A simple program is:
I would like to get the thread ID of both of the threads using this gettid function. I do not want to do the sysCall directly. I want to use this function.
#include <iostream>
#include <boost/thread/thread.hpp>
#include <boost/date_time/date.hpp>
#include <unistd.h>
#include <sys/types.h>
using namespace boost;
using namespace std;
boost::thread thread_obj;
boost::thread thread_obj1;
void func(void)
{
char x;
cout << "enter y to interrupt" << endl;
cin >> x;
pid_t tid = gettid();
cout << "tid:" << tid << endl;
if (x == 'y') {
cout << "x = 'y'" << endl;
cout << "thread interrupt" << endl;
}
}
void real_main() {
cout << "real main thread" << endl;
pid_t tid = gettid();
cout << "tid:" << tid << endl;
boost::system_time const timeout = boost::get_system_time() + boost::posix_time::seconds(3);
try {
boost::this_thread::sleep(timeout);
}
catch (boost::thread_interrupted &) {
cout << "thread interrupted" << endl;
}
}
int main()
{
thread_obj1 = boost::thread(&func);
thread_obj = boost::thread(&real_main);
thread_obj.join();
}
It gives Error on compilation; The use of gettid() has been done according to the man page:
$g++ -std=c++11 -o Intrpt Interrupt.cpp -lboost_system -lboost_thread
Interrupt.cpp: In function ‘void func()’:
Interrupt.cpp:17:25: error: ‘gettid’ was not declared in this scope
pid_t tid = gettid();
This is a silly glibc bug. Work around it like this:
#include <unistd.h>
#include <sys/syscall.h>
#define gettid() syscall(SYS_gettid)
The man page you refer to can be read online here. It clearly states:
Note: There is no glibc wrapper for this system call; see NOTES.
and
NOTES
Glibc does not provide a wrapper for this system call; call it using syscall(2).
The thread ID returned by this call is not the same thing as a POSIX thread ID (i.e., the opaque value returned by pthread_self(3)).
So you can't. The only way to use this function is through the syscall.
But you probably shouldn't anyway. You can use pthread_self() (and compare using pthread_equal(t1, t2)) instead. It's possible that boost::thread has its own equivalent too.
Additional to the solution provided by Glenn Maynard it might be appropriate to check the glibc version and only if it is lower than 2.30 define the suggested macro for gettid().
#if __GLIBC__ == 2 && __GLIBC_MINOR__ < 30
#include <sys/syscall.h>
#define gettid() syscall(SYS_gettid)
#endif
Related
Scenario
I have a C++ function which intakes a parameter as std::chrono::milliseconds. It is basically a timeout value. And, it is a default parameter set to some value by default.
Code
#include <iostream>
#include <chrono>
void Fun(const std::chrono::milliseconds someTimeout = std::chrono::milliseconds(100)) {
if (someTimeout > 0) {
std::cout << "someNumberInMillis is: " << someNumberInMillis.count() << std::endl;
}
}
int main() {
unsigned int someValue = 500;
Fun(std::chrono::milliseconds(someValue))
}
Issue
All of above is okay but, when I call Fun with a value then fails to compile and I get the following error:
No viable conversion from 'bool' to 'std::chrono::milliseconds' (aka
'duration >')
Question:
What am I doing wrong here? I want the caller of Fun to be explicitly aware that it is using std::chrono::milliseconds when it invokes Fun. But the compiler doesn't seem to allow using std::chrono::milliseconds as a parameter?
How use std::chrono::milliseconds as a default parameter?
Environment
Compiler used is clang on macOS High Sierra
With the other syntax errors fixed, this compiles without warnings in GCC 9:
#include <iostream>
#include <chrono>
void Fun(const std::chrono::milliseconds someNumberInMillis
= std::chrono::milliseconds(100))
{
if (someNumberInMillis > std::chrono::milliseconds{0}) {
std::cout << "someNumberInMillis is: " << someNumberInMillis.count()
<< std::endl;
}
}
int main()
{
unsigned int someValue = 500;
Fun(std::chrono::milliseconds(someValue));
}
The following is not possible for any boost output archive:
int foo(){
return 4;
}
ar << static_cast<unsigned int>(foo());
Is there an alternative without out creating a local temporary x=foo().
and why is the underlying archive operator <<(T & t) not const reference , for an output archive such that the above would work?
This seems to work, and I think this is why:
... To help detect such cases, output archive operators expect to be
passed const reference arguments.
It seems worth noting that in your example ar << foo(); does not work either (i.e. it doesn't have to do with your cast).
#include <fstream>
#include <iostream>
#include <boost/serialization/serialization.hpp>
#include <boost/archive/text_iarchive.hpp>
#include <boost/archive/text_oarchive.hpp>
unsigned int foo(){
return 4;
}
int main()
{
{
std::ofstream outputStream("someFile.txt");
boost::archive::text_oarchive outputArchive(outputStream);
outputArchive << static_cast<const int&>(foo());
}
std::ifstream inputStream("someFile.txt");
boost::archive::text_iarchive inputArchive(inputStream);
int readBack;
inputArchive >> readBack;
std::cout << "Read back: " << readBack << std::endl;
return 0;
}
I want to move a stringstream, in the real world application I have some stringstream class data member, which I want to reuse for different string's during operation.
stringstream does not have a copy-assignment or copy constructor, which makes sense. However, according to cppreference.com and cplusplus.com std::stringstream should have a move assignment and swap operation defined. I tried both, and both fail.
Move assignment
#include <string> // std::string
#include <iostream> // std::cout
#include <sstream> // std::stringstream
int main () {
std::stringstream stream("1234");
//stream = std::move(std::stringstream("5678"));
stream.operator=(std::move(std::stringstream("5678")));
//stream.operator=(std::stringstream("5678"));
return 0;
}
source: http://ideone.com/Izyanb
prog.cpp:11:56: error: use of deleted function ‘std::basic_stringstream<char>& std::basic_stringstream<char>::operator=(const std::basic_stringstream<char>&)’
stream.operator=(std::move(std::stringstream("5678")));
The compiler states that there is no copy assignment for all three statements, which is true. However, I fail to see why it is not using the move-assignment, especially since std::move is supposed to return a rvalue reference. Stringstream should have a move assignment, as shown here: http://en.cppreference.com/w/cpp/io/basic_stringstream/operator%3D
PS: I'm working with c++11, hence rvalue-references are part of the 'world'.
Swap
This I found really strange, I copied example code from cplusplus.com and it failed:
// swapping stringstream objects
#include <string> // std::string
#include <iostream> // std::cout
#include <sstream> // std::stringstream
int main () {
std::stringstream foo;
std::stringstream bar;
foo << 100;
bar << 200;
foo.swap(bar);
int val;
foo >> val; std::cout << "foo: " << val << '\n';
bar >> val; std::cout << "bar: " << val << '\n';
return 0;
}
source: http://ideone.com/NI0xMS
cplusplus.com source: http://www.cplusplus.com/reference/sstream/stringstream/swap/
prog.cpp: In function ‘int main()’:
prog.cpp:14:7: error: ‘std::stringstream’ has no member named ‘swap’
foo.swap(bar);
What am I missing? Why can't I move or swap a stringstream? How should I swap or move a stringstream?
This is a missing feature on GCC : see bug 54316 , it has been fixed (you can thank Jonathan Wakely) for the next versions (gcc 5)
Clang with libc++ compiles this code :
int main () {
std::stringstream stream("1234");
std::stringstream stream2 = std::move(std::stringstream("5678"));
return 0;
}
Live demo
And it also compiles the example with std::stringstream::swap
I have an alternative to moving or swapping, one can also clear and set a stringstream to a new string:
#include <string> // std::string
#include <iostream> // std::cout
#include <sstream> // std::stringstream
int main () {
std::stringstream ss("1234");
ss.clear();
ss.str("5678");
int val;
ss >> val; std::cout << "val: " << val << '\n';
return 0;
}
It's a clean work around that does not require one to refactor code, except for the localized section where the swap is changed to a clear() and str().
Trying to invoke hi or std::move on it results in this:
/tmp/cch3DRvH.o: In function `main':
main.cpp:(.text.startup+0x5): undefined reference to `hi(int, char**)'
collect2: error: ld returned 1 exit status
#include <type_traits>
#include <utility>
#include <iostream>
#include <typeinfo>
#include <functional>
int main(int argc, char* argv[])
{
decltype(main) hi;
decltype(main) hi2;
// std::function<int(int, char**)> hi2 = std::move(hi);
// hi(argc, argv);
std::cout << std::boolalpha;
std::cout << std::is_same<decltype(main), decltype(hi)>::value << std::endl;
std::cout << std::is_function<decltype(main)>::value << std::endl;
std::cout << std::is_function<decltype(hi)>::value << std::endl;
std::cout << std::is_same<decltype(std::move(main)), decltype(std::move(hi))>::value << std::endl;
std::cout << std::is_same<decltype(hi2), decltype(hi)>::value << std::endl;
return 0;
}
Output:
main.cpp: In function ‘int main(int, char**)’:
main.cpp:16:54: warning: ISO C++ forbids taking address of function ‘::main’ [-Wpedantic]
std::cout << std::is_same<decltype(std::move(main)), decltype(std::move(hi))>::value << std::endl;
main.cpp:16:54: warning: ISO C++ forbids taking address of function ‘::main’ [-Wpedantic]
true
true
true
true
true
It seems hi is almost the same as main except it doesn't give warnings like when using std::cout << hi. Is it possible to get this program to not output undefined reference tohi(int, char**)'`?
decltype(main) is an alias for the type int(int, char **). You have declared a function hi with this type but did not define it.
To get rid of the undefined reference error, just define the function:
int hi( int, char ** ) {
return 0;
}
The messages that appear before true are not output from your program but from the compiler.
I am trying to use a stringstream object in VC++ (VStudio 2003) butI am getting an error when I use the overloaded << operator to try and set some manipulators.
I am trying the following:
int SomeInt = 1;
stringstream StrStream;
StrStream << std::setw(2) << SomeInt;
This will not compile (error C2593: 'operator <<' is ambiguous).
Does VStudio 2003 support using manipulators in this way?
I know that I can just set the width directly on the stringstream object e.g. StrStream.width(2);
I was wondering why the more usual method doesn't work?
Are you sure you included all of the right headers? The following compiles for me in VS2003:
#include <iostream>
#include <sstream>
#include <iomanip>
int main()
{
int SomeInt = 1;
std::stringstream StrStream;
StrStream << std::setw(2) << SomeInt;
return 0;
}
I love this reference site for stream questions like this.
/Allan
You probably just forgot to include iomanip, but I can't be sure because you didn't include code for a complete program there.
This complete program works fine over here using VS 2003:
#include <sstream>
#include <iomanip>
int main()
{
int SomeInt = 1;
std::stringstream StrStream;
StrStream << std::setw(2) << SomeInt;
}