I study C at university, and also through "Learn C the Hard Way". This question specifically relates to Learn C the Hard Way - Exercise 17
The code I wrote uses the structure database:
#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]; }
In the "additional credit" section of this exercise, it is proposed to "Change the code to accept the parameters for MAX_DATA and MAX_ROWS, save them in the database structure and write to a file, thereby creating a database that can be arbitrarily set."
Ignoring MAX_DATA for now, as I want to focus on one at a time.
Note. Here is my full code
#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, struct Connection *conn) { if(errno) { perror(message); } else { printf("ERROR: %s\n", message); } free(conn->file); free(conn->db); free(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 < sizeof(conn->db->rows) / sizeof(*conn->db->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; char *res = strncpy(addr->name, name, MAX_DATA); addr->name[MAX_DATA - 1] = '\0'; if(!res) die("Name copy failed", conn); res = strncpy(addr->email, email, MAX_DATA); addr->email[MAX_DATA - 1] = '\0'; if(!res) die("Email copy failed", conn); } 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 < sizeof(conn->db->rows) / sizeof(*conn->db->rows); i++) { struct Address *cur = &db->rows[i]; if(cur->set) { Address_print(cur); } } } int main(int argc, char *argv[]) { char *filename = argv[1]; char action = argv[2][0]; int id = 0; struct Connection *conn = Database_open(filename, action); if(argc < 3) die("USAGE: ex17 <dbfile> <action> [action params]", conn); if(argc > 3) id = atoi(argv[3]); if(id >= sizeof(conn->db->rows) / sizeof(*conn->db->rows)) die("There not that many records.", 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, only: c=create, g=get, s=set, d=del, l=list", conn); } Database_close(conn); return 0; }
My first thought was not to create a “database” of the global structure and instead put it in my main function so that it could flow through the rest of the program, but it seems so inconvenient that you need to pass this variable throughout programs. However, this made me realize that I would need to pass a pointer to it through any function call that he needed, which is not an additional loan.
I get the feeling that as a C newbie, I'm missing something. My question may be vague because I do not fully know the terms I'm talking about.
If my question is not clear - how can I add this functionality? I am really at a loss.