C Beginner question: Pointer to a pointer / Can't access values from outside a function

I'm passing a po开发者_C百科inter to a pointer (**resultSet) to my MySQL function.

Here's an excerpt on how I copy the MySQL data from within the function:

int
getItems(char * cmd, char **resultSet)
{

...
MYSQL initialisations and set-up
...



    resultSet = malloc(sizeof(char)*(int)mysql_num_rows);

    while((row = mysql_fetch_row(result))) 
    {

            for (i=0 ; i < mysql_num_fields(result); i++)               
            {   
            printf("%i: \t", i);        
            resultSet[counter] = malloc(sizeof(char)*strlen(row[i])+1);
            strcpy(resultSet[counter], row[i]);
            printf("%s\n", resultSet[counter]);
            }
            printf("---------------------\n");
            counter++;      
    }
...
MYSQL cleaning up
...
return 0;
}

Calling it in main with

getItems(cmd, resultSet);

From within my getItems function this

printf("%s\n", resultSet[0]);

seems to work.

However, if I try to access it from outside my function I get a segmentation fault. Why is this?

---

You probably want:

resultSet = malloc(sizeof(void *)*(int)mysql_num_rows);

Instead of:

resultSet = malloc(sizeof(void)*(int)mysql_num_rows);

as you need pointers not bytes.

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If you want to use resultSet as a return parameter, you need to make the function signature

int getItems(char * cmd, char ***resultSet)

and use it in the function like

*resultSet = malloc(sizeof(char)*(int)mysql_num_rows)

The function call could look like

char** results;
nitems = getItems(somecmd, &results); 

Better and simpler is probably to leave it as it is and make the allocation before the function call.

---

Two problems here:

• As indicated the allocation of resultset is wrong.
• Secondly, strlen(row[i]+1) will calculate the string length of the memory location referenced by row[i]+1. This length will be 1 less then the string length of row[i]. Since you're basically duplicating a null terminated string, use that function: resultSet[counter] = strdup((char *)row[i]);

No need to malloc and strcpy.

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As far as I can see, you have an array of char arrays that you want to fill with your data. Your initial allocation should be changed to:

resultSet = malloc(sizeof(char*)*(int)mysql_num_rows);

To reflect this (notice char*).

Also:

resultSet[counter] = malloc(sizeof(char)*strlen(row[i])+1);

This row is technically correct, but you should change it to:

resultSet[counter] = malloc(sizeof(char) * (strlen(row[i])+1) );

To reflect what you actually want to do (the reason for this is the order that C performs arithmetics, and the first approach would yield wrong results if you tried to do it on any other datatype than char/unsigned char).

Finally, I expect that you want to return the value of the 2d-array somehow. There are two ways to do this:

1. Make the function return a char** (return NULL on failure). You don't need resultSet as an input parameter here.
2. int getItems(char * cmd, char ***resultSet)

Notice that everything with resultSet in the function would have to change to *resultSet. The function can then be called with:

char **result;
int status = getItems(cmd, &result);

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You probably just need to set the return value (or the parameter) to &resultSet. But we could know for sure if you showed the parameters to the function and the calling of the function.