mapped root: null
时间: 2024-05-31 08:07:58 浏览: 17
As an AI language model, I cannot provide a specific answer without more context. However, in general, "mapped root: null" could mean that there is no root path or starting point for a particular mapping process or operation. It could also indicate that there was an error or issue in the mapping process that prevented the root from being properly mapped or identified.
相关问题
[{"key":"id1","value":"1321321"},{"key":"id3","value":"312313"}]。转成{id1:1321321,id3:312313}
您可以使用以下代码将给定的JSON格式转换为{id1:1321321,id3:312313}的形式:
```cpp
#include <iostream>
#include <map>
#include <string>
#include <sstream>
#include <json/json.h> //引入jsoncpp库
int main() {
std::string json = R"([{"key":"id1","value":"1321321"},{"key":"id3","value":"312313"}])";
Json::Value root;
Json::CharReaderBuilder reader;
std::istringstream iss(json);
std::string errs;
// 解析JSON
if (Json::parseFromStream(reader, iss, &root, &errs)) {
std::map<std::string, std::string> result;
// 遍历JSON数组中的每个元素
for (const auto& item : root) {
std::string key = item["key"].asString();
std::string value = item["value"].asString();
result[key = value;
}
// 输出转换后的结果
std::cout << "{";
for (const auto& pair : result) {
std::cout << pair.first << ":" << pair.second << ",";
}
std::cout << "}";
} else {
std::cout << "JSON解析错误: " << errs;
}
return 0;
}
```
请注意,这段代码使用了jsoncpp库来解析和处理JSON。在运行之前,请确保已经正确安装和配置了jsoncpp库。<span class="em">1</span><span class="em">2</span><span class="em">3</span>
#### 引用[.reference_title]
- *1* *2* *3* [c++ std::map value_type、key_type、mapped_type](https://blog.csdn.net/janeqi1987/article/details/100049597)[target="_blank" data-report-click={"spm":"1018.2226.3001.9630","extra":{"utm_source":"vip_chatgpt_common_search_pc_result","utm_medium":"distribute.pc_search_result.none-task-cask-2~all~insert_cask~default-1-null.142^v92^chatsearchT3_1"}}] [.reference_item style="max-width: 100%"]
[ .reference_list ]
Linux下hugetlbpage使用详解
HugeTLB Pages(大页面)是Linux内核的一种特性,它允许我们使用大于4KB的页面大小来管理内存,这可以提高大量内存操作的效率。下面是Linux下HugeTLB Pages的使用详解:
1. 查看系统是否支持HugeTLB Pages
使用以下命令查看系统是否支持HugeTLB Pages:
```
grep Huge /proc/meminfo
```
如果输出中包含HugePages_Total和Hugepagesize,则表示系统支持HugeTLB Pages。
2. 创建HugeTLB Pages
使用以下命令创建HugeTLB Pages:
```
sudo mkdir /mnt/hugepages
sudo mount -t hugetlbfs nodev /mnt/hugepages
```
这将在/mnt/hugepages目录下创建HugeTLB Pages。
3. 分配HugeTLB Pages
使用以下命令分配HugeTLB Pages:
```
sudo sysctl -w vm.nr_hugepages=64
```
这将分配64个HugeTLB Pages。可以根据需要调整该值。
4. 使用HugeTLB Pages
可以使用mmap函数将HugeTLB Pages映射到进程的地址空间中:
```c
#include <sys/mman.h>
#include <fcntl.h>
#include <stdlib.h>
#include <stdio.h>
#define LENGTH (1UL << 30)
int main(int argc, char *argv[]) {
int fd;
void *addr;
fd = open("/mnt/hugepages/test", O_CREAT | O_RDWR, 0755);
if (fd < 0) {
perror("open");
exit(1);
}
addr = mmap(NULL, LENGTH, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_HUGETLB, fd, 0);
if (addr == MAP_FAILED) {
perror("mmap");
exit(1);
}
printf("mapped address: %p\n", addr);
return 0;
}
```
这段代码将/mnt/hugepages/test文件映射到进程的地址空间中,并使用HugeTLB Pages来管理内存。
注意:使用HugeTLB Pages的程序需要以root权限运行。
以上就是Linux下HugeTLB Pages的使用详解。
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