Reading database in C and populating array of structure
0
I am reading from MySQl in C. But after populating structure in C when I print the values these are not printed.
MySQL Table:
mysql> select * from student;
+------+----------+-----------+----------------+-----------+----------------+
| id | name | subject_1 | subject_1_mark | subject_2 | subject_2_mark |
+------+----------+-----------+----------------+-----------+----------------+
| 1 | rock | phy | 45 | cse | 76 |
| 2 | scoot | phy | 76 | cse | 98 |
| 3 | ryan | math | 69 | ece | 99 |
| 4 | ronit | math | 56 | ece | 97 |
| 5 | raunak | che | 65 | mech | 99 |
| 6 | abhinav | che | 69 | mech | 79 |
| 7 | abhishek | mth | 97 | cse | 79 |
| 8 | montu | mth | 79 | cse | 98 |
| 9 | moti | mth | 99 | mech | 98 |
| 10 | jerry | mth | 94 | ml | 98 |
+------+----------+-----------+----------------+-----------+----------------+
MY code:
#include <mysql/mysql.h>
#include <stdio.h>
#include<malloc.h>
/* gcc database.c -L/usr/lib/mysql -lmysqlclient
*
*/
struct student {
char *id;
char *name;
char *subject_1;
char *subject_mark_1;
char *subject_2;
char *subject_mark_2;
struct student *next;
};
struct studentDB {
struct student *next;
};
void main() {
MYSQL *conn;
MYSQL_RES *res;
MYSQL_ROW row;
char *server = "localhost";
char *user = "root";
char *password = "my_sql_password"; /* set me first */
char *database = "new_db";
conn = mysql_init(NULL);
/* Connect to database */
if (!mysql_real_connect(conn, server,
user, password, database, 0, NULL, 0)) {
fprintf(stderr, "%sn", mysql_error(conn));
exit(1);
}
/* send SQL query */
if (mysql_query(conn, "select * from student")) {
fprintf(stderr, "%sn", mysql_error(conn));
exit(1);
}
res = mysql_use_result(conn);
/* output rows from table */
struct studentDB *db = (struct studentDB *)malloc(sizeof(struct studentDB));
db->next = NULL;
printf("MySQL Tables in mysql database:n");
printf("IdtNametsubject_1tsubject_mark_1tsubject_2tsubject_mark_2n");
while ((row = mysql_fetch_row(res)) != NULL){
struct student *record = (struct student *)malloc(sizeof(struct student));
record->id = row[0];
//printf("record->id %sn",record->id);
record->name = row[1];
//printf("record->id %sn",record->name);
record->subject_1 = row[2];
//printf("record->id %sn",record->subject_1);
record->subject_mark_1 = row[3];
//printf("record->id %sn",record->subject_mark_1);
record->subject_2 = row[4];
//printf("record->id %sn",record->subject_2);
record->subject_mark_2 = row[5];
//printf("record->id %sn",record->subject_mark_2);
record->next = db->next;
//printf("current record %pn",record);
db->next = record;
//printf("next record id %pn",db->next->next);
//printf("%s %s %s %s %s %sn",row[0],row[1],row[2],row[3],row[4],row[5]);
}
struct student *ptr = db->next;
//printf("record->id %pn",db->next);
while(ptr!=NULL) {
printf("nrecord id %s",ptr->name);
printf("n");
ptr = ptr->next;
}
/* close connection */
mysql_free_result(res);
mysql_close(conn);
}
My output is:
record id
record id "
record id "
record id "
record id "
record id "
record id "
record id "
record id "
record id "
mysql c database libmysql
edited Nov 22 '18 at 19:59
Govind Parmar
12.5k53563
asked Nov 22 '18 at 19:32
LCPLCP
113
add a comment |
0
I am reading from MySQl in C. But after populating structure in C when I print the values these are not printed.
MySQL Table:
mysql> select * from student;
+------+----------+-----------+----------------+-----------+----------------+
| id | name | subject_1 | subject_1_mark | subject_2 | subject_2_mark |
+------+----------+-----------+----------------+-----------+----------------+
| 1 | rock | phy | 45 | cse | 76 |
| 2 | scoot | phy | 76 | cse | 98 |
| 3 | ryan | math | 69 | ece | 99 |
| 4 | ronit | math | 56 | ece | 97 |
| 5 | raunak | che | 65 | mech | 99 |
| 6 | abhinav | che | 69 | mech | 79 |
| 7 | abhishek | mth | 97 | cse | 79 |
| 8 | montu | mth | 79 | cse | 98 |
| 9 | moti | mth | 99 | mech | 98 |
| 10 | jerry | mth | 94 | ml | 98 |
+------+----------+-----------+----------------+-----------+----------------+
MY code:
#include <mysql/mysql.h>
#include <stdio.h>
#include<malloc.h>
/* gcc database.c -L/usr/lib/mysql -lmysqlclient
*
*/
struct student {
char *id;
char *name;
char *subject_1;
char *subject_mark_1;
char *subject_2;
char *subject_mark_2;
struct student *next;
};
struct studentDB {
struct student *next;
};
void main() {
MYSQL *conn;
MYSQL_RES *res;
MYSQL_ROW row;
char *server = "localhost";
char *user = "root";
char *password = "my_sql_password"; /* set me first */
char *database = "new_db";
conn = mysql_init(NULL);
/* Connect to database */
if (!mysql_real_connect(conn, server,
user, password, database, 0, NULL, 0)) {
fprintf(stderr, "%sn", mysql_error(conn));
exit(1);
}
/* send SQL query */
if (mysql_query(conn, "select * from student")) {
fprintf(stderr, "%sn", mysql_error(conn));
exit(1);
}
res = mysql_use_result(conn);
/* output rows from table */
struct studentDB *db = (struct studentDB *)malloc(sizeof(struct studentDB));
db->next = NULL;
printf("MySQL Tables in mysql database:n");
printf("IdtNametsubject_1tsubject_mark_1tsubject_2tsubject_mark_2n");
while ((row = mysql_fetch_row(res)) != NULL){
struct student *record = (struct student *)malloc(sizeof(struct student));
record->id = row[0];
//printf("record->id %sn",record->id);
record->name = row[1];
//printf("record->id %sn",record->name);
record->subject_1 = row[2];
//printf("record->id %sn",record->subject_1);
record->subject_mark_1 = row[3];
//printf("record->id %sn",record->subject_mark_1);
record->subject_2 = row[4];
//printf("record->id %sn",record->subject_2);
record->subject_mark_2 = row[5];
//printf("record->id %sn",record->subject_mark_2);
record->next = db->next;
//printf("current record %pn",record);
db->next = record;
//printf("next record id %pn",db->next->next);
//printf("%s %s %s %s %s %sn",row[0],row[1],row[2],row[3],row[4],row[5]);
}
struct student *ptr = db->next;
//printf("record->id %pn",db->next);
while(ptr!=NULL) {
printf("nrecord id %s",ptr->name);
printf("n");
ptr = ptr->next;
}
/* close connection */
mysql_free_result(res);
mysql_close(conn);
}
My output is:
record id
record id "
record id "
record id "
record id "
record id "
record id "
record id "
record id "
record id "
mysql c database libmysql
edited Nov 22 '18 at 19:59
Govind Parmar
12.5k53563
asked Nov 22 '18 at 19:32
LCPLCP
113
It's not explicitly said in the documentation how mysql_use_result works with the memory. Does your code work with mysql_store_result? I am not sure if you can really relay on the fact that all memory stays allocated if you read one line at a time. Can you run a debugger and after you store second row in your db->next check how does db->next->next look like? Quite possibly you may need to copy the result to memory allocated by you. I used this library 20 years ago, so I'm sorry that I don't remember all details. :)
– petrch
Nov 22 '18 at 20:05
$6 = {id = 0x555555770fb1 "2", name = 0x555555770fb3 "scoot", subject_1 = 0x555555770fbc "76", subject_mark_1 = 0x555555770fc0 "se", subject_2 = 0x555555770fc3 "98", subject_mark_2 = 0x555555770fc7 "76", next = 0x0}
– LCP
Nov 23 '18 at 3:32
It works if I changed structure to use array:
– LCP
Nov 23 '18 at 3:51
struct student { int id; char name[20]; char subject_1[20]; float subject_mark_1; char subject_2[20]; float subject_mark_2; struct student *next; };
– LCP
Nov 23 '18 at 3:51
I am afraid that "when I used array it works" looks like a memory leak. You maybe do allocate array, but unless you copy the strings into those arrays it possibly works only because of some random memory leak. Internally char[20] is still char* and you just forgetting the memory for the array as you overwrite the pointer. My rule of thumb is unless the documentation says "the function returns an allocated string" or I see it in the source code I don't expect it to be true.
– petrch
Nov 23 '18 at 22:07
add a comment |
0
0
0
I am reading from MySQl in C. But after populating structure in C when I print the values these are not printed.
MySQL Table:
mysql> select * from student;
+------+----------+-----------+----------------+-----------+----------------+
| id | name | subject_1 | subject_1_mark | subject_2 | subject_2_mark |
+------+----------+-----------+----------------+-----------+----------------+
| 1 | rock | phy | 45 | cse | 76 |
| 2 | scoot | phy | 76 | cse | 98 |
| 3 | ryan | math | 69 | ece | 99 |
| 4 | ronit | math | 56 | ece | 97 |
| 5 | raunak | che | 65 | mech | 99 |
| 6 | abhinav | che | 69 | mech | 79 |
| 7 | abhishek | mth | 97 | cse | 79 |
| 8 | montu | mth | 79 | cse | 98 |
| 9 | moti | mth | 99 | mech | 98 |
| 10 | jerry | mth | 94 | ml | 98 |
+------+----------+-----------+----------------+-----------+----------------+
MY code:
#include <mysql/mysql.h>
#include <stdio.h>
#include<malloc.h>
/* gcc database.c -L/usr/lib/mysql -lmysqlclient
*
*/
struct student {
char *id;
char *name;
char *subject_1;
char *subject_mark_1;
char *subject_2;
char *subject_mark_2;
struct student *next;
};
struct studentDB {
struct student *next;
};
void main() {
MYSQL *conn;
MYSQL_RES *res;
MYSQL_ROW row;
char *server = "localhost";
char *user = "root";
char *password = "my_sql_password"; /* set me first */
char *database = "new_db";
conn = mysql_init(NULL);
/* Connect to database */
if (!mysql_real_connect(conn, server,
user, password, database, 0, NULL, 0)) {
fprintf(stderr, "%sn", mysql_error(conn));
exit(1);
}
/* send SQL query */
if (mysql_query(conn, "select * from student")) {
fprintf(stderr, "%sn", mysql_error(conn));
exit(1);
}
res = mysql_use_result(conn);
/* output rows from table */
struct studentDB *db = (struct studentDB *)malloc(sizeof(struct studentDB));
db->next = NULL;
printf("MySQL Tables in mysql database:n");
printf("IdtNametsubject_1tsubject_mark_1tsubject_2tsubject_mark_2n");
while ((row = mysql_fetch_row(res)) != NULL){
struct student *record = (struct student *)malloc(sizeof(struct student));
record->id = row[0];
//printf("record->id %sn",record->id);
record->name = row[1];
//printf("record->id %sn",record->name);
record->subject_1 = row[2];
//printf("record->id %sn",record->subject_1);
record->subject_mark_1 = row[3];
//printf("record->id %sn",record->subject_mark_1);
record->subject_2 = row[4];
//printf("record->id %sn",record->subject_2);
record->subject_mark_2 = row[5];
//printf("record->id %sn",record->subject_mark_2);
record->next = db->next;
//printf("current record %pn",record);
db->next = record;
//printf("next record id %pn",db->next->next);
//printf("%s %s %s %s %s %sn",row[0],row[1],row[2],row[3],row[4],row[5]);
}
struct student *ptr = db->next;
//printf("record->id %pn",db->next);
while(ptr!=NULL) {
printf("nrecord id %s",ptr->name);
printf("n");
ptr = ptr->next;
}
/* close connection */
mysql_free_result(res);
mysql_close(conn);
}
My output is:
record id
record id "
record id "
record id "
record id "
record id "
record id "
record id "
record id "
record id "
mysql c database libmysql
edited Nov 22 '18 at 19:59
Govind Parmar
12.5k53563
asked Nov 22 '18 at 19:32
LCPLCP
113
I am reading from MySQl in C. But after populating structure in C when I print the values these are not printed.
MySQL Table:
mysql> select * from student;
+------+----------+-----------+----------------+-----------+----------------+
| id | name | subject_1 | subject_1_mark | subject_2 | subject_2_mark |
+------+----------+-----------+----------------+-----------+----------------+
| 1 | rock | phy | 45 | cse | 76 |
| 2 | scoot | phy | 76 | cse | 98 |
| 3 | ryan | math | 69 | ece | 99 |
| 4 | ronit | math | 56 | ece | 97 |
| 5 | raunak | che | 65 | mech | 99 |
| 6 | abhinav | che | 69 | mech | 79 |
| 7 | abhishek | mth | 97 | cse | 79 |
| 8 | montu | mth | 79 | cse | 98 |
| 9 | moti | mth | 99 | mech | 98 |
| 10 | jerry | mth | 94 | ml | 98 |
+------+----------+-----------+----------------+-----------+----------------+
MY code:
#include <mysql/mysql.h>
#include <stdio.h>
#include<malloc.h>
/* gcc database.c -L/usr/lib/mysql -lmysqlclient
*
*/
struct student {
char *id;
char *name;
char *subject_1;
char *subject_mark_1;
char *subject_2;
char *subject_mark_2;
struct student *next;
};
struct studentDB {
struct student *next;
};
void main() {
MYSQL *conn;
MYSQL_RES *res;
MYSQL_ROW row;
char *server = "localhost";
char *user = "root";
char *password = "my_sql_password"; /* set me first */
char *database = "new_db";
conn = mysql_init(NULL);
/* Connect to database */
if (!mysql_real_connect(conn, server,
user, password, database, 0, NULL, 0)) {
fprintf(stderr, "%sn", mysql_error(conn));
exit(1);
}
/* send SQL query */
if (mysql_query(conn, "select * from student")) {
fprintf(stderr, "%sn", mysql_error(conn));
exit(1);
}
res = mysql_use_result(conn);
/* output rows from table */
struct studentDB *db = (struct studentDB *)malloc(sizeof(struct studentDB));
db->next = NULL;
printf("MySQL Tables in mysql database:n");
printf("IdtNametsubject_1tsubject_mark_1tsubject_2tsubject_mark_2n");
while ((row = mysql_fetch_row(res)) != NULL){
struct student *record = (struct student *)malloc(sizeof(struct student));
record->id = row[0];
//printf("record->id %sn",record->id);
record->name = row[1];
//printf("record->id %sn",record->name);
record->subject_1 = row[2];
//printf("record->id %sn",record->subject_1);
record->subject_mark_1 = row[3];
//printf("record->id %sn",record->subject_mark_1);
record->subject_2 = row[4];
//printf("record->id %sn",record->subject_2);
record->subject_mark_2 = row[5];
//printf("record->id %sn",record->subject_mark_2);
record->next = db->next;
//printf("current record %pn",record);
db->next = record;
//printf("next record id %pn",db->next->next);
//printf("%s %s %s %s %s %sn",row[0],row[1],row[2],row[3],row[4],row[5]);
}
struct student *ptr = db->next;
//printf("record->id %pn",db->next);
while(ptr!=NULL) {
printf("nrecord id %s",ptr->name);
printf("n");
ptr = ptr->next;
}
/* close connection */
mysql_free_result(res);
mysql_close(conn);
}
My output is:
record id
record id "
record id "
record id "
record id "
record id "
record id "
record id "
record id "
record id "
mysql c database libmysql
mysql c database libmysql
edited Nov 22 '18 at 19:59
Govind Parmar
12.5k53563
asked Nov 22 '18 at 19:32
LCPLCP
113
edited Nov 22 '18 at 19:59
Govind Parmar
12.5k53563
asked Nov 22 '18 at 19:32
LCPLCP
113
edited Nov 22 '18 at 19:59
Govind Parmar
12.5k53563
edited Nov 22 '18 at 19:59
Govind Parmar
12.5k53563
edited Nov 22 '18 at 19:59
Govind Parmar
12.5k53563
12.5k53563
asked Nov 22 '18 at 19:32
LCPLCP
113
asked Nov 22 '18 at 19:32
LCPLCP
113
asked Nov 22 '18 at 19:32
LCPLCP
113
113
It's not explicitly said in the documentation how mysql_use_result works with the memory. Does your code work with mysql_store_result? I am not sure if you can really relay on the fact that all memory stays allocated if you read one line at a time. Can you run a debugger and after you store second row in your db->next check how does db->next->next look like? Quite possibly you may need to copy the result to memory allocated by you. I used this library 20 years ago, so I'm sorry that I don't remember all details. :)
– petrch
Nov 22 '18 at 20:05
$6 = {id = 0x555555770fb1 "2", name = 0x555555770fb3 "scoot", subject_1 = 0x555555770fbc "76", subject_mark_1 = 0x555555770fc0 "se", subject_2 = 0x555555770fc3 "98", subject_mark_2 = 0x555555770fc7 "76", next = 0x0}
– LCP
Nov 23 '18 at 3:32
It works if I changed structure to use array:
– LCP
Nov 23 '18 at 3:51
struct student { int id; char name[20]; char subject_1[20]; float subject_mark_1; char subject_2[20]; float subject_mark_2; struct student *next; };
– LCP
Nov 23 '18 at 3:51
I am afraid that "when I used array it works" looks like a memory leak. You maybe do allocate array, but unless you copy the strings into those arrays it possibly works only because of some random memory leak. Internally char[20] is still char* and you just forgetting the memory for the array as you overwrite the pointer. My rule of thumb is unless the documentation says "the function returns an allocated string" or I see it in the source code I don't expect it to be true.
– petrch
Nov 23 '18 at 22:07
add a comment |
It's not explicitly said in the documentation how mysql_use_result works with the memory. Does your code work with mysql_store_result? I am not sure if you can really relay on the fact that all memory stays allocated if you read one line at a time. Can you run a debugger and after you store second row in your db->next check how does db->next->next look like? Quite possibly you may need to copy the result to memory allocated by you. I used this library 20 years ago, so I'm sorry that I don't remember all details. :)
– petrch
Nov 22 '18 at 20:05
$6 = {id = 0x555555770fb1 "2", name = 0x555555770fb3 "scoot", subject_1 = 0x555555770fbc "76", subject_mark_1 = 0x555555770fc0 "se", subject_2 = 0x555555770fc3 "98", subject_mark_2 = 0x555555770fc7 "76", next = 0x0}
– LCP
Nov 23 '18 at 3:32
It works if I changed structure to use array:
– LCP
Nov 23 '18 at 3:51
struct student { int id; char name[20]; char subject_1[20]; float subject_mark_1; char subject_2[20]; float subject_mark_2; struct student *next; };
– LCP
Nov 23 '18 at 3:51
I am afraid that "when I used array it works" looks like a memory leak. You maybe do allocate array, but unless you copy the strings into those arrays it possibly works only because of some random memory leak. Internally char[20] is still char* and you just forgetting the memory for the array as you overwrite the pointer. My rule of thumb is unless the documentation says "the function returns an allocated string" or I see it in the source code I don't expect it to be true.
– petrch
Nov 23 '18 at 22:07
It's not explicitly said in the documentation how mysql_use_result works with the memory. Does your code work with mysql_store_result? I am not sure if you can really relay on the fact that all memory stays allocated if you read one line at a time. Can you run a debugger and after you store second row in your db->next check how does db->next->next look like? Quite possibly you may need to copy the result to memory allocated by you. I used this library 20 years ago, so I'm sorry that I don't remember all details. :)
– petrch
Nov 22 '18 at 20:05
It's not explicitly said in the documentation how mysql_use_result works with the memory. Does your code work with mysql_store_result? I am not sure if you can really relay on the fact that all memory stays allocated if you read one line at a time. Can you run a debugger and after you store second row in your db->next check how does db->next->next look like? Quite possibly you may need to copy the result to memory allocated by you. I used this library 20 years ago, so I'm sorry that I don't remember all details. :)
– petrch
Nov 22 '18 at 20:05
$6 = {id = 0x555555770fb1 "2", name = 0x555555770fb3 "scoot", subject_1 = 0x555555770fbc "76", subject_mark_1 = 0x555555770fc0 "se", subject_2 = 0x555555770fc3 "98", subject_mark_2 = 0x555555770fc7 "76", next = 0x0}
– LCP
Nov 23 '18 at 3:32
$6 = {id = 0x555555770fb1 "2", name = 0x555555770fb3 "scoot", subject_1 = 0x555555770fbc "76", subject_mark_1 = 0x555555770fc0 "se", subject_2 = 0x555555770fc3 "98", subject_mark_2 = 0x555555770fc7 "76", next = 0x0}
– LCP
Nov 23 '18 at 3:32
It works if I changed structure to use array:
– LCP
Nov 23 '18 at 3:51
It works if I changed structure to use array:
– LCP
Nov 23 '18 at 3:51
struct student { int id; char name[20]; char subject_1[20]; float subject_mark_1; char subject_2[20]; float subject_mark_2; struct student *next; };
– LCP
Nov 23 '18 at 3:51
struct student { int id; char name[20]; char subject_1[20]; float subject_mark_1; char subject_2[20]; float subject_mark_2; struct student *next; };
– LCP
Nov 23 '18 at 3:51
I am afraid that "when I used array it works" looks like a memory leak. You maybe do allocate array, but unless you copy the strings into those arrays it possibly works only because of some random memory leak. Internally char[20] is still char* and you just forgetting the memory for the array as you overwrite the pointer. My rule of thumb is unless the documentation says "the function returns an allocated string" or I see it in the source code I don't expect it to be true.
– petrch
Nov 23 '18 at 22:07
I am afraid that "when I used array it works" looks like a memory leak. You maybe do allocate array, but unless you copy the strings into those arrays it possibly works only because of some random memory leak. Internally char[20] is still char* and you just forgetting the memory for the array as you overwrite the pointer. My rule of thumb is unless the documentation says "the function returns an allocated string" or I see it in the source code I don't expect it to be true.
– petrch
Nov 23 '18 at 22:07
add a comment |
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It's not explicitly said in the documentation how mysql_use_result works with the memory. Does your code work with mysql_store_result? I am not sure if you can really relay on the fact that all memory stays allocated if you read one line at a time. Can you run a debugger and after you store second row in your db->next check how does db->next->next look like? Quite possibly you may need to copy the result to memory allocated by you. I used this library 20 years ago, so I'm sorry that I don't remember all details. :)
– petrch
Nov 22 '18 at 20:05
$6 = {id = 0x555555770fb1 "2", name = 0x555555770fb3 "scoot", subject_1 = 0x555555770fbc "76", subject_mark_1 = 0x555555770fc0 "se", subject_2 = 0x555555770fc3 "98", subject_mark_2 = 0x555555770fc7 "76", next = 0x0}
– LCP
Nov 23 '18 at 3:32
It works if I changed structure to use array:
– LCP
Nov 23 '18 at 3:51
struct student { int id; char name[20]; char subject_1[20]; float subject_mark_1; char subject_2[20]; float subject_mark_2; struct student *next; };
– LCP
Nov 23 '18 at 3:51
I am afraid that "when I used array it works" looks like a memory leak. You maybe do allocate array, but unless you copy the strings into those arrays it possibly works only because of some random memory leak. Internally char[20] is still char* and you just forgetting the memory for the array as you overwrite the pointer. My rule of thumb is unless the documentation says "the function returns an allocated string" or I see it in the source code I don't expect it to be true.
– petrch
Nov 23 '18 at 22:07