write that to the file, thus creating a database that can be arbitrarily sized.
--> 嘗試調整 `MAX_ROWS` 使其變成可變的任意大小，但是在執行以下指令時出現 `Segmentation Fault`
>ex17_ext2
```c=
#include <stdio.h>
#include <assert.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#define MAX_DATA 512
#define MAX_ROWS 100
struct Address {
int id;
int set;
char name[MAX_DATA];
char email[MAX_DATA];
};
struct Database {
int max_rows;
struct Address rows[];
};
struct Connection {
FILE *file;
struct Database *db;
};
void Database_close(struct Connection *conn);
void die(const char *message, struct Connection *conn)
{
if (errno) {
if (conn) {
Database_close(conn);
}
perror(message);
} else {
printf("ERROR: %s\n", message);
if (conn) {
Database_close(conn);
}
}
exit(1);
}
void Address_print(struct Address *addr)
{
printf("%d %s %s\n", addr->id, addr->name, addr->email);
}
void Database_load(struct Connection *conn)
{
int rc = fread(conn->db, sizeof(struct Database), 1, conn->file);
if (rc != 1)
die("Failed to load database.", conn);
}
struct Connection *Database_open(const char *filename, char mode, int max_rows)
{
struct Connection *conn = malloc(sizeof(struct Connection));
if (!conn)
die("Memory error", conn);
conn->db = malloc(sizeof(struct Database) + sizeof(char[max_rows]));
if (!conn->db)
die("Memory error", conn);
if (mode == 'c') {
conn->file = fopen(filename, "w");
} else {
conn->file = fopen(filename, "r+");
if (conn->file) {
Database_load(conn);
}
}
if (!conn->file)
die("Failed to open the file", conn);
return conn;
}
void Database_close(struct Connection *conn)
{
if (conn) {
if (conn->file)
fclose(conn->file);
if (conn->db)
free(conn->db);
free(conn);
}
}
void Database_write(struct Connection *conn)
{
rewind(conn->file);
int rc = fwrite(conn->db, sizeof(struct Database), 1, conn->file);
if (rc != 1)
die("Failed to write database.", conn);
rc = fflush(conn->file);
if (rc == -1)
die("Cannot flush database.", conn);
}
void Database_create(struct Connection *conn, int max_rows)
{
int i = 0;
for (i = 0; i < max_rows; i++) {
// make a prototype to initialize it
struct Address addr = {.id = i, .set = 0};
// then just assign it
conn->db->rows[i] = addr;
}
}
void Database_set(struct Connection *conn, int id, const char *name,
const char *email)
{
struct Address *addr = &conn->db->rows[id];
if (addr->set)
die("Already set, delete it first", conn);
addr->set = 1;
// WARNING: bug, read the "How To Break It" and fix this
char *res = strncpy(addr->name, name, MAX_DATA);
// bug fixed
addr->name[MAX_DATA - 1] = '\0';
// demonstrate the strncpy bug
if (!res)
die("Name copy failed", conn);
res = strncpy(addr->email, email, MAX_DATA);
if (!res)
die("Email copy failed", conn);
// bug fixed
addr->email[MAX_DATA - 1] = '\0';
}
void Database_get(struct Connection *conn, int id)
{
struct Address *addr = &conn->db->rows[id];
if (addr->set) {
Address_print(addr);
} else {
die("ID is not set", conn);
}
}
void Database_delete(struct Connection *conn, int id)
{
struct Address addr = {.id = id, .set = 0};
conn->db->rows[id] = addr;
}
void Database_list(struct Connection *conn)
{
int i = 0;
struct Database *db = conn->db;
for (i = 0; i < MAX_ROWS; i++) {
struct Address *cur = &db->rows[i];
if (cur->set) {
Address_print(cur);
}
}
}
int main(int argc, char *argv[])
{
if (argc < 3)
die("USAGE: ex17 <dbfile> <action> [action params]", NULL);
char *filename = argv[1];
char action = argv[2][0];
// adding this to setting max_rows
int max_rows = 10;
struct Connection *conn = Database_open(filename, action, max_rows);
int id = 0;
if (argc > 3) id = atoi(argv[3]);
if (id >= MAX_ROWS) die("There's not that many record.", conn);
switch(action) {
case 'c':
Database_create(conn, max_rows);
Database_write(conn);
break;
case 'g':
if (argc != 4)
die("Need an id to get", conn);
Database_get(conn, id);
break;
case 's':
if (argc != 6)
die("Need id, name, email to set", conn);
Database_set(conn, id, argv[4], argv[5]);
Database_write(conn);
break;
case 'd':
if (argc != 4)
die("Need id to delete", conn);
Database_delete(conn, id);
Database_write(conn);
break;
case 'l':
Database_list(conn);
break;
default:
die("Invalid action: c=create, g=get, s=set, d=del, l=list", conn);
}
Database_close(conn);
return 0;
}
```
```
$ ./ex17_ext2 db.dat c
Segmentation fault
```
與ext1比較，修改了：
```
$ diff ex17_ext1.c ex17_ext2.c
18c18,19
< struct Address rows[MAX_ROWS];
---
> int max_rows;
> struct Address rows[];
57c58
< struct Connection *Database_open(const char *filename, char mode)
---
> struct Connection *Database_open(const char *filename, char mode, int max_rows)
63c64
< conn->db = malloc(sizeof(struct Database));
---
> conn->db = malloc(sizeof(struct Database) + sizeof(char[max_rows]));
107c108
< void Database_create(struct Connection *conn)
---
> void Database_create(struct Connection *conn, int max_rows)
111c112
< for (i = 0; i < MAX_ROWS; i++) {
---
> for (i = 0; i < max_rows; i++) {
113a115
>
180c182,185
< struct Connection *conn = Database_open(filename, action);
---
>
> // adding this to setting max_rows
> int max_rows = 10;
> struct Connection *conn = Database_open(filename, action, max_rows);
188c193
< Database_create(conn);
---
> Database_create(conn, max_rows);
```
使用gdb 把過程顯示出來
```
$ gdb --batch --ex run --ex bt --ex q --args ./ex17_ext2 db.dat c
Program received signal SIGSEGV, Segmentation fault.
0x0000007ff7eb1d88 in __GI_rewind (fp=0x55555632e0) at rewind.c:34
34 rewind.c: No such file or directory.
#0 0x0000007ff7eb1d88 in __GI_rewind (fp=0x55555632e0) at rewind.c:34
#1 0x0000005555550e3c in Database_write (conn=0x55555632a0) at ex17_ext2.c:97
#2 0x00000055555512d0 in main (argc=3, argv=0x7ffffff998) at ex17_ext2.c:194
A debugging session is active.
Inferior 1 [process 1305537] will be killed.
Quit anyway? (y or n) [answered Y; input not from terminal]
```
錯誤發生在 `Database_write` 函式裡呼叫 `rewind（）` 這個處理 `FILE` 類內建函式把 `steam` 指向的位置重置為開頭時發生錯誤，但想好久還是不懂為什麼會錯，難道是 conn->file 這邊由 `->` 讀取 `file` 時產生問題？
再仔細檢查，發現是自己打錯了!
```c
conn->db = malloc(sizeof(struct Database) + sizeof(char[max_rows]));
```
應該改成
```c
conn->db = malloc(sizeof(struct Database) + sizeof(struct Address[max_rows]));
```
由於參考 `C99 規格書` 在 6.7.2.1.16 [FAM(Flexible Array Members)](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf#page=115) 時忘記改為正確的類型。
這讓我好奇，先回憶 `struct Connection`
```c
struct Connection {
FILE *file;
struct Database *db;
};
```
有 `FILE *file` 及 `struct Database *db`
為什麼在 `struct Connection` `conn` 的成員 `db` 呼叫 `malloc` ，分配空間時輸入不對的類別，會造成後來使用 `rewind` 讀取 `conn` 的成員 `file` 時發生找不到的錯誤?
後來找到解答，由於前面分配錯的空間（ + sizeof(char[max_rows]），這樣是分配 `max_rows` 個 `char` 的空間，但是再回看 `struct Database` :
```c
struct Database {
int max_rows;
struct Address rows[];
}
```
這邊rows是 `struct Address` type ， 所以儘管分配錯誤的空間大小 ( + sizeof(char[max_rows])) ，但是當存取 rows[i] 對應的 element 的時候，它會跳 i 個 `struct Address` type， 所以整個記憶體位置就不對了。
而實際看看問題出在哪裡：
```
./ex17_ext2 db.dat c
```
第一個參數 `db.dat` 代表一個檔案名稱，第二個參數 `c` 是產生第一個檔案，裡面存 `MAX_ROWS` 筆 `struct Address`，而這兩個參數在執行過程會依序呼叫 `Database_open` 、 `Database_create` 、 `Database_write`。而錯誤發生在 `Database_write`裡面的 `rewind()` ，經過 `gdb` 執行過程找到問題，是這樣產生的：
1. 在 `Database_open` ，先 `malloc` 分配出可用記憶體空間給 `conn` ， 之後 `malloc` 分配空間給 `conn->db` ， 再來 `fopen` 建立 conn->file，
在 `gdb` 裡顯示 `conn` 、 `conn->db` 、 `conn->file` 的記憶體位置，原先能正常執行時各個變數記憶體位置可能如下：
```
1: conn = (struct Connection *) 0x55555632a0
2: conn->db = (struct Database *) 0x55555632c0
3: conn->file = (FILE *) 0x555557c5f0
```
而分配錯誤空間後各個變數記憶體位置可能如下：
```
1: conn = (struct Connection *) 0x55555632a0
2: conn->db = (struct Database *) 0x55555632c0
3: conn->file = (FILE *) 0x55555632e0
```
可以看出前者 `conn->file` 的位置跟 `conn->db` 兩個變數的 差值 比 後者 兩個變數的差值還大，先是由於分配錯誤空間大小導致，這邊還沒有問題，問題發生在之後 `conn->create` 的時候用新產生的 `struct Address` `assign` 給 rows的時候，由此把 `conn->file` 的內容也改寫掉導致。
2. 由此也可觀察出 `struct` 裡 `element` 的位置其記憶體位址不一定從上到下，回看用 `malloc` 建立 `conn->db` 再建立 `conn->file` 可以看出 後者的記憶體位置是接在 `conn->db` 之後而不是之前。
最後放上能運行的：
ex17_ext2.c
```c
#include <stdio.h>
#include <assert.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#define MAX_DATA 512
#define MAX_ROWS 100
struct Address {
int id;
int set;
char name[MAX_DATA];
char email[MAX_DATA];
};
struct Database {
int max_rows;
struct Address rows[];
};
struct Connection {
FILE *file;
struct Database *db;
};
void Database_close(struct Connection *conn);
void die(const char *message, struct Connection *conn)
{
if (errno) {
if (conn) {
Database_close(conn);
}
perror(message);
} else {
printf("ERROR: %s\n", message);
if (conn) {
Database_close(conn);
}
}
exit(1);
}
void Address_print(struct Address *addr)
{
printf("%d %s %s\n", addr->id, addr->name, addr->email);
}
void Database_load(struct Connection *conn, int max_rows)
{
int rc = fread(conn->db, sizeof(struct Database) + sizeof(struct Address[max_rows]), 1, conn->file);
if (rc != 1)
die("Failed to load database.", conn);
}
struct Connection *Database_open(const char *filename, char mode, int max_rows)
{
struct Connection *conn = malloc(sizeof(struct Connection));
if (!conn)
die("Memory error", conn);
conn->db = malloc(sizeof(struct Database) + sizeof(struct Address[max_rows]));
if (!conn->db)
die("Memory error", conn);
conn->db->max_rows = max_rows;
if (mode == 'c') {
conn->file = fopen(filename, "w");
} else {
conn->file = fopen(filename, "r+");
if (conn->file) {
Database_load(conn, max_rows);
}
}
if (!conn->file)
die("Failed to open the file", conn);
return conn;
}
void Database_close(struct Connection *conn)
{
if (conn) {
if (conn->file)
fclose(conn->file);
if (conn->db)
free(conn->db);
free(conn);
}
}
void Database_write(struct Connection *conn, int max_rows)
{
rewind(conn->file);
int rc = fwrite(conn->db, sizeof(struct Database) + sizeof(struct Address[max_rows]), 1, conn->file);
if (rc != 1)
die("Failed to write database.", conn);
rc = fflush(conn->file);
if (rc == -1)
die("Cannot flush database.", conn);
}
void Database_create(struct Connection *conn, int max_rows)
{
int i = 0;
for (i = 0; i < max_rows; i++) {
// make a prototype to initialize it
struct Address addr = {.id = i, .set = 0};
// then just assign it
conn->db->rows[i] = addr;
}
}
void Database_set(struct Connection *conn, int id, const char *name,
const char *email)
{
struct Address *addr = &conn->db->rows[id];
if (addr->set)
die("Already set, delete it first", conn);
addr->set = 1;
// WARNING: bug, read the "How To Break It" and fix this
char *res = strncpy(addr->name, name, MAX_DATA);
// bug fixed
addr->name[MAX_DATA - 1] = '\0';
// demonstrate the strncpy bug
if (!res)
die("Name copy failed", conn);
res = strncpy(addr->email, email, MAX_DATA);
if (!res)
die("Email copy failed", conn);
// bug fixed
addr->email[MAX_DATA - 1] = '\0';
}
void Database_get(struct Connection *conn, int id)
{
struct Address *addr = &conn->db->rows[id];
if (addr->set) {
Address_print(addr);
} else {
die("ID is not set", conn);
}
}
void Database_delete(struct Connection *conn, int id)
{
struct Address addr = {.id = id, .set = 0};
conn->db->rows[id] = addr;
}
void Database_list(struct Connection *conn, int max_rows)
{
int i = 0;
struct Database *db = conn->db;
for (i = 0; i < max_rows; i++) {
struct Address *cur = &db->rows[i];
if (cur->set) {
Address_print(cur);
}
}
}
int main(int argc, char *argv[])
{
if (argc < 3)
die("USAGE: ex17 <dbfile> <action> [action params]", NULL);
char *filename = argv[1];
char action = argv[2][0];
// adding this to setting max_rows
int max_rows = 10;
struct Connection *conn = Database_open(filename, action, max_rows);
int id = 0;
if (argc > 3) id = atoi(argv[3]);
if (id >= MAX_ROWS) die("There's not that many record.", conn);
switch(action) {
case 'c':
Database_create(conn, max_rows);
Database_write(conn, max_rows);
break;
case 'g':
if (argc != 4)
die("Need an id to get", conn);
Database_get(conn, id);
break;
case 's':
if (argc != 6)
die("Need id, name, email to set", conn);
Database_set(conn, id, argv[4], argv[5]);
Database_write(conn, max_rows);
break;
case 'd':
if (argc != 4)
die("Need id to delete", conn);
Database_delete(conn, id);
Database_write(conn, max_rows);
break;
case 'l':
Database_list(conn, max_rows);
break;
default:
die("Invalid action: c=create, g=get, s=set, d=del, l=list", conn);
}
Database_close(conn);
return 0;
}
```
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