i want to understand how dynamic memory allocation done for result of sqlite3_get_table with malloc ?

[Qt embedded developer]

I have seen that my sqlite query in string format passed to below function

intSettingList=0;
memset(sqlQuerySendBuff, 0x00, sizeof (sqlQuerySendBuff));
struct Result_Set table1;

sprintf(sqlQuerySendBuff, "SELECT IFNULL(strKey,'0'), IFNULL(strValue,'0') from tblWebSetting where bActive = 1 and bDeleted = 0");
    
table1 = get_result(sqlQuerySendBuff);

Now i am confused in below malloc based statements

ResultSet get_result(const char* fmt, ...) {

    int ret = 0;
    int nrow = 0, ncol = 0, i = 0, j = 0, count = 0;
    char *err_messg;
    char **result;
    char ***recordset;
    ResultSet resultSet_Table;
    char RET[10];

    char sql_string[QUERY_BUFF_SIZE]; //this honestly needs to be more elegant; will do for now
    va_list args;
    va_start(args, fmt);
    sql_string[0] = '\0';
    printf("Temp called 3\n");
    ret = vsprintf(sql_string, fmt, args);
    printf("Temp called 4\n");
    va_end(args);
    printf("\n%s\n", sql_string);

    //initialize resultSet_Table;
    resultSet_Table.rows = 0;
    resultSet_Table.cols = 0;
    resultSet_Table.recordset = NULL;

    //        if(sqlite3_open("/home/ap/pub/CONFIG.DB", &StructSql) == SQLITE_OK)
    //        {

    printf("Database Opened\n");
    // CTOS_LCDTPrintXY(1, 1, "DB Opened");


    ret = sqlite3_get_table(
            StructSql,
            sql_string,
            &result,
            &nrow,
            &ncol,
            &err_messg
            );
    printf("nrow=%d ncol=%d\n", nrow, ncol);



    recordset = (char ***) malloc(nrow * sizeof (char **));
    for (count = ncol; count < ((nrow + 1) * ncol); count++) {

        printf("Enter in LOOP\n");
        recordset[i] = (char **) malloc(ncol * sizeof (char *));
        for (j = 0; j < ncol; j++) {
            printf("%s ", result[count]);
            recordset[i][j] = (char *) malloc((strlen(result[count]) + 1));
            strcpy(recordset[i][j], result[count]);

            if (j != (ncol - 1))
                count++;
        }
        i++;
        printf("\n");
    }

    printf("Enter in outer loop\n");
    sqlite3_free_table(result);

    if (ret != SQLITE_OK) {
        printf("SQL error: %s\n", err_messg);
        sprintf(RET, "%d", ret);
        // InsertErrorLog
//        if (strcmp(strErrorLogDescription, "") != 0) {
//            InsertErrorlog("", 0, 0, err_messg, strErrorLogDescription);
//        }
        success = 0;
    } else {
        resultSet_Table.rows = nrow;
        resultSet_Table.cols = ncol;
        resultSet_Table.recordset = recordset;
        success = 1;
    }
    //        }
    //	if(sqlite3_close(StructSql) == SQLITE_OK)
    //        {
    //            printf("Database Closed\n");
    //            //CTOS_LCDTPrintXY(1, 2, "DB Closed");
    //        }
    memset(strErrorLogDescription, 0x00, sizeof (strErrorLogDescription));
    sqlite3_int64 intBeforeReleaseInBG = sqlite3_memory_used();
    printf("intBeforeReleaseInBG =  %lld\n", intBeforeReleaseInBG);
    int intReleaseMemory = sqlite3_release_memory(10000);		
    printf("intReleaseMemory =  %d\n", intReleaseMemory);
    sqlite3_int64 intAfterReleaseInBG = sqlite3_memory_used();
    //sqlite3_db_release_memory(StructSql);
    sqlite3_int64 intSoftHeap = sqlite3_soft_heap_limit64(250000);
    printf("intSoftHeap =  %lld\n", intSoftHeap);
    printf("intAfterReleaseInBG =  %lld\n", intAfterReleaseInBG);
    return resultSet_Table;
}

I want to understand why malloc() used like below :

recordset = (char ***) malloc(nrow * sizeof (char **));

recordset[i] = (char **) malloc(ncol * sizeof (char *));

recordset[i][j] = (char *) malloc((strlen(result[count]) + 1));

[JonB]

@Qt-embedded-developer
Well you need to look up the documentation for sqlite3_get_table() to see what it says! At some level these are required to allocate the area of memory to which the later code is going to do the strcpy()s etc. from the result set returned.

recordset = (char ***) malloc(nrow * sizeof (char **));    // space for `nrow`s of `char **`

recordset[i] = (char **) malloc(ncol * sizeof (char *));    // space for `ncol`s of `char *`

recordset[i][j] = (char *) malloc((strlen(result[count]) + 1));    // space for the length of a C string in `result[count]`, with its terminating '\0' byte

[JonB]

@Qt-embedded-developer
It is a shame that you are programming with C++ and Qt and still using malloc(). That really belongs in C programs, C++ introduced new from the outset.

Having said that, what is your question? void *malloc(size_t size) takes a size in bytes and returns a generic void *. So you specify how many bytes are required for whatever you want to store there, and you cast the returned pointer to whatever you are assigning it to.

[Qt embedded developer]

@JonB Yes i know that how to use malloc but what is logic behind memory allocation for result received from sqlite3_get_table ?

recordset = (char ***) malloc(nrow * sizeof (char **));

recordset[i] = (char **) malloc(ncol * sizeof (char *));

recordset[i][j] = (char *) malloc((strlen(result[count]) + 1));

[JonB]

@Qt-embedded-developer
Well you need to look up the documentation for sqlite3_get_table() to see what it says! At some level these are required to allocate the area of memory to which the later code is going to do the strcpy()s etc. from the result set returned.

recordset = (char ***) malloc(nrow * sizeof (char **));    // space for `nrow`s of `char **`

recordset[i] = (char **) malloc(ncol * sizeof (char *));    // space for `ncol`s of `char *`

recordset[i][j] = (char *) malloc((strlen(result[count]) + 1));    // space for the length of a C string in `result[count]`, with its terminating '\0' byte

[Qt embedded developer]

@JonB Sorry may be i am wrong and illogical. can you able tell me this logic is some what similar to 2d array like or 3d array of string like ?

[JonB]

@Qt-embedded-developer

• recordset is a 2D array/table. It holds all the rows, and is indexed by row number. There are a total of nrows allocated.
• recordset[row] is one row. It holds an array of the columns in the row. There are a total of ncols allocated.
• recorset[row][col] is one cell/value, in one column of one row. The column values are char *, i.e. C-strings. For each one the length of the string plus 1 byte for '\0' is alloacted.

rows -> columns -> cells/values of type C-string.

[Qt embedded developer]

@JonB Hi can you give example for deallocate memory for recordset ?

[JonB]

@Qt-embedded-developer
This is absolutely basic C! If you malloc(), what function do you have to call to free the allocated memory?

[Qt embedded developer]

@JonB i have to call free() to deallocate memory.

void free(void *ptr);

i have to free recordset memory inside below part else condition

if (ret != SQLITE_OK) {
       printf("SQL error: %s\n", err_messg);
       sprintf(RET, "%d", ret);
       // InsertErrorLog
//        if (strcmp(strErrorLogDescription, "") != 0) {
//            InsertErrorlog("", 0, 0, err_messg, strErrorLogDescription);
//        }
       success = 0;
   } else {
       resultSet_Table.rows = nrow;
       resultSet_Table.cols = ncol;
       resultSet_Table.recordset = recordset;
       success = 1;
   }

[JonB]

@Qt-embedded-developer
So write the code to call free()s corresponding to where you did your malloc()s. What else is there to ask/say?

[Qt embedded developer]

@JonB

i have allocated memory like below so to where and how to write free() for this all allocated memory.

recordset = (char ***) malloc(nrow * sizeof (char **));
    for (count = ncol; count < ((nrow + 1) * ncol); count++) {

        printf("Enter in LOOP\n");
        recordset[i] = (char **) malloc(ncol * sizeof (char *));
        for (j = 0; j < ncol; j++) {
            printf("%s ", result[count]);
            recordset[i][j] = (char *) malloc((strlen(result[count]) + 1));
            strcpy(recordset[i][j], result[count]);

            if (j != (ncol - 1))
                count++;
        }
        i++;
        printf("\n");
    }

---

in else part if i wrote code like below. is it right ?
```
free(recordset);
for (count = ncol; count < ((nrow + 1) * ncol); count++) {

    printf("Enter in LOOP\n");
    free(recordset[i]); 
    for (j = 0; j < ncol; j++) {
        free(recordset[i][j]);
    }
    i++;
    printf("\n");
}

[JonB]

@Qt-embedded-developer
No, that is not right.

You are freeing the items in the same order as you allocated them. Once you have done free(recordset) you must not then do free(recordset[i]), and once you have done free(recordset[i]) you must not then do free(recordset[i][j]).

Please stop and think about it/why. This is basic C. free() things in the opposite order to how you built your structure with malloc(). Do not free() something if you then go into it to access/free() something else.

[Qt embedded developer]

@JonB means i need to write code like below:

int i = 0;

for (count = ncol; count < ((nrow + 1) * ncol); count++) {

    printf("Enter in LOOP\n");
  
    for (j = 0; j < ncol; j++) {
        free(recordset[i][j]);
    }
  free(recordset[i]); 
    i++;
    printf("\n");
}

free(recordset);

And can you elaborate below sentence:
Do not free() something if you then go into it to access/free() something else.

[JonB]

@Qt-embedded-developer
In principle you at least now have the free()s in the right order.

I don't know why you have an i and a count, whether you go through the right items, and why you use (nrow + 1) (the + 1). But that's a coding issue for you to sort out. I have answered your questions, over to you to implement now.

Do not free() something if you then go into it to access/free() something else.

Exactly what I wrote; If you go e.g.

free(recordset);
free(recordset[i]);  // or anything to `recordset[i]`

think about what state recordset[i] might be in after you have free()d recordset.

[Qt embedded developer]

@JonB said in i want to understand how dynamic memory allocation done for result of sqlite3_get_table with malloc ?:

over to you to implement now

Hi

here below i allocated memory for string element in array index
recordset[i][j] = (char *) malloc((strlen(result[count]) + 1));

[mpergand]

Yeah good C exercise :)

       int rows=2, cols=3;
       const char* R1[]={"r1 c1","r1 c2","r1,c3"};
       const char* R2[]={"r2 c1","r2 c2","r2,c3"};
       const char** data[]={ R1,R2};
           
       char*** recordset;  // array of pointers (rows) to array of pointers (cols)
       recordset=(char***) malloc(sizeof(char***)*rows);
       
   	for (int r = 0; r < rows; ++r) 
   		{
   		recordset[r]=(char**) malloc(sizeof(char**)*cols);
   		for (int c = 0; c < cols; ++c) 
   			{
   			recordset[r][c]=(char*) malloc(sizeof(char)*(strlen(data[r][c])+1));
   			strcpy(recordset[r][c],data[r][c]);
   			}
   		}
   
   	for(int r=0 ; r<rows; r++)
   		std::cout<<recordset[r][0]<<", "<<recordset[r][1]<<", "<<recordset[r][2]<<", "<<std::endl;
   
   	// delete
   	for (int r = 0; r < rows; ++r) 
   		{
   		for (int c = 0; c < cols; ++c) 
   			{
   			free(recordset[r][c]);
   			}
   			
   		free(recordset[r]);
   		}
   		
   	free(recordset);

[JonB]

@mpergand said in i want to understand how dynamic memory allocation done for result of sqlite3_get_table with malloc ?:

  	recordset[r][c]=(char*) malloc(sizeof(char*));
  	strcpy(recordset[r][c],data[r][c]);

Ummm, really...? ;-)

[mpergand]

@JonB
Bien vu ;)