lock_ios
An iostreams manipulator to provide thread safety to individual I/O streams.
by David Krauss (potatoswatter)
Usage
#include "lock_ios.h"
using namespace s6_lock_ios;
int main() {
std::cout << mutex_init; // Or << mutex_init_own.
std::thread t( print_one_thing );
print_two_things();
}
void print_one_thing() {
std::cout << lock_ios() << "One thing.\n";
// cout is unlocked at the end of the statement. (The lock is a temporary object.)
}
void print_two_things() {
ios_lock lock; // Same type as std::unique_lock< std::recursive_mutex >.
std::cout << lock_ios( lock ) << "The first thing.\n";
std::cout << The second thing.\n";
} // cout is unlocked when lock is destroyed or lock.unlock() is called.
There is one mutex per stream, created by mutex_init
. It is not a global lock.
The mutex is "recursive," so a function may lock a stream which is already locked by a calling function.
Input streams are supported as well.
(For example, getline( std::cin >> lock_ios() )
is reasonable usage.)
Setup
Prepare a stream for sharing by applying either mutex_init
or mutex_init_own
.
This will create and attach an internal mutex.
mutex_init
is safe for concurrency. It allows several contending threads to start sharing
the stream. This is achieved by a critical section.
mutex_init_own
may be called by a thread before it starts sharing a stream with others.
Typically it is lock-free.
Some streams, such as std::cerr
and std::cin
, flush other streams via std::ios::tie()
.
These setup routines clear tie
associations.
Unlike most manipulators, these require that the stream is in a good state (per !s.bad()
).
Upon failure, they set badbit
and throw std::ios_base::failure
(even if the stream has
not set exceptions
).
These setup routines do not lock anything. lock_ios
must be called subsequently to do so.
I/O
Always lock a shared stream before any access. It's not just that output from racing
insertions appears mixed or interleaved. Any concurrent accesses incur data races, which
(aside from the four standard streams under sync_with_stdio
) carry undefined behavior.
Unlike most manipulators, lock_ios
works regardless of the stream's state.
As with any access, locking is necessary before checking badbit
or failbit
.
If lock_ios
is attempted when no mutex has been initialized, a system_error
is thrown
as if a std::unique_lock
was locked without a mutex. (badbit
is not set.)
Limitations
This library requires the ios_base::pword
function to be thread-safe, which is only
realistic as long as it does not allocate memory. User-defined manipulators, such as this
library, tend to cause it to allocate memory. To be safe, either avoid other such formatting
libraries, or ensure that each stream has been suitably exposed to the library (its pword
s
and iword
s are allocated) before concurrent use.
Note that such libraries using pword
are inherently unsafe for concurrency. It is not
an incompatibility with this library, in particular.
Caveats
Stream buffers are not locked individually. For example, both std::cerr
and std::clog
may be locked at the same time by different threads, and they will race with each other.
(However, this is not typically true of cerr
and cout
, though they might funnel into
the same file once the data leaves the process.)
A lock operation is unsequenced with respect to evaluation of its statement's subexpressions.
Avoid interactions between the lock and the items after the <<
or >>
operator.
Be careful when locking several streams at the same time. The locks should be acquired in a consistent order, or a deadlock could arise.
A named lock object only locks a single stream. Inserting lock_ios(lock)
to a second
stream, when lock
is already locking something, will first unlock lock
.
Do not destroy a stream while it is locked.
Feedback
Please report issues on GitHub.
For other feedback, my email address may also be found there.