Dynamically allocated string matrix in C - c

Question

I'm trying to create a string matrix in C to stores the results of a sql callback. For some reason, it always crashes on the 12th reallocation of "data" even though the memory address of data is the same.
Thanks.
int row_index;
static int db_select_cb(void *p_data ,int argc, char **argv, char **azColName){
char ***data = (char ***)p_data;
data = (char ***)realloc(data,sizeof(char **)*(row_index+1));
data[row_index] = (char **)malloc(sizeof(char *)*(argc));
for(int col_index = 0;col_index < argc;col_index++){
data[row_index][col_index] = (char *)malloc(sizeof(char)*(strlen(argv[col_index])+1));
strcpy(data[row_index][col_index],argv[col_index]);
}
row_index++;
return 0;
}
char ***db_select(sqlite3 *conn,unsigned char *zSQL){
row_index = 0;
char ***data = (char ***)malloc(sizeof(char ***)*(row_index+1));
char *err = 0;
int cerr = sqlite3_exec(conn,zSQL,db_select_cb,(void*)data,&err);
if(cerr){
printf(":: SQL ERROR IN \"db_select\" || %s ||\n", err);
sqlite3_free(err);
return 0;
}
return data;
}
Thanks for your help guys. The problem was that I needed to pass a reference to the matrix to the callback as realloc was modifying data. Here's what ended up working.
int row_index;
static int db_select_cb(void *p_data ,int argc, char **argv, char **azColName){
char ****data = (char ****)p_data;
*data = realloc(*data,sizeof(char **)*(row_index+1));
(*data)[row_index] = malloc(sizeof(char *)*(argc));
for(int col_index = 0;col_index < argc;col_index++){
(*data)[row_index][col_index] = malloc(sizeof(char)*(strlen(argv[col_index])+1));
strcpy((*data)[row_index][col_index],argv[col_index]);
}
row_index++;
return 0;
}
char ***db_select(sqlite3 *conn,unsigned char *zSQL){
row_index = 0;
char ***data = malloc(sizeof(char **)*(row_index+1));
char *err = 0;
int cerr = sqlite3_exec(conn,zSQL,db_select_cb,(void*)&data,&err);
if(cerr){
printf(":: SQL ERROR IN \"db_select\" || %s ||\n", err);
sqlite3_free(err);
return 0;
}
return data;
}
Here is an updated solution using structs which as Groo pointed out is the only way to keep track of the row and columns sizes.
typedef struct{
char ***data;
int row_size;
int *col_size;
}Table;
static int db_select_cb(void *p_table ,int argc, char **argv, char **azColName){
Table **table = (Table **)p_table;
(*table)->data = realloc((*table)->data,sizeof(char **)*((*table)->row_size+1));
(*table)->data[(*table)->row_size] = malloc(sizeof(char *)*(argc));
(*table)->col_size = realloc((*table)->col_size,sizeof(int)*((*table)->row_size+1));
int col_index;
for(col_index = 0;col_index < argc;col_index++){
(*table)->data[(*table)->row_size][col_index] = malloc(sizeof(char)*(strlen(argv[col_index])+1));
strcpy((*table)->data[(*table)->row_size][col_index],argv[col_index]);
}
(*table)->col_size[(*table)->row_size] = col_index;
(*table)->row_size++;
return 0;
}
Table *db_select(sqlite3 *conn,unsigned char *zSQL){
Table *table = malloc(sizeof(Table));
table->row_size = 0;
table->data = NULL;
table->col_size = NULL;
char *err = 0;
int cerr = sqlite3_exec(conn,zSQL,db_select_cb,(void*)&table,&err);
if(cerr){
printf(":: SQL ERROR IN \"db_select\" || %s ||\n", err);
sqlite3_free(err);
return 0;
}
return table;
}

Answer 1

Your life would be much easier if you would create a couple of sanely named structs and some tiny helper functions.
First of all, your db_select function returns an allocated data, but sets a global variable row_index. Number of columns is lost forever. But this already indicates that you need a struct - you want to pack all information that this function needs to give you into a single "coherent" block.
So, you might say a row is a bunch of columns:
typedef struct {
char *cols;
int cols_count;
} Row;
And a table is a bunch of rows:
typedef struct {
Row * rows;
int rows_count;
} Table;
And now you handle allocation and housekeeping separately (note: I am writing this in the browser, haven't even checked if it will compile):
// allocates a new table
Table * Table_create(void) {
Table * table = calloc(1, sizeof *table);
return table;
}
// creates a new child row in the table, with the specified number of cols
Row * Row_create(Table *table, int numCols) {
table = realloc(table, table->rows_count * sizeof *table);
table->rows_count++;
Row * newRow = &table->rows[table->rows_count - 1];
newRow->cols = calloc(numCols * sizeof *newRow->cols);
newRow->cols_count = numCols;
return newRow;
}
Sqlite functionality now looks quite simpler:
// this obviously allocates a new table, so somebody will have to
// free it at some point
Table * table_fetch_from_db(sqlite3 * conn, unsigned char * sql) {
Table * table = Table_create();
if (sqlite3_exec(conn, sql, load_single_row, table, NULL)) {
// handle error
}
return table;
}
int load_single_row(void *args, int numCols, char **cols, char **colNames) {
// we passed a Table* as args
Table * table = (Table*)args;
// allocate a new row inside table
Row * row = Row_create(table, numCols);
for (int i = 0; i < numCols; i++) {
int single_col_len = strlen(cols[col_index]);
row->cols[i] = malloc(single_col_len * sizeof *row->cols[i]);
strcpy(row->cols[i], cols[i]);
}
return 0;
}
If you're using C99, this code might be slightly simplified using flexible array members, because you don't need to allocate the struct and the inner array separately.
Note that I haven't tested any of this, it lacks functions for freeing tables, and possibly won't fix your actual issue. :)