{"id":6783,"date":"2012-08-23T07:00:00","date_gmt":"2012-08-23T07:00:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/oldnewthing\/2012\/08\/23\/wait-you-never-said-that-i-had-to-initialize-the-object-before-i-used-it\/"},"modified":"2012-08-23T07:00:00","modified_gmt":"2012-08-23T07:00:00","slug":"wait-you-never-said-that-i-had-to-initialize-the-object-before-i-used-it","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/oldnewthing\/20120823-00\/?p=6783","title":{"rendered":"Wait, you never said that I had to initialize the object before I used it!"},"content":{"rendered":"

\nA customer reported that they were having trouble creating\n\nslim reader\/writer locks<\/a>\nat runtime.\nThey simplified the issue to a short program:\n<\/p>\n

\n#include <windows.h>\n#include <iostream>\nusing namespace std; \/\/ this is just a quick test app\nint a = 10;\n\/\/ This version works\nint working_version()\n{\n SRWLOCK lock;\n AcquireSRWLockExclusive(&lock);\n cout<<\"Acquired exclusively\"<<endl;\n a++;\n ReleaseSRWLockExclusive(&lock);\n}\n\/\/ This one doesn't\nint broken_version_1()\n{\n SRWLOCK *lock = new SRWLOCK;\n AcquireSRWLockExclusive(lock);\n cout<<\"Acquired exclusively\"<<endl;\n a++;\n ReleaseSRWLockExclusive(lock);\n \/\/ ignore the memory leak - this is just a quick test app\n}\n\/\/ This one doesn't either\nint broken_version_2()\n{\n SRWLOCK *lock = new SRWLOCK[2];\n AcquireSRWLockExclusive(&lock[0]);\n cout<<\"Acquired exclusively\"<<endl;\n a++;\n ReleaseSRWLockExclusive(&lock[0]);\n \/\/ ignore the memory leak - this is just a quick test app\n}\nint main(int argc, char **argv)\n{\n switch (argv[1][0]) {\n case '0': working_version(); break;\n case '1': broken_version_1(); break;\n case '2': broken_version_2(); break;\n }\n cout<<\"a=\"<<a<<endl;\n return 0;\n}\n<\/pre>\n
\n“What is the correct way of creating an SRWLOCK<\/code>\nvia the new<\/code> operator?”\n<\/p>\n
\nIt wasn’t long before somebody noted that nowhere in the code\nis the function\nInitialize­SRW­Lock<\/code> called.\n<\/p>\n
\n“Oh, yeah, thanks for catching that.\nIt looks like one needs to initialize SRW locks which are\ncreated via the new<\/code> operator.\nOtherwise it’s not required.”\n<\/p>\n
\nNo, the function is always<\/i> required.\nIt’s just that you got lucky in the local variable case\nand the initial stack garbage looks enough like an initialized SRW lock\nthat you don’t notice the problem.\n<\/p>\n
\nMSDN doesn’t say\n“You must initialize an SRW lock before using it”\nbecause the statement was believed to be so obvious\nthat it never occurred to anybody that somebody would\nthink the opposite was true.\nI mean, what’s the point of having an\nInitialize­SRW­Lock<\/code> function if initialization\nis not required?\nThink of it as one of the\n\nground rules for programming<\/a>:\nIf an object has an initialization method,\nyou must initialize the object before using it.\n<\/p>\n
\nBut just to be sure, I’ve submitted a documentation change request\nto add the requirement.\n<\/p>\n
\nBonus chatter<\/b>:\nA common coding pattern is to wrap the low-level C-style object\ninside a C++style RAII-style object.\n<\/p>\n
\nBonus chatter 2<\/b>:\nIf you’re creating a highly-concurrent system, then you should\nprobably put each lock on its own cache line.<\/p>\n","protected":false},"excerpt":{"rendered":"
A customer reported that they were having trouble creating slim reader\/writer locks at runtime. They simplified the issue to a short program: #include <windows.h> #include <iostream> using namespace std; \/\/ this is just a quick test app int a = 10; \/\/ This version works int working_version() { SRWLOCK lock; AcquireSRWLockExclusive(&lock); cout<<“Acquired exclusively”<<endl; a++; ReleaseSRWLockExclusive(&lock); […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-6783","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
A customer reported that they were having trouble creating slim reader\/writer locks at runtime. They simplified the issue to a short program: #include <windows.h> #include <iostream> using namespace std; \/\/ this is just a quick test app int a = 10; \/\/ This version works int working_version() { SRWLOCK lock; AcquireSRWLockExclusive(&lock); cout<<“Acquired exclusively”<<endl; a++; ReleaseSRWLockExclusive(&lock); […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/6783","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=6783"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/6783\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=6783"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=6783"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=6783"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}