Problem with allocation and initializing in dynamic 2dimentional array.

alireza976 · 07-01-2011, 03:13 PM

hello

I want to fetch all of the values from a table of ''mysql'' database and put them in a dynamic 2dimentional array, in this code.

Quote:

#include <stdio.h>
#include <mysql/mysql.h>
#include <mysql/errmsg.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char **argv)
{
MYSQL *conn;
MYSQL_RES *result;
MYSQL_ROW row;

char myquery[300]="select * from pageinfo;",
int i,
x,
j=0,
num_fields,
num_rows=0,
u=0;

if((conn=mysql_init(NULL))==NULL)
{
printf("\nFailed to initate MySQL connection");
// exit(1);
}

if (!mysql_real_connect(conn,"127.0.0.1","root","1","aaa",0,"/var/run/mysqld/mysqld.sock",0))
{
printf( "Failed to connect to MySQL: Error: %s\n", mysql_error(conn));
// exit(1);
}

printf("\nLogged on to database sucessfully\n\n");
printf("\n\n%s\n\n", myquery);
if (mysql_query(conn,myquery))
{
printf("\n\nQuery error.\n\n");
}
else
{ //Query OK.
result = mysql_store_result(conn); //unsigned int num_fields.
num_rows = mysql_num_rows(result); //Returns the number of rows in the result set.
num_fields = mysql_num_fields(result); //Returns the number of columns in a result set.

char** array;
array=(char**)malloc(num_rows*sizeof(char));
while(j<(num_rows*num_fields))
{
array[j]=(char*)malloc(60*sizeof(char));
j++;
}

while((row = mysql_fetch_row(result)))
{
for(i = 0; i < num_fields; i++)
{
strcpy(array[u],row[i]);
//sprintf(array[u],"%s",row[i]);
printf("[%s] ",array[u]);
u++;
}
}
printf("\n\nFetched ok\n\n");
printf("\n\n%d\n\n",u);
mysql_free_result(result);
}
mysql_close(conn);
}

Reading information process does work correctly but when program get the second row, there is a ''segmentation fault'' error and it causes the program to stop.
The trouble can be either memory allocation or entering the informaition to array.
Could anybody help me to solve this problem?

Thanks

bastl · 07-01-2011, 06:30 PM

To use an array of records you should specify a structure that defines that record (mybe a whole row if records of a row don't have same length) and make thereof an array.
And then it is best to trop that struct and make a class for that what you want to do, because variable arrays are name space depending (heap).

Allocate a memory as a whole block not only parts of it, because arrays can only access a continuous memory block (''segmentation fault'') and calling malloc a second time could separate that memory blocks from the other.

Variable allocation:
1)Current allocation is the allocation before + additional needed memory.
2)Copy contens of the old allocation to the new allocation + new record.
3)Free old allocation.

dwhitney67 · 07-02-2011, 12:43 PM

Fix this:

Code:

char** array;
array=(char**)malloc(num_rows*sizeof(char));

Should be:

Code:

char** array;
array=(char**)malloc(num_rows*sizeof(char*));   // you need to allocate pointers, not chars
...

or since you are using C-99

Code:

char** array=(char**)malloc(num_rows*sizeof(char*));
...

Feel free to sprinkle some white-space between operators to make your code easier to read. Also, post code using CODE tags, not QUOTE tags.

P.S. Don't use strcpy(); use strncpy() instead, so that you can specify the size of the destination. The size should be N-1 bytes, not N!

alireza976 · 07-03-2011, 03:25 AM

ok

Thanks