C++ Library Reference

Chapter 4

Using Classic iostreams in a Multithreading Environment

This chapter describes how to use the iostream classes of the libC and libiostream libraries for input-output (I/O) in a multithreaded environment. It also provides examples of how to extend functionality of the library by deriving from the iostream classes. This chapter is not a guide for writing multithreaded code in C++, however.

The discussion here applies only to the old iostreams (libC and libiostream) and does not apply to libCstd, the new iostream that is part of the C++ Standard Library.

4.1 Multithreading

Multithreading (MT) is a powerful facility that can speed up applications on multiprocessors; it can also simplify the structuring of applications on both multiprocessors and uniprocessors. The iostream library has been modified to allow its interfaces to be used by applications in a multithreaded environment by programs that utilize the multithreading capabilities when running Solaris version 2.6, 7, or 8 of the Solaris operating environment. Applications that utilize the single-threaded capabilities of previous versions of the library are not affected by the behavior of the modified iostream interfaces.

A library is defined to be MT-safe if it works correctly in an environment with threads. Generally, this "correctness" means that all of its public functions are reentrant. The iostream library provides protection against multiple threads that attempt to modify the state of objects (that is, instances of a C++ class) shared by more than one thread. However, the scope of MT-safety for an iostream object is confined to the period in which the object's public member function is executing.

---

Caution – An application is not automatically guaranteed to be MT-safe because it uses MT-safe objects from the libC library. An application is defined to be MT-safe only when it executes as expected in a multithreaded environment.

---

4.2 Organization of the MT-Safe iostream Library

The organization of the MT-safe iostream library is slightly different from other versions of the iostream library. The exported interface of the library refers to the public and protected member functions of the iostream classes and the set of base classes available, and is consistent with other versions; however, the class hierarchy is different. See Section 4.3 "Interface Changes to the iostream Library" for details.

The original core classes have been renamed with the prefix unsafe_. TABLE 4-1 lists the classes that are the core of the iostream package.

TABLE 4-1   Core Classes

Class	Description
stream_MT	The base class for MT-safe classes.
streambuf	The base class for buffers.
unsafe_ios	A class that contains state variables that are common to the various stream classes; for example, error and formatting state.
unsafe_istream	A class that supports formatted and unformatted conversion from sequences of characters retrieved from the streambufs.
unsafe_ostream	A class that supports formatted and unformatted conversion to sequences of characters stored into the streambufs.
unsafe_iostream	A class that combines unsafe_istream and unsafe_ostream classes for bidirectional operations.

Each MT-safe class is derived from the base class stream_MT. Each MT-safe class, except streambuf, is also derived from the existing unsafe_ base class. Here are some examples:

class streambuf: public stream_MT { ... }; class ios: virtual public unsafe_ios, public stream_MT { ... }; class istream: virtual public ios, public unsafe_istream { ... };

The class stream_MT provides the mutual exclusion (mutex) locks required to make each iostream class MT-safe; it also provides a facility that dynamically enables and disables the locks so that the MT-safe property can be dynamically changed. The basic functionality for I/O conversion and buffer management are organized into the unsafe_ classes; the MT-safe additions to the library are confined to the derived classes. The MT-safe version of each class contains the same protected and public member functions as the unsafe_ base class. Each member function in the MT-safe version class acts as a wrapper that locks the object, calls the same function in the unsafe_ base class, and unlocks the object.

---

Note – The class streambuf is not derived from an unsafe class. The public and protected member functions of class streambuf are reentrant by locking. Unlocked versions, suffixed with _unlocked, are also provided.

---

4.2.1 Public Conversion Routines

A set of reentrant public functions that are MT-safe have been added to the iostream interface. A user-specified buffer is an additional argument to each function. These functions are described as follows.

TABLE 4-2   Reentrant Public Functions 

Function	Description
char *oct_r (char *buf,         int buflen,         long num,         int width)	Returns a pointer to the ASCII string that represents the number in octal. A width of nonzero is assumed to be the field width for formatting. The returned value is not guaranteed to point to the beginning of the user-provided buffer.
char *hex_r (char *buf,         int buflen,         long num,         int width)	Returns a pointer to the ASCII string that represents the number in hexadecimal. A width of nonzero is assumed to be the field width for formatting. The returned value is not guaranteed to point to the beginning of the user-provided buffer.
char *dec_r (char *buf,         int buflen,         long num,         int width)	Returns a pointer to the ASCII string that represents the number in decimal. A width of nonzero is assumed to be the field width for formatting. The returned value is not guaranteed to point to the beginning of the user-provided buffer.
char *chr_r (char *buf,         int buflen,         long num,         int width)	Returns a pointer to the ASCII string that contains character chr. If the width is nonzero, the string contains width blanks followed by chr. The returned value is not guaranteed to point to the beginning of the user-provided buffer.
char *form_r (char *buf,         int buflen,         long num,         int width)	Returns a pointer of the string formatted by sprintf, using the format string format and any remaining arguments. The buffer must have sufficient space to contain the formatted string.

---

Caution – The public conversion routines of the iostream library (oct, hex, dec, chr, and form) that are present to ensure compatibility with an earlier version of libC are not MT-safe.

---

4.2.2 Compiling and Linking with the MT-Safe libC Library

When you build an application that uses the iostream classes of the libC library to run in a multithreaded environment, compile and link the source code of the application using the -mt option. This option passes -D_REENTRANT to the preprocessor and -lthread to the linker.

---

Note – Use -mt (rather than -lthread) to link with libC and libthread. This option ensures proper linking order of the libraries. Using -lthread improperly could cause your application to work incorrectly.

---

Single-threaded applications that use iostream classes do not require special compiler or linker options. By default, the compiler links with the libC library.

4.2.3 MT-Safe iostream Restrictions

The restricted definition of MT-safety for the iostream library means that a number of programming idioms used with iostream are unsafe in a multithreaded environment using shared iostream objects.

4.2.3.1 Checking Error State

To be MT-safe, error checking must occur in a critical region with the I/O operation that causes the error. The following example illustrates how to check for errors:

CODE EXAMPLE 4-1   Checking Error State 

#include <iostream.h> enum iostate { IOok, IOeof, IOfail }; iostate read_number(istream& istr, int& num) { stream_locker sl(istr, stream_locker::lock_now); istr >> num; if (istr.eof()) return IOeof; if (istr.fail()) return IOfail; return IOok; }

In this example, the constructor of the stream_locker object sl locks the istream object istr. The destructor of sl, called at the termination of read_number, unlocks istr.

4.2.3.2 Obtaining Characters Extracted by Last Unformatted Input Operation

To be MT-safe, the gcount function must be called within a thread that has exclusive use of the istream object for the period that includes the execution of the last input operation and gcount call. The following example shows a call to gcount:

CODE EXAMPLE 4-2   Calling gcount

void fetch_line(istream& istr, char* line, int& linecount) { stream_locker sl(istr, stream_locker::lock_defer); sl.lock(); // lock the stream istr istr >> line; linecount = istr.gcount(); sl.unlock(); // unlock istr ... }

In this example, the lock and unlock member functions of class stream_locker define a mutual exclusion region in the program.

4.2.3.3 User-Defined I/O Operations

To be MT-safe, I/O operations defined for a user-defined type that involve a specific ordering of separate operations must be locked to define a critical region. The following example shows a user-defined I/O operation:

CODE EXAMPLE 4-3   User-Defined I/O Operations  

class mystream: public istream { // other definitions... int getRecord(char* name, int& id, float& gpa); }; int mystream::getRecord(char* name, int& id, float& gpa) { stream_locker sl(this, stream_locker::lock_now); *this >> name; *this >> id; *this >> gpa; return this->fail() == 0; }

4.2.4 Performance

Using the MT-safe classes in this version of the libC library results in some amount of performance overhead, even in a single-threaded application; however, if you use the unsafe_ classes of libC, this overhead can be avoided.

The scope resolution operator can be used to execute member functions of the base unsafe_ classes; for example:

cout.unsafe_ostream::put('4');

cin.unsafe_istream::read(buf, len);

---

Note – The unsafe_ classes cannot be safely used in multithreaded applications.

---

Instead of using unsafe_ classes, you can make the cout and cin objects unsafe and then use the normal operations. A slight performance deterioration results. The following example shows how to use unsafe cout and cin:

CODE EXAMPLE 4-4   Disabling MT-Safety

//disable mt-safety cout.set_safe_flag(stream_MT::unsafe_object); //disable mt-safety cin.set_safe_flag(stream_MT::unsafe_object); cout.put(`4'); cin.read(buf, len);

When an iostream object is MT-safe, mutex locking is provided to protect the object's member variables. This locking adds unnecessary overhead to an application that only executes in a single-threaded environment. To improve performance, you can dynamically switch an iostream object to and from MT-safety. The following example makes an iostream object MT-unsafe:

CODE EXAMPLE 4-5   Switching to MT-Unsafe

fs.set_safe_flag(stream_MT::unsafe_object);// disable MT-safety .... do various i/o operations

You can safely use an MT-unsafe stream in code where an iostream is not shared by threads; for example, in a program that has only one thread, or in a program where each iostream is private to a thread.

If you explicitly insert synchronization into the program, you can also safely use MT-unsafe iostreams in an environment where an iostream is shared by threads. The following example illustrates the technique:

CODE EXAMPLE 4-6   Using Synchronization with MT-Unsafe Objects

generic_lock() ; fs.set_safe_flag(stream_MT::unsafe_object) ; ... do various i/o operations generic_unlock() ;

where the generic_lock and generic_unlock functions can be any synchronization mechanism that uses such primitives as mutex, semaphores, or reader/writer locks.

---

Note – The stream_locker class provided by the libC library is the preferred mechanism for this purpose.

---

See Section 4.6 "Object Locks" for more information.

4.3 Interface Changes to the iostream Library

This section describes the interface changes made to the iostream library to make it MT-Safe.

4.3.1 New Classes

The following table lists the new classes added to the libC interfaces.

CODE EXAMPLE 4-7   New Classes  

stream_MT stream_locker unsafe_ios unsafe_istream unsafe_ostream unsafe_iostream unsafe_fstreambase unsafe_strstreambase

4.3.2 New Class Hierarchy

The following table lists the new class hierarchy added to the iostream interfaces.

CODE EXAMPLE 4-8   New Class Hierarchy

class streambuf : public stream_MT { ... }; class unsafe_ios { ... }; class ios : virtual public unsafe_ios, public stream_MT { ... }; class unsafe_fstreambase : virtual public unsafe_ios { ... }; class fstreambase : virtual public ios, public unsafe_fstreambase { ... }; class unsafe_strstreambase : virtual public unsafe_ios { ... }; class strstreambase : virtual public ios, public unsafe_strstreambase { ... }; class unsafe_istream : virtual public unsafe_ios { ... }; class unsafe_ostream : virtual public unsafe_ios { ... }; class istream : virtual public ios, public unsafe_istream { ... }; class ostream : virtual public ios, public unsafe_ostream { ... }; class unsafe_iostream : public unsafe_istream, public unsafe_ostream { ... };

4.3.3 New Functions

The following table lists the new functions added to the iostream interfaces.

CODE EXAMPLE 4-9   New Functions 

class streambuf { public: int sgetc_unlocked(); void sgetn_unlocked(char *, int); int snextc_unlocked(); int sbumpc_unlocked(); void stossc_unlocked(); int in_avail_unlocked(); int sputbackc_unlocked(char); int sputc_unlocked(int); int sputn_unlocked(const char *, int); int out_waiting_unlocked(); protected: char* base_unlocked(); char* ebuf_unlocked(); int blen_unlocked(); char* pbase_unlocked(); char* eback_unlocked(); char* gptr_unlocked(); char* egptr_unlocked(); char* pptr_unlocked(); void setp_unlocked(char*, char*); void setg_unlocked(char*, char*, char*); void pbump_unlocked(int); void gbump_unlocked(int); void setb_unlocked(char*, char*, int); int unbuffered_unlocked(); char *epptr_unlocked(); void unbuffered_unlocked(int); int allocate_unlocked(int); }; class filebuf : public streambuf { public: int is_open_unlocked(); filebuf* close_unlocked(); filebuf* open_unlocked(const char*, int, int = filebuf::openprot); filebuf* attach_unlocked(int); }; class strstreambuf : public streambuf { public: int freeze_unlocked(); char* str_unlocked(); }; unsafe_ostream& endl(unsafe_ostream&); unsafe_ostream& ends(unsafe_ostream&); unsafe_ostream& flush(unsafe_ostream&); unsafe_istream& ws(unsafe_istream&); unsafe_ios& dec(unsafe_ios&); unsafe_ios& hex(unsafe_ios&); unsafe_ios& oct(unsafe_ios&); char* dec_r (char* buf, int buflen, long num, int width) char* hex_r (char* buf, int buflen, long num, int width) char* oct_r (char* buf, int buflen, long num, int width) char* chr_r (char* buf, int buflen, long chr, int width) char* str_r (char* buf, int buflen, const char* format, int width = 0); char* form_r (char* buf, int buflen, const char* format, ...)

4.4 Global and Static Data

Global and static data in a multithreaded application are not safely shared among threads. Although threads execute independently, they share access to global and static objects within the process. If one thread modifies such a shared object, all the other threads within the process observe the change, making it difficult to maintain state over time. In C++, class objects (instances of a class) maintain state by the values in their member variables. If a class object is shared, it is vulnerable to changes made by other threads.

When a multithreaded application uses the iostream library and includes iostream.h, the standard streams--cout, cin, cerr, and clog-- are, by default, defined as global shared objects. Since the iostream library is MT-safe, it protects the state of its shared objects from access or change by another thread while a member function of an iostream object is executing. However, the scope of MT-safety for an object is confined to the period in which the object's public member function is executing. For example,

int c; cin.get(c);

gets the next character in the get buffer and updates the buffer pointer in ThreadA. However, if the next instruction in ThreadA is another get call, the libC library does not guarantee to return the next character in the sequence. It is not guaranteed because, for example, ThreadB may have also executed the get call in the intervening period between the two get calls made in ThreadA.

See Section 4.6 "Object Locks" for strategies for dealing with the problems of shared objects and multithreading.

4.5 Sequence Execution

Frequently, when iostream objects are used, a sequence of I/O operations must be MT-safe. For example, the code:

cout << " Error message:" << errstring[err_number] << "\n";

involves the execution of three member functions of the cout stream object. Since cout is a shared object, the sequence must be executed atomically as a critical section to work correctly in a multithreaded environment. To perform a sequence of operations on an iostream class object atomically, you must use some form of locking.

The libC library now provides the stream_locker class for locking operations on an iostream object. See Section 4.6 "Object Locks" for information about the stream_locker class.

4.6 Object Locks

The simplest strategy for dealing with the problems of shared objects and multithreading is to avoid the issue by ensuring that iostream objects are local to a thread. For example,

• Declare objects locally within a thread's entry function.
• Declare objects in thread-specific data. (For information on how to use thread specific data, see the thr_keycreate(3T) man page.)
• Dedicate a stream object to a particular thread. The object thread is private by convention.

However, in many cases, such as default shared standard stream objects, it is not possible to make the objects local to a thread, and an alternative strategy is required.

To perform a sequence of operations on an iostream class object atomically, you must use some form of locking. Locking adds some overhead even to a single-threaded application. The decision whether to add locking or make iostream objects private to a thread depends on the thread model chosen for the application: Are the threads to be independent or cooperating?

• If each independent thread is to produce or consume data using its own iostream object, the iostream objects are private to their respective threads and locking is not required.
• If the threads are to cooperate (that is, they are to share the same iostream object), then access to the shared object must be synchronized and some form of locking must be used to make sequential operations atomic.

4.6.1 Class stream_locker

The iostream library provides the stream_locker class for locking a series of operations on an iostream object. You can, therefore, minimize the performance overhead incurred by dynamically enabling or disabling locking in iostream objects.

Objects of class stream_locker can be used to make a sequence of operations on a stream object atomic. For example, the code shown in the example below seeks to find a position in a file and reads the next block of data.

CODE EXAMPLE 4-10   Example of Using Locking Operations 

{ const int len = 128; char buf[len]; int offset = 48; stream_locker s_lock(fs, stream_locker::lock_now); . . . . .// open file fs.seekg(offset, ios::beg); fs.read(buf, len); }

In this example, the constructor for the stream_locker object defines the beginning of a mutual exclusion region in which only one thread can execute at a time. The destructor, called after the return from the function, defines the end of the mutual exclusion region. The stream_locker object ensures that both the seek to a particular offset in a file and the read from the file are performed together, atomically, and that ThreadB cannot change the file offset before the original ThreadA reads the file.

An alternative way to use a stream_locker object is to explicitly define the mutual exclusion region. In the following example, to make the I/O operation and subsequent error checking atomic, lock and unlock member function calls of a vbstream_locker object are used.

CODE EXAMPLE 4-11   Making I/O Operation and Error Checking Atomic

{ ... stream_locker file_lck(openfile_stream, stream_locker::lock_defer); .... file_lck.lock(); // lock openfile_stream openfile_stream << "Value: " << int_value << "\n"; if(!openfile_stream) { file_error("Output of value failed\n"); return; } file_lck.unlock(); // unlock openfile_stream }

For more information, see the stream_locker(3CC4) man page.

4.7 MT-Safe Classes

You can extend or specialize the functionality of the iostream classes by deriving new classes. If objects instantiated from the derived classes will be used in a multithreaded environment, the classes must be MT-safe.

Considerations when deriving MT-safe classes include:

• Making a class object MT-safe by protecting the internal state of the object from multiple-thread modification. To do this, serialize access to member variables in public and protected member functions with mutex locks.
• Making a sequence of calls to member functions of an MT-safe base class atomic, using a stream_locker object.
• Avoiding locking overhead by using the _unlocked member functions of streambuf within critical regions defined by stream_locker objects.
• Locking the public virtual functions of class streambuf in case the functions are called directly by an application. These functions are: xsgetn, underflow, pbackfail, xsputn, overflow, seekoff, and seekpos.
• Extending the formatting state of an ios object by using the member functions iword and pword in class ios. However, a problem can occur if more than one thread is sharing the same index to an iword or pword function. To make the threads MT-safe, use an appropriate locking scheme.
• Locking member functions that return the value of a member variable greater in size than a char.

4.8 Object Destruction

Before an iostream object that is shared by several threads is deleted, the main thread must verify that the subthreads are finished with the shared object. The following example shows how to safely destroy a shared object.

CODE EXAMPLE 4-12    Destroying a Shared Object 

#include <fstream.h> #include <thread.h> fstream* fp; void *process_rtn(void*) { // body of sub-threads which uses fp... } multi_process(const char* filename, int numthreads) { fp = new fstream(filename, ios::in); // create fstream object // before creating threads. // create threads for (int i=0; i<numthreads; i++) thr_create(0, STACKSIZE, process_rtn, 0, 0, 0); ... // wait for threads to finish for (int i=0; i<numthreads; i++) thr_join(0, 0, 0); delete fp; // delete fstream object after fp = NULL; // all threads have completed. }

4.9 An Example Application

The following code provides an example of a multiply-threaded application that uses iostream objects from the libC library in an MT-safe way.

The example application creates up to 255 threads. Each thread reads a different input file, one line at a time, and outputs the line to an output file, using the standard output stream, cout. The output file, which is shared by all threads, is tagged with a value that indicates which thread performed the output operation.

CODE EXAMPLE 4-13   Using iostream Objects in an MT-Safe Way 

// create tagged thread data // the output file is of the form: // <tag><string of data>\n // where tag is an integer value in a unsigned char. // Allows up to 255 threads to be run in this application // <string of data> is any printable characters // Because tag is an integer value written as char, // you need to use od to look at the output file, suggest: // od -c out.file |more #include <stdlib.h> #include <stdio.h> #include <iostream.h> #include <fstream.h> #include <thread.h> struct thread_args { char* filename; int thread_tag; }; const int thread_bufsize = 256; // entry routine for each thread void* ThreadDuties(void* v) { // obtain arguments for this thread thread_args* tt = (thread_args*)v; char ibuf[thread_bufsize]; // open thread input file ifstream instr(tt->filename); stream_locker lockout(cout, stream_locker::lock_defer); while(1) { // read a line at a time instr.getline(ibuf, thread_bufsize - 1, '\n'); if(instr.eof()) break; // lock cout stream so the i/o operation is atomic lockout.lock(); // tag line and send to cout cout << (unsigned char)tt->thread_tag << ibuf << "\n"; lockout.unlock(); } return 0; } int main(int argc, char** argv) { // argv: 1+ list of filenames per thread if(argc < 2) { cout << "usage: " << argv[0] << " <files..>\n"; exit(1); } int num_threads = argc - 1; int total_tags = 0; // array of thread_ids thread_t created_threads[thread_bufsize]; // array of arguments to thread entry routine thread_args thr_args[thread_bufsize]; int i; for( i = 0; i < num_threads; i++) { thr_args[i].filename = argv[1 + i]; // assign a tag to a thread - a value less than 256 thr_args[i].thread_tag = total_tags++; // create threads thr_create(0, 0, ThreadDuties, &thr_args[i], THR_SUSPENDED, &created_threads[i]); } for(i = 0; i < num_threads; i++) { thr_continue(created_threads[i]); } for(i = 0; i < num_threads; i++) { thr_join(created_threads[i], 0, 0); } return 0; }