Can't transmate string through MariaDB connect/c Prepared Statement - c

Question

I'm using "MariaDB Connector/C" for my homework, but I got a problem: I always get an empty string when I pass in a string parameter, the db table is:
MariaDB none#(none):test> SELECT * FROM t3
a
b
0
abc
1
bcd
2
af
3 rows in set
Time: 0.010s
MariaDB none#(none):test> DESC t3
Field
Type
Null
Key
Default
Extra
a
int(11)
NO
PRI
b
char(10)
YES
2 rows in set
Time: 0.011s
And the code I use to test:
#include <mysql/mysql.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main()
{
MYSQL *mysql;
mysql = mysql_init(NULL);
if (!mysql_real_connect(mysql,NULL , "none", "linux", "test", 0,"/tmp/mariadb.sock",0)){
printf( "Error connecting to database: %s",mysql_error(mysql));
} else
printf("Connected...\n");
if(mysql_real_query(mysql,"SET CHARACTER SET utf8",(unsigned int)sizeof("SET CHARACTER SET utf8"))){
printf("Failed to set Encode!\n");
}
char query_stmt_2[]="select * from t3 where b=?";
MYSQL_STMT *stmt2 = mysql_stmt_init(mysql);
if(mysql_stmt_prepare(stmt2, query_stmt_2, -1))
{
printf("STMT2 prepare failed.\n");
}
MYSQL_BIND instr_bind;
char instr[50]="abc";
my_bool in_is_null = 0;
my_bool in_error = 0;
instr_bind.buffer_type = MYSQL_TYPE_STRING;
instr_bind.buffer = &instr[0];
char in_ind = STMT_INDICATOR_NTS;
instr_bind.u.indicator = &in_ind;
unsigned long instr_len=sizeof(instr);
// instr_bind.length = &instr_len;
// instr_bind.buffer_length=instr_len;
instr_bind.is_null = &in_is_null;
instr_bind.error = &in_error;
MYSQL_BIND out_bind[2];
memset(out_bind, 0, sizeof(out_bind));
int out_int[2];
char outstr[50];
my_bool out_int_is_null[2]={0,0};
my_bool out_int_error[2]={0,0};
unsigned long out_int_length[2]={0,0};
out_bind[0].buffer = out_int+0;
out_bind[0].buffer_type = MYSQL_TYPE_LONG;
out_bind[0].is_null = out_int_is_null+0;
out_bind[0].error = out_int_error+0;
out_bind[0].length = out_int_length+0;
out_bind[1].buffer = outstr;
out_bind[1].buffer_type = MYSQL_TYPE_STRING;
out_bind[1].buffer_length = 50;
out_bind[1].is_null = out_int_is_null+1;
out_bind[1].error = out_int_error+1;
out_bind[1].length = out_int_length+1;
if(mysql_stmt_bind_param(stmt2, &instr_bind) ||
mysql_stmt_bind_result(stmt2, out_bind)){
printf("Bind error\n");
}
if(mysql_stmt_execute(stmt2))
{
printf("Exec error: %s",mysql_stmt_error(stmt2));
}
if(mysql_stmt_store_result(stmt2)){
printf("Store result error!\n");
printf("%s\n",mysql_stmt_error(stmt2));
}
while(!mysql_stmt_fetch(stmt2))
{
printf("%d\t%s\n", out_int[0], outstr);
}
mysql_stmt_close(stmt2);
end:
mysql_close(mysql);
}
I only got an empty result:
❯ ./Exec/test/stmt_test
Connected...
I have been in trouble with this for two days, and tomorrow is the deadline, I'm very anxious. Can you help? Thanks a lot!

Answer 1

1) General
Avoid "it was hard to write, so it should be hard to read" code
add variable declarations at the beginning of the function, not in the middle of code (Wdeclaration-after-statement)
don't use c++ comments in C
set character set with api function mysql_set_character_set()
write proper error handling, including mysql_error/mysql_stmt_error results and don't continue executing subsequent code after error.
always initialize MYSQL_BIND
2) input bind buffer
u.indicator is used for bulk operations and doesn't make sense here
bind.is_null is not required, since you specified a valid buffer address
buffer_length is not set (in comments)
3) Output bind buffer
Always bind output parameters after mysql_stmt_execute(), since mysql_stmt_prepare can't always determine the number of parameters, e.g. when calling a stored procedure: In this case mysql_stmt_bind_param will return an error.
binding an error indicator doesn't make much sense without setting MYSQL_REPORT_DATA_TRUNCATION (mysql_optionsv)
For some examples how to deal with prepared statements check the file ps.c of MariaDB Connector/C unit tests