[英]Dynamically sized structs - Learn C The Hard Way Ex17
I'm having trouble with an exercise in Learn C The Hard Way. 我在学习C艰难的方式练习时遇到了麻烦。 The exercise provides a simple database program which has a fixed size and number of rows.
该练习提供了一个简单的数据库程序,它具有固定的大小和行数。 Below you can see the structs that form the database.
您可以在下面看到构成数据库的结构。
#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;
};
the task is to change 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. 任务是更改代码以接受MAX_DATA的参数,MAX_ROWS将它们存储在数据库结构中,并将其写入文件,从而创建一个可以任意调整大小的数据库。
I understand how to accept MAX_DATA and MAX_ROWS from the user, as command line arguments - in a function defined lower down in the file. 我理解如何接受来自用户的MAX_DATA和MAX_ROWS,作为命令行参数 - 在文件中定义的较低的函数中。 Once I have these values I am not sure how to store them in the database struct and write to a file.
一旦我有了这些值,我不知道如何将它们存储在数据库结构中并写入文件。
Appreciate anyone who is able to help. 感谢任何能够提供帮助的人。 You can find the rest of the code here: http://c.learncodethehardway.org/book/ex17.html
您可以在此处找到其余代码: http : //c.learncodethehardway.org/book/ex17.html
One way is to change your arrays into pointers. 一种方法是将数组更改为指针。 Then you could write an alloc_db function which would use the max_row and max_data values to allocate the needed memory.
然后你可以写一个alloc_db函数,它将使用max_row和max_data值来分配所需的内存。
struct Address {
int id;
int set;
char* name;
char* email;
};
struct Database {
struct Address* rows;
unsigned int max_row;
unsigned int max_data;
};
struct Connection {
FILE *file;
struct Database *db;
};
Okay I managed to finally get this program working, I've summarized below. 好吧,我设法最终使这个程序工作,我在下面总结。 I hope this might help someone also stuck on ex17.
我希望这可能有助于某人也坚持使用ex17。
First, I removed the MAX_DATA and MAX_ROWS constants and changed the structs like so: 首先,我删除了MAX_DATA和MAX_ROWS常量并更改了结构,如下所示:
struct Address {
int id;
int set;
char *name;
char *email;
};
struct Database {
int max_data;
int max_rows;
struct Address **rows;
};
struct Connection {
FILE *file;
struct Database *db;
};
I assign max_data
and max_rows
to the new variables in the struct and them write them to the file. 我将
max_data
和max_rows
分配给max_data
中的新变量,然后将它们写入文件。
conn->db->max_data = max_data;
conn->db->max_rows = max_rows;
int rc = fwrite(&conn->db->max_data, sizeof(int), 1, conn->file);
rc = fwrite(&conn->db->max_rows, sizeof(int), 1, conn->file);
Now I can run my program and replace MAX_ROWS
& MAX_DATA
with conn->db->max_rows
& conn->db->max_data
. 现在我可以运行我的程序并用
conn->db->max_rows
& conn->db->max_data
替换MAX_ROWS
和MAX_DATA
。
#include <stdio.h>
#include <assert.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
struct Address {
int id;
int set;
char* name;
char* email;
};
struct Database {
int MAX_ROWS;
int MAX_DATA;
struct Address* rows;
};
struct Connection {
FILE* file;
struct Database* db;
};
void Database_close(struct Connection* conn)
{
if(conn) {
for(size_t i = 0; i < conn->db->MAX_ROWS; i++) {
struct Address* row = &conn->db->rows[i];
if(row->name) free(row->name);
if(row->email) free(row->email);
}
if(conn->db->rows) free(conn->db->rows);
if(conn->db) free(conn->db);
if(conn->file) fclose(conn->file);
free(conn);
}
}
void die(char* message, struct Connection* conn)
{
if(errno) {
perror(message);
} else {
printf("ERROR: %s\n", message);
}
Database_close(conn);
exit(1);
}
void diec(const char* message)
{
if(errno) {
perror(message);
} else {
printf("ERROR: %s\n", message);
}
printf("Something Wrong\n");
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)
{
assert(conn->db && conn->file);
if(fread(conn->db, sizeof(struct Database), 1, conn->file) != 1)
die("Failed to load database.", conn);
conn->db->rows = (struct Address*)malloc(sizeof(struct Address) * conn->db->MAX_ROWS);
for(size_t i = 0; i < conn->db->MAX_ROWS; i++) {
struct Address* row = &conn->db->rows[i];
if(fread(&row->id, sizeof(int), 1, conn->file) != 1)
die("Failed to load id.", conn);
if(fread(&row->set, sizeof(int), 1, conn->file) != 1)
die("Failed to load set.", conn);
row->name = malloc(conn->db->MAX_DATA);
row->email = malloc(conn->db->MAX_DATA);
if(fread(row->name, conn->db->MAX_DATA, 1, conn->file) != 1)
die("Faile to load name", conn);
if(fread(row->email, conn->db->MAX_DATA, 1, conn->file) != 1)
die("Faile to load email", 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_create(struct Connection* conn)
{
printf("MAX_ROWS: ");
scanf("%d", &conn->db->MAX_ROWS);
if (conn->db->MAX_ROWS<=0) die("MAX_ROWS must be positive", conn);
printf("MAX_DATA: ");
scanf("%d", &conn->db->MAX_DATA);
if (conn->db->MAX_DATA<=0) die("MAX_DATA must be positive", conn);
conn->db->rows = (struct Address*)malloc(sizeof(struct Address)*conn->db->MAX_ROWS);
for(size_t i = 0; i < conn->db->MAX_ROWS; i++) {
char* a = (char*)malloc(conn->db->MAX_DATA);
memset(a, 0, conn->db->MAX_DATA);
char* b = (char*)malloc(conn->db->MAX_DATA);
memset(b, 0, conn->db->MAX_DATA);
struct Address addr = {.id = i, .set = 0, .name = a, .email = b};
conn->db->rows[i] = addr;
}
}
void Database_write(struct Connection* conn)
{
rewind(conn->file);
if(fwrite(conn->db, sizeof(struct Database), 1, conn->file) != 1)
die("Failed to write database.", conn);
for (size_t i = 0; i < conn->db->MAX_ROWS; i++) {
if(fwrite(&((conn->db->rows[i]).id), sizeof(int), 1, conn->file) != 1)
die("Failed to write id.", conn);
if(fwrite(&((conn->db->rows[i]).set), sizeof(int), 1, conn->file) != 1)
die("Failed to write set.", conn);
if(fwrite((conn->db->rows[i]).name, conn->db->MAX_DATA, 1, conn->file) != 1)
die("Failed to write name.", conn);
if(fwrite((conn->db->rows[i]).email, conn->db->MAX_DATA, 1, conn->file) != 1)
die("Failed to write email.", conn);
}
fflush(conn->file);
}
void Database_set(struct Connection* conn, int id, char* name, char* email)
{
struct Address* addr = &conn->db->rows[id];
if(addr->set) die("Already set, delete it first", conn);
addr->set = 1;
name[conn->db->MAX_DATA - 1] = '\0';
email[conn->db->MAX_DATA - 1] = '\0';
strncpy(addr->name, name, conn->db->MAX_DATA);
strncpy(addr->email, email, conn->db->MAX_DATA);
}
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)
{
char* a = (char*)malloc(conn->db->MAX_DATA);
char* b = (char*)malloc(conn->db->MAX_DATA);
free(conn->db->rows[id].name);
free(conn->db->rows[id].email);
struct Address addr = {.id = id, .set = 0, .name = a, .email = b};
conn->db->rows[id] = addr;
}
void Database_list(struct Connection* conn)
{
for(size_t i = 0; i < conn->db->MAX_ROWS; i++) {
if(conn->db->rows[i].set) {
Address_print(&(conn->db->rows[i]));
}
}
}
void Database_find(struct Connection* conn, char* keyword) {
int i=0;
int count=0;
while (i < conn->db->MAX_ROWS) {
while ( i<conn->db->MAX_ROWS) {
if(conn->db->rows[i].set==1){
if(strstr(conn->db->rows[i].name, keyword) != NULL || strstr(conn->db->rows[i].email, keyword) != NULL){
break;
}
}
i++;
}
if(i >= conn->db->MAX_ROWS) break;
Address_print(&(conn->db->rows[i]));
count++;
i++;
}
if (count==0) {
printf("Try some other words\n");
}
}
int main(int argc, char* argv[])
{
if(argc < 3) diec("USAGE: ex17 <dbfile> <action> [action params]");
int id = 0;
if(argc > 3) id = atoi(argv[3]);
char* filename = argv[1];
char action = argv[2][0];
struct Connection* conn = Database_open(filename, action);
switch(action) {
case 'c':
Database_create(conn);
Database_write(conn);
printf("\nDone\n");
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;
case 'f':
if(argc != 4) die("Need keyword to search.", conn);
Database_find(conn, argv[3]);
break;
default:
die("Invalid action, only: c=create, g=get, s=set, d=del, l=list, f=find", conn);
}
Database_close(conn);
return 0;
}
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