[SOLVED] how to memory allocate in function and call it by reference for array structure

golden_boy615 · 07-02-2011, 06:29 AM

hello
I am writing a function(in C language in Linux) to initialize an array of a structure from a mysql table by calling a function by reference but I have a problem with call this function by reference . this is my program:

PHP Code:



#include <stdio.h> 
#include <stdlib.h> 
#include <string.h> 
#include <ctype.h> 
#include <errno.h> 
#include <mysql/mysql.h> 
#include <mysql/errmsg.h> 
 
struct ATable { 
    int    id; 
    char   sid[60]; 
}; 
 
void Init_Table(MYSQL *conn,struct ATable* ATable) 
{ 
    MYSQL_ROW row; 
    MYSQL_RES *result; 
    int num_rows; 
    char q="Select id,sid From table"; 
    unsigned int Curent_Row=0; 
    if(mysql_query(conn,q)==0) 
    { 
        result=mysql_store_result(conn); 
        if(result!=NULL) 
        { 
            num_rows = mysql_num_rows(result);      
[B][I]            ATable=(struct ATable*)malloc(num_rows*sizeof(struct ATable));[/I][/B] 
            while((row=mysql_fetch_row(result))!=NULL) 
            { 
                ATable[Curent_Row].id=atoi(row[0]); 
                sprintf(ATable[Curent_Row].sid,"%s",row[10] ? row[10] : "NULL"); 
                Curent_Row++; 
            } 
        } 
        mysql_free_result(result); 
    } 
    else if(mysql_field_count(conn)>0) 
    { 
        fprintf(stderr,"No result set,mysql_store_result() faild : %s\n",mysql_error(conn)); 
    } 
    else 
    { 
        printf("Query was not meant to return data.\n"); 
    } 
} 
 
int main() 
{ 
    MYSQL *conn; 
    struct ATable *ATable; 
    unsigned int Curent_Row=0 ; 
    conn = mysql_init(NULL); 
    if (!mysql_real_connect(conn, "127.0.0.1","root","1","database",0,"/var/run/mysqld/mysqld.sock",0))  
    { 
        fprintf(stderr, "%s\n", mysql_error(conn)); 
        return(0); 
    } 
    Init_Table(conn,ATable); 
    while (Curent_Row<127) 
    { 
        printf("id=%d sid=%s  \n\n",ATable[Curent_Row].id,ATable[Curent_Row].sid); 
        Curent_Row++; 
    } 
    return 0; 
}

I understood that the problem is with my "malloc" inside function but I don't know how to solve it I mean I understood that when function allocate a memory for local ATable variable and when it comes out of function it frees that memory so I get segmentation fault in accessing that array.
what can I do for such a problem ?
how can I malloc inside a function and use it outside a function with calling that function with reference or something like this?

johnsfine · 07-02-2011, 06:52 AM

Tagging your C code as CODE might make this thread more readable than (what you did) tagging it as PHP.

Quote:

Originally Posted by golden_boy615

hello
I am writing a function(in C language in Linux) to initialize an array of a structure from a mysql table by calling a function by reference but I have a problem with call this function by reference .

What do you think you mean by "call ... by reference"?

In C++, you can pass parameters by reference. In C you can't.

If your code were passing ATable by reference, it would still have a memory leak, but it would successfully get the data transferred out of the function (which seems to be your main issue).

Since C has no pass by reference, you can get the same semantic effect (with an uglier syntax) by adding an extra layer of indirection.

You have:

Quote:

Code:

void Init_Table(MYSQL *conn,struct ATable* ATable)
...
        ATable=(struct ATable*)malloc(num_rows*sizeof(struct ATable));
...
        ATable[Curent_Row].id=atoi(row[0]);
        sprintf(ATable[Curent_Row].sid,"%s",row[10] ? row[10] : "NULL");
...
...
    Init_Table(conn,ATable);

An extra layer of indirection would be:

Code:

void Init_Table(MYSQL *conn,struct ATable** ATable)
...
        *ATable=(struct ATable*)malloc(num_rows*sizeof(struct ATable));
...
        (*ATable)[Curent_Row].id=atoi(row[0]);
        sprintf((*ATable)[Curent_Row].sid,"%s",row[10] ? row[10] : "NULL");
...
...
    Init_Table(conn,&ATable);

I added an extra * to every use of ATable in Init_Table and added & to the point where the outside ATable is passed into the function. That has the same effect as pass by reference would.

Quote:

when it comes out of function it frees that memory

False. With or without my change, your code never frees that memory. That is a memory leak.

Without my change, your code loses the pointer to the memory on exit from the function. ATable inside the function is not the same variable as ATable outside the function. So your code had only ATable inside the function pointing to the allocated memory. ATable outside the function never pointed to anything, so it seg faulted on use.

My change gets ATable outside the function to point to the allocated memory. You should also make a change to free that memory when you are done with it.

bastl · 07-02-2011, 04:12 PM

Deffining something outside a function is like static (inside) so global deffined but runtime specific to a name space.

Incidental: Keep in mind if that name space gets not tuched at runtime (created on the heap - new, malloc) that global deffinition don't exist (runtime error could occure)!!!

To access a static deffined class (struct) inside a name space you have to recast that class - redeffine the access. Such access deffinitions are normal be done in haeder files so you simple can include them and the compiler does know how something is accessed and handeled. (That is what I think you mean with reference)

A struct is the same like a class and can have a init, a exit and member functions - oh sorry this is C++ specific I guess. So a class instead should be the tidiest way.

atable.h :

Code:

#ifndef ATABLE_H
#define ATABLE_H   // name space ATABLE_H

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

class ATable
	{
	public:

	ATable(MYSQL *conn);   //init function

	~ATable();   //exit function
	
	struct record_t   // definition of type record_t
		{
		int    id; 
		char   sid[60];
		} *table_1;   // define a pointer of record_t for allocation: table_1 = new record_t[200];
	
	int add_record( record_t* );
	int del_record( record_t* );
	...
	// all methods/members you need
	};

#endif   // ATABLE_H

atable.c according ...

alireza976 · 07-03-2011, 03:21 AM

Quote:

Originally Posted by johnsfine

Tagging your C code as CODE might make this thread more readable than (what you did) tagging it as PHP.

Code:

void Init_Table(MYSQL *conn,struct ATable** ATable)
...
        *ATable=(struct ATable*)malloc(num_rows*sizeof(struct ATable));
...
        (*ATable)[Curent_Row].id=atoi(row[0]);
        sprintf((*ATable)[Curent_Row].sid,"%s",row[10] ? row[10] : "NULL");
...
...
    Init_Table(conn,&ATable);

Hello

how can free the memory from this code?

bastl · 07-03-2011, 02:20 PM

Quote:

Originally Posted by alireza976

Hello
how can free the memory from this code?

maybe you mean:

Code:

free(  ATable );