What is the lifetime of a static variable in a C function

Question

If a variable is declared as static in a function s scope it is only initialized once and retains its value between function calls  What exactly is its lifetime  When do its constructor and destructor get called   void foo           static string plonk    When will I die

User · Answer

The existing explanations aren t really complete without the actual rule from the Standard  found in 6 7      The zero-initialization of all block-scope variables with static storage duration or thread storage duration is performed before any other initialization takes place   Constant initialization of a block-scope entity with static storage duration  if applicable  is performed before its block is first entered   An implementation is permitted to perform early initialization of other block-scope variables with static or thread storage duration under the same conditions that an implementation is permitted to statically initialize a variable with static or thread storage duration in namespace scope   Otherwise such a variable is initialized the first time control passes through its declaration  such a variable is considered initialized upon the  completion  of  its  initialization    If  the  initialization  exits  by  throwing  an  exception   the  initialization   is  not  complete   so  it  will  be  tried  again  the  next  time  control  enters  the  declaration    If  control  enters the declaration concurrently while the variable is being initialized  the concurrent execution shall wait for completion of the initialization   If control re-enters the declaration recursively while the variable is being initialized  the behavior is undefined

User · Answer

Motti is right about the order  but there are some other things to consider   Compilers typically use a hidden flag variable to indicate if the local statics have already been initialized  and this flag is checked on every entry to the function  Obviously this is a small performance hit  but what s more of a concern is that this flag is not guaranteed to be thread-safe   If you have a local static as above  and foo is called from multiple threads  you may have race conditions causing plonk to be initialized incorrectly or even multiple times  Also  in this case plonk may get destructed by a different thread than the one which constructed it   Despite what the standard says  I d be very wary of the actual order of local static destruction  because it s possible that you may unwittingly rely on a static being still valid after it s been destructed  and this is really difficult to track down

User · Answer

FWIW  Codegear C  Builder doesn t destruct in the expected order according to the standard   C   gt  sample exe 1 2 Created in foo Created in if Destroyed in foo Destroyed in if       which is another reason not to rely on the destruction order

User · Answer

The Static variables are come into play once the program execution starts and it remain available till the program execution ends   The Static variables are created in the Data Segment of the Memory

User · Answer

The lifetime of function static variables begins the first time 0  the program flow encounters the declaration and it ends at program termination  This means that the run-time must perform some book keeping in order to destruct it only if it was actually constructed  Additionally  since the standard says that the destructors of static objects must run in the reverse order of the completion of their construction 1   and the order of construction may depend on the specific program run  the order of construction must be taken into account  Example struct emitter       string str      emitter const string amp  s    str s    cout  lt  lt   quot Created  quot   lt  lt  str  lt  lt  endl         emitter     cout  lt  lt   quot Destroyed  quot   lt  lt  str  lt  lt  endl        void foo bool skip first         if   skip first          static emitter a  quot in if quot        static emitter b  quot in foo quot       int main int argc  char           foo argc    2       if  argc    3          foo false      Output   C  gt sample exe Created in foo Destroyed in foo C  gt sample exe  1 Created in if Created in foo Destroyed in foo Destroyed in if C  gt sample exe 1 2 Created in foo Created in if Destroyed in if Destroyed in foo   0  Since C  98 2  has no reference to multiple threads how this will be behave in a multi-threaded environment is unspecified  and can be problematic as Roddy mentions   1  C  98 section 3 6 3 1  basic start term   2  In C  11 statics are initialized in a thread safe way  this is also known as Magic Statics

User · Answer

Motti is right about the order  but there are some other things to consider   Compilers typically use a hidden flag variable to indicate if the local statics have already been initialized  and this flag is checked on every entry to the function  Obviously this is a small performance hit  but what s more of a concern is that this flag is not guaranteed to be thread-safe   If you have a local static as above  and foo is called from multiple threads  you may have race conditions causing plonk to be initialized incorrectly or even multiple times  Also  in this case plonk may get destructed by a different thread than the one which constructed it   Despite what the standard says  I d be very wary of the actual order of local static destruction  because it s possible that you may unwittingly rely on a static being still valid after it s been destructed  and this is really difficult to track down

User · Answer

Motti is right about the order  but there are some other things to consider   Compilers typically use a hidden flag variable to indicate if the local statics have already been initialized  and this flag is checked on every entry to the function  Obviously this is a small performance hit  but what s more of a concern is that this flag is not guaranteed to be thread-safe   If you have a local static as above  and foo is called from multiple threads  you may have race conditions causing plonk to be initialized incorrectly or even multiple times  Also  in this case plonk may get destructed by a different thread than the one which constructed it   Despite what the standard says  I d be very wary of the actual order of local static destruction  because it s possible that you may unwittingly rely on a static being still valid after it s been destructed  and this is really difficult to track down

User · Answer

The lifetime of function static variables begins the first time 0  the program flow encounters the declaration and it ends at program termination  This means that the run-time must perform some book keeping in order to destruct it only if it was actually constructed  Additionally  since the standard says that the destructors of static objects must run in the reverse order of the completion of their construction 1   and the order of construction may depend on the specific program run  the order of construction must be taken into account  Example struct emitter       string str      emitter const string amp  s    str s    cout  lt  lt   quot Created  quot   lt  lt  str  lt  lt  endl         emitter     cout  lt  lt   quot Destroyed  quot   lt  lt  str  lt  lt  endl        void foo bool skip first         if   skip first          static emitter a  quot in if quot        static emitter b  quot in foo quot       int main int argc  char           foo argc    2       if  argc    3          foo false      Output   C  gt sample exe Created in foo Destroyed in foo C  gt sample exe  1 Created in if Created in foo Destroyed in foo Destroyed in if C  gt sample exe 1 2 Created in foo Created in if Destroyed in if Destroyed in foo   0  Since C  98 2  has no reference to multiple threads how this will be behave in a multi-threaded environment is unspecified  and can be problematic as Roddy mentions   1  C  98 section 3 6 3 1  basic start term   2  In C  11 statics are initialized in a thread safe way  this is also known as Magic Statics

User · Answer

The lifetime of function static variables begins the first time 0  the program flow encounters the declaration and it ends at program termination  This means that the run-time must perform some book keeping in order to destruct it only if it was actually constructed  Additionally  since the standard says that the destructors of static objects must run in the reverse order of the completion of their construction 1   and the order of construction may depend on the specific program run  the order of construction must be taken into account  Example struct emitter       string str      emitter const string amp  s    str s    cout  lt  lt   quot Created  quot   lt  lt  str  lt  lt  endl         emitter     cout  lt  lt   quot Destroyed  quot   lt  lt  str  lt  lt  endl        void foo bool skip first         if   skip first          static emitter a  quot in if quot        static emitter b  quot in foo quot       int main int argc  char           foo argc    2       if  argc    3          foo false      Output   C  gt sample exe Created in foo Destroyed in foo C  gt sample exe  1 Created in if Created in foo Destroyed in foo Destroyed in if C  gt sample exe 1 2 Created in foo Created in if Destroyed in if Destroyed in foo   0  Since C  98 2  has no reference to multiple threads how this will be behave in a multi-threaded environment is unspecified  and can be problematic as Roddy mentions   1  C  98 section 3 6 3 1  basic start term   2  In C  11 statics are initialized in a thread safe way  this is also known as Magic Statics

User · Answer

The existing explanations aren t really complete without the actual rule from the Standard  found in 6 7      The zero-initialization of all block-scope variables with static storage duration or thread storage duration is performed before any other initialization takes place   Constant initialization of a block-scope entity with static storage duration  if applicable  is performed before its block is first entered   An implementation is permitted to perform early initialization of other block-scope variables with static or thread storage duration under the same conditions that an implementation is permitted to statically initialize a variable with static or thread storage duration in namespace scope   Otherwise such a variable is initialized the first time control passes through its declaration  such a variable is considered initialized upon the  completion  of  its  initialization    If  the  initialization  exits  by  throwing  an  exception   the  initialization   is  not  complete   so  it  will  be  tried  again  the  next  time  control  enters  the  declaration    If  control  enters the declaration concurrently while the variable is being initialized  the concurrent execution shall wait for completion of the initialization   If control re-enters the declaration recursively while the variable is being initialized  the behavior is undefined

User · Answer

FWIW  Codegear C  Builder doesn t destruct in the expected order according to the standard   C   gt  sample exe 1 2 Created in foo Created in if Destroyed in foo Destroyed in if       which is another reason not to rely on the destruction order

User · Answer

The lifetime of function static variables begins the first time 0  the program flow encounters the declaration and it ends at program termination  This means that the run-time must perform some book keeping in order to destruct it only if it was actually constructed  Additionally  since the standard says that the destructors of static objects must run in the reverse order of the completion of their construction 1   and the order of construction may depend on the specific program run  the order of construction must be taken into account  Example struct emitter       string str      emitter const string amp  s    str s    cout  lt  lt   quot Created  quot   lt  lt  str  lt  lt  endl         emitter     cout  lt  lt   quot Destroyed  quot   lt  lt  str  lt  lt  endl        void foo bool skip first         if   skip first          static emitter a  quot in if quot        static emitter b  quot in foo quot       int main int argc  char           foo argc    2       if  argc    3          foo false      Output   C  gt sample exe Created in foo Destroyed in foo C  gt sample exe  1 Created in if Created in foo Destroyed in foo Destroyed in if C  gt sample exe 1 2 Created in foo Created in if Destroyed in if Destroyed in foo   0  Since C  98 2  has no reference to multiple threads how this will be behave in a multi-threaded environment is unspecified  and can be problematic as Roddy mentions   1  C  98 section 3 6 3 1  basic start term   2  In C  11 statics are initialized in a thread safe way  this is also known as Magic Statics

User · Answer

FWIW  Codegear C  Builder doesn t destruct in the expected order according to the standard   C   gt  sample exe 1 2 Created in foo Created in if Destroyed in foo Destroyed in if       which is another reason not to rely on the destruction order

User · Answer

The Static variables are come into play once the program execution starts and it remain available till the program execution ends   The Static variables are created in the Data Segment of the Memory

User · Answer

FWIW  Codegear C  Builder doesn t destruct in the expected order according to the standard   C   gt  sample exe 1 2 Created in foo Created in if Destroyed in foo Destroyed in if       which is another reason not to rely on the destruction order

User · Answer

Motti is right about the order  but there are some other things to consider   Compilers typically use a hidden flag variable to indicate if the local statics have already been initialized  and this flag is checked on every entry to the function  Obviously this is a small performance hit  but what s more of a concern is that this flag is not guaranteed to be thread-safe   If you have a local static as above  and foo is called from multiple threads  you may have race conditions causing plonk to be initialized incorrectly or even multiple times  Also  in this case plonk may get destructed by a different thread than the one which constructed it   Despite what the standard says  I d be very wary of the actual order of local static destruction  because it s possible that you may unwittingly rely on a static being still valid after it s been destructed  and this is really difficult to track down

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