我有一个简单的函数，可以将一些HTTP状态代码映射到字符串：
const std::string & status_name(int code)
{
static std::unordered_map<int, std::string> status_table {
{ 0, "UNKNOWN" },
{ 100, "Continue" },
{ 101, "Switching Protocols" },
{ 200, "OK" },
{ 201, "Created" },
{ 202, "Accepted" },
{ 203, "Non-Authoritative Information" },
{ 204, "No Content" },
{ 205, "Reset Content" },
{ 206, "Partial Content" },
{ 300, "Multiple Choices" },
{ 301, "Moved Permanently" },
{ 302, "Found" },
{ 303, "See Other" },
{ 304, "Not Modified" },
{ 305, "Use Proxy" },
{ 307, "Temporary Redirect" },
{ 400, "Bad Request" },
{ 401, "Unauthorized" },
{ 402, "Payment Required" },
{ 403, "Forbidden" },
{ 404, "Not Found" },
{ 405, "Method Not Allowed" },
{ 406, "Not Acceptable" },
{ 407, "Proxy Authentication Required" },
{ 408, "Request Timeout" },
{ 409, "Conflict" },
{ 410, "Gone" },
{ 411, "Length Required" },
{ 412, "Precondition Failed" },
{ 413, "Request Entity Too Large" },
{ 414, "Request-URI Too Long" },
{ 415, "Unsupported Media Type" },
{ 416, "Requested Range Not Satisfiable" },
{ 417, "Expectation Failed" },
{ 499, "Rotten session" },
{ 500, "Internal Server Error" },
{ 501, "Not Implemented" },
{ 502, "Bad Gateway" },
{ 503, "Service Unavailable" },
{ 504, "Gateway Timeout" },
{ 505, "HTTP Version Not Supported" },
};
auto it = status_table.find(code);
if (it != status_table.end())
return it->second;
return status_table.at(0);
}
有一天，在我们的测试服务器上，发生了崩溃。分析核心转储似乎在std::out_of_range中引发了status_table.at(0);异常。另一个奇怪的事情是有问题的状态代码是200。崩溃的程序滥用多个线程，所以我的胃感觉告诉我这个问题与线程有关。
我真的很困惑甚至可能。它是 C ++ 11 ，这个版本的标准提供了有关静态变量初始化的线程安全性的保证。编译器 clang-3.3 ，架构为x64，CPU为 Intel（R）Xeon（R）CPU E5506 。
只有两件事情浮现在脑海中：
status_table但是对象本身被看作是空的。问题更新：
在做了一个小测试用例后，我能够轻松地重现问题：
#include <thread>
#include <atomic>
#include <string>
#include <unordered_map>
#include <vector>
#include <memory>
#include <iostream>
#include <unistd.h>
const std::string & status_name1(int code)
{
static std::unordered_map<int, std::string> status_table {
{ 0, "UNKNOWN" },
{ 100, "Continue" },
{ 101, "Switching Protocols" },
{ 200, "OK" },
{ 201, "Created" },
{ 202, "Accepted" },
{ 203, "Non-Authoritative Information" },
{ 204, "No Content" },
{ 205, "Reset Content" },
{ 206, "Partial Content" },
{ 300, "Multiple Choices" },
{ 301, "Moved Permanently" },
{ 302, "Found" },
{ 303, "See Other" },
{ 304, "Not Modified" },
{ 305, "Use Proxy" },
{ 307, "Temporary Redirect" },
{ 400, "Bad Request" },
{ 401, "Unauthorized" },
{ 402, "Payment Required" },
{ 403, "Forbidden" },
{ 404, "Not Found" },
{ 405, "Method Not Allowed" },
{ 406, "Not Acceptable" },
{ 407, "Proxy Authentication Required" },
{ 408, "Request Timeout" },
{ 409, "Conflict" },
{ 410, "Gone" },
{ 411, "Length Required" },
{ 412, "Precondition Failed" },
{ 413, "Request Entity Too Large" },
{ 414, "Request-URI Too Long" },
{ 415, "Unsupported Media Type" },
{ 416, "Requested Range Not Satisfiable" },
{ 417, "Expectation Failed" },
{ 499, "Rotten session" },
{ 500, "Internal Server Error" },
{ 501, "Not Implemented" },
{ 502, "Bad Gateway" },
{ 503, "Service Unavailable" },
{ 504, "Gateway Timeout" },
{ 505, "HTTP Version Not Supported" },
};
auto it = status_table.find(code);
if (it != status_table.end())
return it->second;
return status_table.at(0);
}
const std::string & status_name2(int code)
{
static std::unordered_map<int, std::string> status_table {
{ 0, "UNKNOWN" },
{ 100, "Continue" },
{ 101, "Switching Protocols" },
{ 200, "OK" },
{ 201, "Created" },
{ 202, "Accepted" },
{ 203, "Non-Authoritative Information" },
{ 204, "No Content" },
{ 205, "Reset Content" },
{ 206, "Partial Content" },
{ 300, "Multiple Choices" },
{ 301, "Moved Permanently" },
{ 302, "Found" },
{ 303, "See Other" },
{ 304, "Not Modified" },
{ 305, "Use Proxy" },
{ 307, "Temporary Redirect" },
{ 400, "Bad Request" },
{ 401, "Unauthorized" },
{ 402, "Payment Required" },
{ 403, "Forbidden" },
{ 404, "Not Found" },
{ 405, "Method Not Allowed" },
{ 406, "Not Acceptable" },
{ 407, "Proxy Authentication Required" },
{ 408, "Request Timeout" },
{ 409, "Conflict" },
{ 410, "Gone" },
{ 411, "Length Required" },
{ 412, "Precondition Failed" },
{ 413, "Request Entity Too Large" },
{ 414, "Request-URI Too Long" },
{ 415, "Unsupported Media Type" },
{ 416, "Requested Range Not Satisfiable" },
{ 417, "Expectation Failed" },
{ 499, "Rotten session" },
{ 500, "Internal Server Error" },
{ 501, "Not Implemented" },
{ 502, "Bad Gateway" },
{ 503, "Service Unavailable" },
{ 504, "Gateway Timeout" },
{ 505, "HTTP Version Not Supported" },
};
auto it = status_table.find(code);
if (it != status_table.end())
return it->second;
return status_table.at(0);
}
const std::string & status_name3(int code)
{
static std::unordered_map<int, std::string> status_table {
{ 0, "UNKNOWN" },
{ 100, "Continue" },
{ 101, "Switching Protocols" },
{ 200, "OK" },
{ 201, "Created" },
{ 202, "Accepted" },
{ 203, "Non-Authoritative Information" },
{ 204, "No Content" },
{ 205, "Reset Content" },
{ 206, "Partial Content" },
{ 300, "Multiple Choices" },
{ 301, "Moved Permanently" },
{ 302, "Found" },
{ 303, "See Other" },
{ 304, "Not Modified" },
{ 305, "Use Proxy" },
{ 307, "Temporary Redirect" },
{ 400, "Bad Request" },
{ 401, "Unauthorized" },
{ 402, "Payment Required" },
{ 403, "Forbidden" },
{ 404, "Not Found" },
{ 405, "Method Not Allowed" },
{ 406, "Not Acceptable" },
{ 407, "Proxy Authentication Required" },
{ 408, "Request Timeout" },
{ 409, "Conflict" },
{ 410, "Gone" },
{ 411, "Length Required" },
{ 412, "Precondition Failed" },
{ 413, "Request Entity Too Large" },
{ 414, "Request-URI Too Long" },
{ 415, "Unsupported Media Type" },
{ 416, "Requested Range Not Satisfiable" },
{ 417, "Expectation Failed" },
{ 499, "Rotten session" },
{ 500, "Internal Server Error" },
{ 501, "Not Implemented" },
{ 502, "Bad Gateway" },
{ 503, "Service Unavailable" },
{ 504, "Gateway Timeout" },
{ 505, "HTTP Version Not Supported" },
};
auto it = status_table.find(code);
if (it != status_table.end())
return it->second;
return status_table.at(0);
}
const std::string & status_name4(int code)
{
static std::unordered_map<int, std::string> status_table {
{ 0, "UNKNOWN" },
{ 100, "Continue" },
{ 101, "Switching Protocols" },
{ 200, "OK" },
{ 201, "Created" },
{ 202, "Accepted" },
{ 203, "Non-Authoritative Information" },
{ 204, "No Content" },
{ 205, "Reset Content" },
{ 206, "Partial Content" },
{ 300, "Multiple Choices" },
{ 301, "Moved Permanently" },
{ 302, "Found" },
{ 303, "See Other" },
{ 304, "Not Modified" },
{ 305, "Use Proxy" },
{ 307, "Temporary Redirect" },
{ 400, "Bad Request" },
{ 401, "Unauthorized" },
{ 402, "Payment Required" },
{ 403, "Forbidden" },
{ 404, "Not Found" },
{ 405, "Method Not Allowed" },
{ 406, "Not Acceptable" },
{ 407, "Proxy Authentication Required" },
{ 408, "Request Timeout" },
{ 409, "Conflict" },
{ 410, "Gone" },
{ 411, "Length Required" },
{ 412, "Precondition Failed" },
{ 413, "Request Entity Too Large" },
{ 414, "Request-URI Too Long" },
{ 415, "Unsupported Media Type" },
{ 416, "Requested Range Not Satisfiable" },
{ 417, "Expectation Failed" },
{ 499, "Rotten session" },
{ 500, "Internal Server Error" },
{ 501, "Not Implemented" },
{ 502, "Bad Gateway" },
{ 503, "Service Unavailable" },
{ 504, "Gateway Timeout" },
{ 505, "HTTP Version Not Supported" },
};
auto it = status_table.find(code);
if (it != status_table.end())
return it->second;
return status_table.at(0);
}
const std::string & status_name5(int code)
{
static std::unordered_map<int, std::string> status_table {
{ 0, "UNKNOWN" },
{ 100, "Continue" },
{ 101, "Switching Protocols" },
{ 200, "OK" },
{ 201, "Created" },
{ 202, "Accepted" },
{ 203, "Non-Authoritative Information" },
{ 204, "No Content" },
{ 205, "Reset Content" },
{ 206, "Partial Content" },
{ 300, "Multiple Choices" },
{ 301, "Moved Permanently" },
{ 302, "Found" },
{ 303, "See Other" },
{ 304, "Not Modified" },
{ 305, "Use Proxy" },
{ 307, "Temporary Redirect" },
{ 400, "Bad Request" },
{ 401, "Unauthorized" },
{ 402, "Payment Required" },
{ 403, "Forbidden" },
{ 404, "Not Found" },
{ 405, "Method Not Allowed" },
{ 406, "Not Acceptable" },
{ 407, "Proxy Authentication Required" },
{ 408, "Request Timeout" },
{ 409, "Conflict" },
{ 410, "Gone" },
{ 411, "Length Required" },
{ 412, "Precondition Failed" },
{ 413, "Request Entity Too Large" },
{ 414, "Request-URI Too Long" },
{ 415, "Unsupported Media Type" },
{ 416, "Requested Range Not Satisfiable" },
{ 417, "Expectation Failed" },
{ 499, "Rotten session" },
{ 500, "Internal Server Error" },
{ 501, "Not Implemented" },
{ 502, "Bad Gateway" },
{ 503, "Service Unavailable" },
{ 504, "Gateway Timeout" },
{ 505, "HTTP Version Not Supported" },
};
auto it = status_table.find(code);
if (it != status_table.end())
return it->second;
return status_table.at(0);
}
const std::string & status_name6(int code)
{
static std::unordered_map<int, std::string> status_table {
{ 0, "UNKNOWN" },
{ 100, "Continue" },
{ 101, "Switching Protocols" },
{ 200, "OK" },
{ 201, "Created" },
{ 202, "Accepted" },
{ 203, "Non-Authoritative Information" },
{ 204, "No Content" },
{ 205, "Reset Content" },
{ 206, "Partial Content" },
{ 300, "Multiple Choices" },
{ 301, "Moved Permanently" },
{ 302, "Found" },
{ 303, "See Other" },
{ 304, "Not Modified" },
{ 305, "Use Proxy" },
{ 307, "Temporary Redirect" },
{ 400, "Bad Request" },
{ 401, "Unauthorized" },
{ 402, "Payment Required" },
{ 403, "Forbidden" },
{ 404, "Not Found" },
{ 405, "Method Not Allowed" },
{ 406, "Not Acceptable" },
{ 407, "Proxy Authentication Required" },
{ 408, "Request Timeout" },
{ 409, "Conflict" },
{ 410, "Gone" },
{ 411, "Length Required" },
{ 412, "Precondition Failed" },
{ 413, "Request Entity Too Large" },
{ 414, "Request-URI Too Long" },
{ 415, "Unsupported Media Type" },
{ 416, "Requested Range Not Satisfiable" },
{ 417, "Expectation Failed" },
{ 499, "Rotten session" },
{ 500, "Internal Server Error" },
{ 501, "Not Implemented" },
{ 502, "Bad Gateway" },
{ 503, "Service Unavailable" },
{ 504, "Gateway Timeout" },
{ 505, "HTTP Version Not Supported" },
};
auto it = status_table.find(code);
if (it != status_table.end())
return it->second;
return status_table.at(0);
}
const std::string & status_name7(int code)
{
static std::unordered_map<int, std::string> status_table {
{ 0, "UNKNOWN" },
{ 100, "Continue" },
{ 101, "Switching Protocols" },
{ 200, "OK" },
{ 201, "Created" },
{ 202, "Accepted" },
{ 203, "Non-Authoritative Information" },
{ 204, "No Content" },
{ 205, "Reset Content" },
{ 206, "Partial Content" },
{ 300, "Multiple Choices" },
{ 301, "Moved Permanently" },
{ 302, "Found" },
{ 303, "See Other" },
{ 304, "Not Modified" },
{ 305, "Use Proxy" },
{ 307, "Temporary Redirect" },
{ 400, "Bad Request" },
{ 401, "Unauthorized" },
{ 402, "Payment Required" },
{ 403, "Forbidden" },
{ 404, "Not Found" },
{ 405, "Method Not Allowed" },
{ 406, "Not Acceptable" },
{ 407, "Proxy Authentication Required" },
{ 408, "Request Timeout" },
{ 409, "Conflict" },
{ 410, "Gone" },
{ 411, "Length Required" },
{ 412, "Precondition Failed" },
{ 413, "Request Entity Too Large" },
{ 414, "Request-URI Too Long" },
{ 415, "Unsupported Media Type" },
{ 416, "Requested Range Not Satisfiable" },
{ 417, "Expectation Failed" },
{ 499, "Rotten session" },
{ 500, "Internal Server Error" },
{ 501, "Not Implemented" },
{ 502, "Bad Gateway" },
{ 503, "Service Unavailable" },
{ 504, "Gateway Timeout" },
{ 505, "HTTP Version Not Supported" },
};
auto it = status_table.find(code);
if (it != status_table.end())
return it->second;
return status_table.at(0);
}
const std::string & status_name8(int code)
{
static std::unordered_map<int, std::string> status_table {
{ 0, "UNKNOWN" },
{ 100, "Continue" },
{ 101, "Switching Protocols" },
{ 200, "OK" },
{ 201, "Created" },
{ 202, "Accepted" },
{ 203, "Non-Authoritative Information" },
{ 204, "No Content" },
{ 205, "Reset Content" },
{ 206, "Partial Content" },
{ 300, "Multiple Choices" },
{ 301, "Moved Permanently" },
{ 302, "Found" },
{ 303, "See Other" },
{ 304, "Not Modified" },
{ 305, "Use Proxy" },
{ 307, "Temporary Redirect" },
{ 400, "Bad Request" },
{ 401, "Unauthorized" },
{ 402, "Payment Required" },
{ 403, "Forbidden" },
{ 404, "Not Found" },
{ 405, "Method Not Allowed" },
{ 406, "Not Acceptable" },
{ 407, "Proxy Authentication Required" },
{ 408, "Request Timeout" },
{ 409, "Conflict" },
{ 410, "Gone" },
{ 411, "Length Required" },
{ 412, "Precondition Failed" },
{ 413, "Request Entity Too Large" },
{ 414, "Request-URI Too Long" },
{ 415, "Unsupported Media Type" },
{ 416, "Requested Range Not Satisfiable" },
{ 417, "Expectation Failed" },
{ 499, "Rotten session" },
{ 500, "Internal Server Error" },
{ 501, "Not Implemented" },
{ 502, "Bad Gateway" },
{ 503, "Service Unavailable" },
{ 504, "Gateway Timeout" },
{ 505, "HTTP Version Not Supported" },
};
auto it = status_table.find(code);
if (it != status_table.end())
return it->second;
return status_table.at(0);
}
int main(int argc, char ** argv)
{
std::vector<std::thread> thread_pool;
auto guard_variable = std::make_shared<std::atomic<int>>(0);
for (int i = 0; i < 8; ++i) {
thread_pool.push_back(std::thread([guard_variable] {
while (!guard_variable->load(std::memory_order_relaxed)) {
/// Noop.
}
int i = 0;
i += status_name1(200).length();
i += status_name2(200).length();
i += status_name3(200).length();
i += status_name4(200).length();
i += status_name5(200).length();
i += status_name6(200).length();
i += status_name7(200).length();
i += status_name8(200).length();
std::cout << i << std::endl;
}));
}
usleep(50000);
guard_variable->store(1, std::memory_order_relaxed);
for (auto && thread : thread_pool) {
thread.join();
}
return 0;
}
此代码除了在该测试计算机上以外的其他地方工作。无论我使用什么版本的 clang （3.3,3.6），这个测试程序会立即崩溃，在不同的status_name*函数中的不同时间崩溃。
什么可能导致这种行为？ CPU缓存不一致还是 libc ++ 中的错误？
答案 0 :(得分：1)
Clang支持＆＃34;魔法静电＆＃34; （N2660 动态初始化和并发破坏），因为每http://clang.llvm.org/cxx_status.html有2.9个。
而不是编译器或您的直接代码中的错误，代码中其他地方的错误更有可能破坏程序状态，导致status_table显示为已初始化但为空。