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我有樸素簡單的功能，一些HTTP狀態代碼映射到字符串：C++ 11功能的靜態變量初始化古怪
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，我們的測試服務器上，已經發生碰撞。分析核心轉儲似乎std::out_of_range異常被拋出status_table.at(0);。另一個奇怪的是，有問題的狀態碼是200。程序崩潰濫用多線程，所以我的胃感覺告訴我，這個問題是線程相關的。
我真的很困惑它甚至有可能。它是C++ 11和這個版本的標準提供了關於靜態變量初始化的線程安全的保證。編譯器是clang-3.3，架構是x64，CPU是Intel（R）CPU E5506。
的只有兩件事情浮現在腦海中：
- 有一個在我錯過看到代碼的一些bug;
- 這個怪異與CPU緩存的非一致性有關：導致崩潰的線程看到了防護變量，表示
status_table的「初始化」狀態，但是對象本身被看作是空的。
更新的問題：
做一個小的測試情況下，我能夠很容易地重現該問題後：
#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;
}
此代碼按預期工作在其他地方比其他測試機器。不管是什麼版本鐺我用（3.3,3.6）這個測試程序立即崩潰，在不同的時間立即在不同的status_name*函數中。
什麼可能導致此行爲？是CPU緩存不一致還是它的錯誤libC++？
@JohnZwinck請參閱http://en.cppreference.com/w/cpp/container/unordered_map/at – ecatmur
您應該使用'const char *'作爲映射類型而不是'std :: string'。無論如何，這是因爲你存儲的是文字。 –
@JohnZwinck我完全錯過了所有觀點......我使用'std :: string'，因爲我想'const std :: string＆'作爲函數調用的結果，即收集'靜態'字符串。 – GreenScape