mysql_stmt_close时隔离错误

Question

我正在尝试使用C API查询mysql 5.5。
dbutil.c 包含样板代码：

#include "../include/dbutil.h"
#include "../include/logging.h"
#include "../include/common.h"
MYSQL get_connection(char *host, char *user, char *passwd, char *db) {
    MYSQL conn;
    mysql_init(&conn);
    if (!mysql_real_connect(&conn, host, user, passwd, db, 0, NULL, 0)) {
        log_to_console("Cannot connect to MySQL server: %s", mysql_error(&conn));
        exit(1);
    }
    return conn;
}
MYSQL_STMT prepare_stmt(MYSQL *conn, char *sql) {
    MYSQL_STMT *stmtP = mysql_stmt_init(conn);
    if (!stmtP) {
        log_to_console("Cannot create statement, out of memory.");
        exit(1);
    }
    MYSQL_STMT stmt = *stmtP;
    if (mysql_stmt_prepare(&stmt, sql, strlen(sql))) {
        log_to_console("Preparing statement failed: %s", mysql_error(conn));
        exit(1);
    }
    return stmt;
}
void bind_param(MYSQL_BIND *param, enum enum_field_types fieldType, char* buffer, my_bool *isNull, unsigned long *length) {
    param->buffer_type = fieldType;
    param->buffer = buffer;
    param->is_null = isNull;
    param->length = length;
}
void bind_string_param(MYSQL_BIND *param, int stringSize, char* buffer, my_bool *isNull, unsigned long *length) {
    bind_param(param, MYSQL_TYPE_STRING, buffer, isNull, length);
    param->buffer_length = stringSize;
}
void bind_result(MYSQL_BIND *result, enum enum_field_types fieldType, char* buffer, my_bool *isNull, unsigned long *length) {
    bind_param(result, fieldType, buffer, isNull, length);
}
void bind_string_result(MYSQL_BIND *result, int stringSize, char* buffer, my_bool *isNull, unsigned long *length) {
    bind_string_param(result, stringSize, buffer, isNull, length);
}

下面是实际运行查询的代码：

#include "../include/dbutil.h"
int main() {
    MYSQL conn = get_connection("localhost", "user", "pass", "test");
    MYSQL_STMT stmt = prepare_stmt(&conn, "SELECT hostname from hosts where id = ?");
    MYSQL_BIND param[1], result[1];
    memset(param, 0, sizeof(param));
    memset(result, 0, sizeof(result));
    int in = 1;
    unsigned long length = 200;
    my_bool isNull[2];
    isNull[0] = 0;
    char out[200];
    bind_param(&param[0], MYSQL_TYPE_LONG, (char *) &in, &isNull[0], (unsigned long *)0);
    bind_string_result(&result[0], 200, (char *) &out, &isNull[1], &length);
    mysql_stmt_bind_param(&stmt, param);
    mysql_stmt_execute(&stmt);
    mysql_stmt_bind_result(&stmt, result);
    mysql_stmt_store_result(&stmt);
    mysql_stmt_fetch(&stmt);
    printf("Restult %sn", out);
    mysql_stmt_close(&stmt);
    mysql_close(&conn);
    return 0;
}

语句执行并返回预期结果，但代码在命中时出现段错误

mysql_stmt_close(&stmt);

我已经浏览了核心转储，发现段错误是由libc free（）语句引起的，我认为该语句是在libmysql.c的free_root（）中调用的。

my_bool STDCALL mysql_stmt_close(MYSQL_STMT *stmt)
{
  MYSQL *mysql= stmt->mysql;
  int rc= 0;
  DBUG_ENTER("mysql_stmt_close");
  free_root(&stmt->result.alloc, MYF(0));
  free_root(&stmt->mem_root, MYF(0));
  free_root(&stmt->extension->fields_mem_root, MYF(0));
...

具体来说。核心转储表示 *stmt.extension 是发生语句关闭的时间。任何想法为什么会这样？

Answer 1

听起来代码/库正在尝试free()一些已经发布的内存。

经过一番谷歌搜索，我找到了这个链接。在调用 mysql_stmt_close() 之前，您似乎应该检查以确保stmt不是 NULL。如果stmt已经为 NULL，则库已经清理了它，无需调用 mysql_stmt_close() 。

Answer 2

在此处查看 out 参数：

char out[200];
...
bind_string_result(&result[0], 200, (char *) &out, &isNull[1], &length);

如果您看到bind_string_result声明：

void bind_string_result(MYSQL_BIND *result, int stringSize, char* buffer, my_bool *isNull, unsigned long *length) {

该函数期望在您发送指向char *的指针时接收 char * 类型的参数，换句话说，char **

编译器不会给您任何警告，因为您要将参数强制转换为char *但它可能会在内存中产生冲突。 尝试调用函数为：

bind_string_result(&result[0], 200, out, &isNull[1], &length);