Pointer to one structure to another, writing and reading from file gives SegFault
I am clearing my C skills from Learn C The Hard Way, I am currently in the 17th Exercise .
I am doing the Extra Credits part. Generating the database code listed on this page to get I am trying
"Change the code to accept the parameters for MAX_DATA and MAX_ROWS, store them in the database structure and write this to a file, thus creating a database that can be arbitrarily defined."
So, I commented out the #define directives and changed the address and database structures as stated:
struct Address {
int id;
int set;
char *name;
char *email;
};
struct Database {
int MAX_DATA;
int MAX_ROWS;
struct Address *rows;
};
This way I can get the MAX_DATA and MAX_ROWS parameters from the user to create a database of his liking. Other things that I changed in the code -
Database_create function:
void Database_create(struct Connection *conn, int MAX_DATA, int MAX_ROWS)
{
int i = 0;
conn->db->MAX_DATA = MAX_DATA;
conn->db->MAX_ROWS = MAX_ROWS;
conn->db->rows = malloc(sizeof(struct Address) * MAX_ROWS);
for(i = 0; i < MAX_ROWS; i++) {
struct Address addr = {.id = i, .set = 0};
conn->db->rows[i] = addr;
}
}
The part where I get "Invalid Read 4 Bytes" and SegFault:
void Database_set(struct Connection *conn, int id, const char *name, const char *email)
{
struct Address *addr = &conn->db->rows[id];
int MAX_DATA = conn->db->MAX_DATA;
if(addr->set) die("Already set, delete it first");
/* This if statement gives error for addr->set */
addr->set = 1;
addr->name = malloc(sizeof(char) * MAX_DATA);
addr->email = malloc(sizeof(char) * MAX_DATA);
// 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");
}
I know this piece of code is not a whole piece, but I cannot paste all the code here. So, I posted it here: http://pastebin.com/EbKShT3r I can create and write a database for the first run using the "c" option. However, adding entries using the "s" option results in a segmentation fault.
EDIT : So finally I got this program working with the solution given by @WhizCraig However, to free up memory, here's what I'm trying:
void Database_close(struct Connection *conn)
{
int i;
if (conn) {
int MAX_ROWS = conn->db->MAX_ROWS;
for (i=0; i<MAX_ROWS; i++) {
struct Address *row = conn->db->rows+i;
if (row->set) {
free(row->name);
free(row->email);
}
}
free(conn->db->rows);
if(conn->file) fclose(conn->file);
if(conn->db) free(conn->db);
free(conn);
}
}
And, I get errors in Valgrind, there is a memory leak. I don't understand the error in the above code. However, the main problem seems to be solved :)
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struct Address {
int id;
int set;
char *name;
char *email;
};
struct Database {
int MAX_DATA;
int MAX_ROWS;
Address **rows; // USE ARRAY OF POINTERS
};
void Database_create(Connection *conn, int MAX_DATA, int MAX_ROWS)
{
int i = 0;
conn->db->MAX_DATA = MAX_DATA;
conn->db->MAX_ROWS = MAX_ROWS;
conn->db->rows = (Address**)malloc(sizeof(Address*) * MAX_ROWS);
for(i = 0; i < MAX_ROWS; i++) {
conn->db->rows[i] = (Address*)malloc(sizeof(Address));
conn->db->rows[i]->id = i;
conn->db->rows[i]->set = 0;
}
}
void Database_set(Connection *conn, int id, const char *name, const char *email)
{
if (!(conn && conn->db && conn->db->rows && conn->db->rows[id])) return;
Address *addr = conn->db->rows[id];
int MAX_DATA = conn->db->MAX_DATA;
if(addr->set == 0) die("Already set, delete it first");
addr->set = 1;
addr->name = malloc(sizeof(char) * MAX_DATA);
addr->email = malloc(sizeof(char) * MAX_DATA);
char *res = strncpy(addr->name, name, MAX_DATA);
if(!res) die("Name copy failed");
res = strncpy(addr->email, email, MAX_DATA);
if(!res) die("Email copy failed");
}
void Database_close(Connection *conn)
{
size_t i;
if(conn) {
if (con->db && conn->db->rows) {
for (i = 0; i < conn->db->MAX_ROWS; i++) {
Address *cur = conn->db->rows[i];
free(cur);
}
}
if(conn->file) fclose(conn->file);
if(conn->db) free(conn->db);
free(conn);
}
}
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I modified the Rohit code posted here: http://pastebin.com/MvLXkDCz to fix the memory leaks. It goes through Valgrind without any error. The following are the relevant changes:
void Database_close(Connection *conn)
{
size_t i;
if(conn) {
if(conn->db && conn->db->rows) {
for(i = 0; i < conn->db->max_rows; i++) {
Address *cur = conn->db->rows[i];
free(cur->name);
free(cur->email);
free(cur);
}
free(conn->db->rows);
}
if(conn->file) fclose(conn->file);
if(conn->db) free(conn->db);
free(conn);
}
}
void Database_delete(Connection *conn, int id)
{
conn->db->rows[id]->set = 0;
}
You can find the complete program here: https://github.com/sookoor/Learn-C-the-Hard-Way/blob/master/ex17.c
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