Dynamically sized structs - Learn C The Hard Way Ex17

问题

I'm having trouble with an exercise in Learn C The Hard Way. The exercise provides a simple database program which has a fixed size and number of rows. Below you can see the structs that form the database.

#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.

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. Once I have these values I am not sure how to store them in the database struct and write to a file.

Appreciate anyone who is able to help. You can find the rest of the code here: http://c.learncodethehardway.org/book/ex17.html

回答1:

One way is to change your arrays into pointers. Then you could write an alloc_db function which would use the max_row and max_data values to allocate the needed memory.

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;
};

回答2:

Okay I managed to finally get this program working, I've summarized below. I hope this might help someone also stuck on ex17.

First, I removed the MAX_DATA and MAX_ROWS constants and changed the structs like so:

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.

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.

回答3:

#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;
}

来源：https://stackoverflow.com/questions/21269700/dynamically-sized-structs-learn-c-the-hard-way-ex17

标签

database

struct