我正在使用“MariaDB Connector/C”做作业，但遇到了一个问题：当我传入一个字符串参数时，总是得到一个空字符串，数据库表是：

MariaDB none@(none):test> SELECT * FROM t3

| 一种|最佳|
| - -|- -|
| 第0页|abc公司|
| 一个|二进制编码|
| 2个|自动增益控制|
第一个
| 字段|型号|空值|关键字|默认值|额外的|
| - -|- -|- -|- -|- -|- -|
| 一种|整数（11）|否|优先级|||
| 最佳|充电器（10）|是的||||

2 rows in set
Time: 0.011s

而我用来测试的代码：

# 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);

}

我只得到了一个空的结果：

❯ ./Exec/test/stmt_test
Connected...

这两天我一直在为这事发愁，明天就是最后期限了，我很着急。你能帮忙吗？谢谢！