dynamic arrays in structs c

[trent.josephsen]

You're very close. Here's a question for you that might help you find the solution: where and how are you going to call free() on the pointer returned by the malloc() in getRows()?

It might help, if you're stuck, to draw pictures to represent your data structures. E.g., `conn' is an arrow (a pointer) that points to a box (a struct) that contains `file' and a pointer `db', which is an arrow pointing to another box that contains `rows', etc.

[wingnut2626]

is *conn->db->rows a pointer to rows? or is it an array of conn->db->rows

[wingnut2626]

just found out about calloc.....going to mess around with this....

[wingnut2626]

/*
 * ex17.c
 * 
 * Copyright 2014 Ronald C. Reid <ronald@laptop1>
 * 
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 * 
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 * 
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
 * MA 02110-1301, USA.
 * 
 * 
 */


#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 {
    struct Address *rows;
};

struct Connection {
    FILE *file;
    struct Database *db;
};

int getTotal(const char *ptr)
{
    int i;
    int total = 0;
    for(i = 0; *ptr++ != '\0'; i++){
        total++;
    }
    
    return total;
}

int getTotalRows(struct Address *addr)
{
    int i;
    int total = 0;
    for(i = 0; !addr++; i++){
        total++;
    }
    return total;
}

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;
    int x;
    printf("How many items do you want in the database?\n");
    scanf("%d", &x);
    conn->db->rows = calloc(x, sizeof(struct Address));
    for(i = 0; i < x; i++) {
        printf("Run number %d\n", i);
        getchar();
        // make a prototype to initialize it
        struct Address addr = {.id = i, .set = 0};
        printf("Debug statement. line 149 after the prototype of Address is set.");
        getchar();
        // then just assign it
        conn->db->rows[i] = addr;
    }
    printf("DEBUG.  Line 150.  I is now %d\n", i);
    getchar(); 
}

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, getTotal(name));
    // demonstrate the strncpy bug
    if(!res) die("Name copy failed");

    res = strncpy(addr->email, email, getTotal(email));// need total function here to get size of email!
    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 < getTotalRows(conn->db->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 >= getTotalRows(conn->db->rows)) die("There's not that many records.");//need that function to compute total number of rows here!

    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, only: c=create, g=get, s=set, d=del, l=list");
    }

    Database_close(conn);

    return 0;
}

GETTING BETTER!!!!!

[trent.josephsen]

Congrats!

(Don't forget to free() it...)

Strictly speaking, calloc() isn't necessary here; since you don't need it zeroed, malloc(x * sizeof(struct Address)) would work as well (marginally faster, probably). But the most important part is figuring out how to declare and initialize the pointer and it looks like you figured that out already.