6 Basics [basic]

6.9 Program execution [basic.exec]

6.9.3 Start and termination [basic.start]

6.9.3.1 main function [basic.start.main]

A program shall contain exactly one function called main that belongs to the global scope.

Executing a program starts a main thread of execution ([intro.multithread], [thread.threads]) in which the main function is invoked.

It is implementation-defined whether a program in a freestanding environment is required to define a main function.

[Note 1:

In a freestanding environment, startup and termination is implementation-defined; startup contains the execution of constructors for non-local objects with static storage duration; termination contains the execution of destructors for objects with static storage duration.

— end note]

An implementation shall not predefine the main function.

Its type shall have C++ language linkage and it shall have a declared return type of type int, but otherwise its type is implementation-defined.

An implementation shall allow both

(2.1)
a function of () returning int and
(2.2)
a function of (int, pointer to pointer to char) returning int

as the type of main ([dcl.fct]).

In the latter form, for purposes of exposition, the first function parameter is called argc and the second function parameter is called argv, where argc shall be the number of arguments passed to the program from the environment in which the program is run.

If argc is nonzero these arguments shall be supplied in argv[0] through argv[argc-1] as pointers to the initial characters of null-terminated multibyte strings (ntmbss) ([multibyte.strings]) and argv[0] shall be the pointer to the initial character of a ntmbs that represents the name used to invoke the program or "".

The value of argc shall be non-negative.

The value of argv[argc] shall be 0.

Recommended practice: Any further (optional) parameters should be added after argv.

The function main shall not be used within a program.

The linkage ([basic.link]) of main is implementation-defined.

A program that defines main as deleted or that declares main to be inline, static, constexpr, or consteval is ill-formed.

The function main shall not be a coroutine ([dcl.fct.def.coroutine]).

The main function shall not be declared with a linkage-specification ([dcl.link]).

A program that declares

(3.1)
a variable main that belongs to the global scope, or
(3.2)
a function main that belongs to the global scope and is attached to a named module, or
(3.3)
a function template main that belongs to the global scope, or
(3.4)
an entity named main with C language linkage (in any namespace)

is ill-formed.

The name main is not otherwise reserved.

[Example 1:

Member functions, classes, and enumerations can be called main, as can entities in other namespaces.

— end example]

Terminating the program without leaving the current block (e.g., by calling the function std::exit(int) ([support.start.term])) does not destroy any objects with automatic storage duration ([class.dtor]).

If std::exit is invoked during the destruction of an object with static or thread storage duration, the program has undefined behavior.

A return statement ([stmt.return]) in main has the effect of leaving the main function (destroying any objects with automatic storage duration) and calling std::exit with the return value as the argument.

If control flows off the end of the compound-statement of main, the effect is equivalent to a return with operand 0 (see also [except.handle]).

6.9.3.2 Static initialization [basic.start.static]

Variables with static storage duration are initialized as a consequence of program initiation.

Variables with thread storage duration are initialized as a consequence of thread execution.

Within each of these phases of initiation, initialization occurs as follows.

Constant initialization is performed if a variable or temporary object with static or thread storage duration is constant-initialized ([expr.const]).

If constant initialization is not performed, a variable with static storage duration ([basic.stc.static]) or thread storage duration ([basic.stc.thread]) is zero-initialized ([dcl.init]).

Together, zero-initialization and constant initialization are called static initialization; all other initialization is dynamic initialization.

All static initialization strongly happens before ([intro.races]) any dynamic initialization.

[Note 1:

The dynamic initialization of non-block variables is described in [basic.start.dynamic]; that of static block variables is described in [stmt.dcl].

— end note]

An implementation is permitted to perform the initialization of a variable with static or thread storage duration as a static initialization even if such initialization is not required to be done statically, provided that

(3.1)
the dynamic version of the initialization does not change the value of any other object of static or thread storage duration prior to its initialization, and
(3.2)
the static version of the initialization produces the same value in the initialized variable as would be produced by the dynamic initialization if all variables not required to be initialized statically were initialized dynamically.

[Note 2:

As a consequence, if the initialization of an object obj1 refers to an object obj2 potentially requiring dynamic initialization and defined later in the same translation unit, it is unspecified whether the value of obj2 used will be the value of the fully initialized obj2 (because obj2 was statically initialized) or will be the value of obj2 merely zero-initialized.

For example, inline double fd() { return 1.0; } extern double d1; double d2 = d1; // unspecified: // either statically initialized to 0.0 or // dynamically initialized to 0.0 if d1 is // dynamically initialized, or 1.0 otherwise double d1 = fd(); // either initialized statically or dynamically to 1.0

— end note]

6.9.3.3 Dynamic initialization of non-block variables [basic.start.dynamic]

Dynamic initialization of a non-block variable with static storage duration is unordered if the variable is an implicitly or explicitly instantiated specialization, is partially-ordered if the variable is an inline variable that is not an implicitly or explicitly instantiated specialization, and otherwise is ordered.

[Note 1:

A non-inline explicit specialization of a templated variable has ordered initialization.

— end note]

A declaration D is appearance-ordered before a declaration E if

(2.1)
D appears in the same translation unit as E, or
(2.2)
the translation unit containing E has an interface dependency on the translation unit containing D,

in either case prior to E.

Dynamic initialization of non-block variables V and W with static storage duration are ordered as follows:

(3.1)
If V and W have ordered initialization and the definition of V is appearance-ordered before the definition of W, or if V has partially-ordered initialization, W does not have unordered initialization, and for every definition E of W there exists a definition D of V such that D is appearance-ordered before E, then
- (3.1.1)
  if the program does not start a thread ([intro.multithread]) other than the main thread ([basic.start.main]) or V and W have ordered initialization and they are defined in the same translation unit, the initialization of V is sequenced before the initialization of W;
- (3.1.2)
  otherwise, the initialization of V strongly happens before the initialization of W.
(3.2)
Otherwise, if the program starts a thread other than the main thread before either V or W is initialized, it is unspecified in which threads the initializations of V and W occur; the initializations are unsequenced if they occur in the same thread.
(3.3)
Otherwise, the initializations of V and W are indeterminately sequenced.

[Note 2:

This definition permits initialization of a sequence of ordered variables concurrently with another sequence.

— end note]

A non-initialization odr-use is an odr-use ([basic.def.odr]) not caused directly or indirectly by the initialization of a non-block static or thread storage duration variable.

It is implementation-defined whether the dynamic initialization of a non-block non-inline variable with static storage duration is sequenced before the first statement of main or is deferred.

If it is deferred, it strongly happens before any non-initialization odr-use of any non-inline function or non-inline variable defined in the same translation unit as the variable to be initialized.39

It is implementation-defined in which threads and at which points in the program such deferred dynamic initialization occurs.

Recommended practice: An implementation should choose such points in a way that allows the programmer to avoid deadlocks.

[Example 1: // - File 1 - #include "a.h" #include "b.h" B b; A::A() { b.Use(); } // - File 2 - #include "a.h" A a; // - File 3 - #include "a.h" #include "b.h" extern A a; extern B b; int main() { a.Use(); b.Use(); }

It is implementation-defined whether either a or b is initialized before main is entered or whether the initializations are delayed until a is first odr-used in main.

In particular, if a is initialized before main is entered, it is not guaranteed that b will be initialized before it is odr-used by the initialization of a, that is, before A::A is called.

If, however, a is initialized at some point after the first statement of main, b will be initialized prior to its use in A::A.

— end example]

It is implementation-defined whether the dynamic initialization of a non-block inline variable with static storage duration is sequenced before the first statement of main or is deferred.

If it is deferred, it strongly happens before any non-initialization odr-use of that variable.

It is implementation-defined in which threads and at which points in the program such deferred dynamic initialization occurs.

It is implementation-defined whether the dynamic initialization of a non-block non-inline variable with thread storage duration is sequenced before the first statement of the initial function of a thread or is deferred.

If it is deferred, the initialization associated with the entity for thread t is sequenced before the first non-initialization odr-use by t of any non-inline variable with thread storage duration defined in the same translation unit as the variable to be initialized.

It is implementation-defined in which threads and at which points in the program such deferred dynamic initialization occurs.

If the initialization of a non-block variable with static or thread storage duration exits via an exception, the function std::terminate is called ([except.terminate]).

39)39)

A non-block variable with static storage duration having initialization with side effects is initialized in this case, even if it is not itself odr-used ([basic.def.odr], [basic.stc.static]).

6.9.3.4 Termination [basic.start.term]

Constructed objects ([dcl.init]) with static storage duration are destroyed and functions registered with std::atexit are called as part of a call to std::exit ([support.start.term]).

The call to std::exit is sequenced before the destructions and the registered functions.

[Note 1:

Returning from main invokes std::exit ([basic.start.main]).

— end note]

Constructed objects with thread storage duration within a given thread are destroyed as a result of returning from the initial function of that thread and as a result of that thread calling std::exit.

The destruction of all constructed objects with thread storage duration within that thread strongly happens before destroying any object with static storage duration.

If the completion of the constructor or dynamic initialization of an object with static storage duration strongly happens before that of another, the completion of the destructor of the second is sequenced before the initiation of the destructor of the first.

If the completion of the constructor or dynamic initialization of an object with thread storage duration is sequenced before that of another, the completion of the destructor of the second is sequenced before the initiation of the destructor of the first.

If an object is initialized statically, the object is destroyed in the same order as if the object was dynamically initialized.

For an object of array or class type, all subobjects of that object are destroyed before any block variable with static storage duration initialized during the construction of the subobjects is destroyed.

If the destruction of an object with static or thread storage duration exits via an exception, the function std::terminate is called ([except.terminate]).

If a function contains a block variable of static or thread storage duration that has been destroyed and the function is called during the destruction of an object with static or thread storage duration, the program has undefined behavior if the flow of control passes through the definition of the previously destroyed block variable.

[Note 2:

Likewise, the behavior is undefined if the block variable is used indirectly (e.g., through a pointer) after its destruction.

— end note]

If the completion of the initialization of an object with static storage duration strongly happens before a call to std::atexit (see <cstdlib>, [support.start.term]), the call to the function passed to std::atexit is sequenced before the call to the destructor for the object.

If a call to std::atexit strongly happens before the completion of the initialization of an object with static storage duration, the call to the destructor for the object is sequenced before the call to the function passed to std::atexit.

If a call to std::atexit strongly happens before another call to std::atexit, the call to the function passed to the second std::atexit call is sequenced before the call to the function passed to the first std::atexit call.

If there is a use of a standard library object or function not permitted within signal handlers ([support.runtime]) that does not happen before ([intro.multithread]) completion of destruction of objects with static storage duration and execution of std::atexit registered functions ([support.start.term]), the program has undefined behavior.

[Note 3:

If there is a use of an object with static storage duration that does not happen before the object's destruction, the program has undefined behavior.

Terminating every thread before a call to std::exit or the exit from main is sufficient, but not necessary, to satisfy these requirements.

These requirements permit thread managers as static-storage-duration objects.

— end note]

Calling the function std::abort() declared in <cstdlib> terminates the program without executing any destructors and without calling the functions passed to std::atexit() or std::at_quick_exit().