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title TARGET DECK FILE TAGS tags
Storage Obsidian::STEM c17::storage
c17

Overview

An object has a storage duration that determines its lifetime. There are four storage durations: static, thread, automatic, and allocated.

%%ANKI Cloze An {object} has a {storage duration} that determines its {lifetime}. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What are the four storage durations? Back: Static, thread, automatic, and allocated. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What is an object's lifetime? Back: The portion of a program execution during which storage is guaranteed to be reserved for the object. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What is the result of accessing an object within its lifetime? Back: The value last stored within the object. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What is the result of accessing an object outside its lifetime? Back: Undefined behavior. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

Static

An object whose identifier is declared without storage-class specifier _Thread_local, and either with external or internal linkage or with the storage-class specifier static, has static storage duration.

Its lifetime is the entire execution of the program and its stored value is initialized only once, prior to program startup.

%%ANKI Basic What is the storage duration of an object whose identifer has external linkage? Back: Either static or thread. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic When; does an object whose identifier has external linkage have a static storage duration? Back: When the identifier is declared without storage-class specifier _Thread_local. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic When; does an object whose identifier has external linkage have a thread storage duration? Back: When the identifier is declared with storage-class specifier _Thread_local. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Which storage class specifier(s) can be used to declare an object with static storage duration? Back: extern and static. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. What is the storage duration of the object with identifier x?

int x;

Back: Static. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. What is the storage duration of the object with identifier x?

extern int x;

Back: Static. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. What is the storage duration of the object with identifier x?

static int x;

Back: Static. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. What is the storage duration of the object with identifier x?

_Thread_local int x;

Back: Thread. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. What is the storage duration of the object with identifier x?

void foo() { static int x; }

Back: Static. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What is the lifetime of an object with static storage duration? Back: The entire program execution. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic When is an object with static storage duration initialized? Back: Prior to program startup (before main is called). Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic How many times can object with static storage duration be initialized? Back: Only once. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic An object with static storage duration must have what linkage? Back: Internal or external. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

Thread Local

An object whose identifier is declared with the storage-class specified _Thread_local has thread storage duration.

Its lifetime is the entire execution of the thread for which it is created, and its stored value is initialized when the thread is started.

%%ANKI Basic Which keyword declares an object with thread local storage duration? Back: _Thread_local Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Cloze {1:_Thread_local} is a {2:keyword} whereas {2:thread_local} is a {1:macro}. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Which library is thread_local defined in? Back: <threads.h> Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Which storage class specifier(s) can be used to declare an object with thread storage duration? Back: _Thread_local Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What is the lifetime of an object with thread storage duration? Back: The entire execution fo the thread for which it is created. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic When is an object with thread storage duration initialized? Back: When its corresponding thread is started. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic An object with thread storage duration must have what linkage? Back: Internal or external. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

Automatic

An object whose identifier is declared with no linkage and without the storage-class specifier static has automatic storage duration, as do some compound literals.

%%ANKI Basic Which keyword is used to declare automatic storage duration? Back: auto Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Which storage class specifier is essentially never used in modern C? Back: auto Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Which storage class specifier(s) can be used to declare an object with automatic storage duration? Back: auto Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. What is the storage duration of the object with identifier x?

void foo() { int x; }

Back: Automatic. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. What is the storage duration of the object with identifier x?

void foo(int x) { }

Back: Automatic. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic An object with automatic storage duration must have what linkage? Back: None. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic What are the three possible lifetimes of an automatic object? Back: Enclosing block, declaration scope, temporary. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

For these objects that do not have a derived#Variable-Length type, its lifetime extends from entry into its enclosing block until execution of that block ends. Its initial value is indeterminate. If initialization is specified, it is performed each time the declaration or compound literal is reached in the execution of the block.

For these objects that do have a VLA type, its lifetime extends from the declaration of the object until execution of the program leaves the scope of the declaration.

%%ANKI Basic Consider non-temporary object with automatic storage duration. What property determines its lifetime? Back: Whether or not the object has a VLA type. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic When is the lifetime of an object with automatic storage duration that of its enclosing block? Back: When it is a non-temporary object with non-VLA type. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider non-temporary, non-VLA object with automatic storage duration. What is its lifetime? Back: From entry into its enclosing block until execution of that block ends. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic When does the lifetime of an object with automatic storage duration match its declaration's scope? Back: When it is a non-temporary object with VLA type. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. What is the lifetime of the object with identifier x?

void foo(int x) { }

Back: From start of execution to end of execution of foo. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. What is the lifetime of the object with identifier x?

void foo() {
  int z;
  int y;
  int x;
}

Back: Between the start and end of foo's execution. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. What is the value of x at line 1?

void foo() {
  int z = 0;  // Line 1
  int y = 1;  // Line 2
  int x = 2;  // Line 3
}

Back: Indeterminate. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. What is the value of x at line 3?

void foo() {
  int z = 0;  // Line 1
  int y = 1;  // Line 2
  int x = 2;  // Line 3
}

Back: 2 Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. What is the lifetime of the object referred to by identifier x?

void foo() {
  int z;
  { int y; int x; }
  int a;
}

Back: From the start to the end of execution of its surrounding block. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. Does x's lifetime extend to line 1?

void foo() {
  int z = 0;  // Line 1
  int y = 1;  // Line 2
  int x = 2;  // Line 3
}

Back: Yes. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. Does x's lifetime extend to line 1?

void foo() {
  int z = 0;      // Line 1
  int y = 1;      // Line 2
  { int x = 2; }  // Line 3
}

Back: No. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. Does x's lifetime extend to line 1?

void foo() {
  int z = 0;      // Line 1
  { int y = 1; }  // Line 2
  int x = 2;      // Line 3
}

Back: Yes. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider non-temporary, VLA object with automatic storage duration. What is its lifetime? Back: From declaration of the VLA to when it leaves its scope. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. Does x's lifetime extend to line 1?

void foo(int n) {
  int z = 0;  // Line 1
  int y = 1;  // Line 2
  int x[n];   // Line 3
}

Back: No. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. Does x's lifetime extend to line 3?

void foo(int n) {
  int x[n];   // Line 1
  int z = 0;  // Line 2
  int y = 1;  // Line 3
}

Back: Yes. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. What is the lifetime of the object with identifier x?

void foo(int n) {
  int z;
  int y;
  int x[n];
}

Back: Between its declaration and when foo's execution ends. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. Does x's lifetime extend to line 3?

void foo(int n) {
  { int x[n]; }  // Line 1
  int z = 0;     // Line 2
  int y = 1;     // Line 3
}

Back: No. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider the following translation unit. Does x's lifetime extend to line 3?

void foo(int n) {
  int x[n];       // Line 1
  int z = 0;      // Line 2
  { int y = 1; }  // Line 3
}

Back: Yes. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

A non-lvalue expression with structure or union type, where the structure or union contains a member with array type refers to an object with automatic storage duration and temporary lifetime. Its lifetime lasts for the duration of the expression evaluation it is found within.

%%ANKI Basic When is the lifetime of an object with automatic storage duration temporary? Back: When specifying a non-lvalue expression with struct/union type containing an array member. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider an object with temporary lifetime and automatic storage duration. What must the object's type be? Back: Either a struct or union. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

%%ANKI Basic Consider an object with temporary lifetime and automatic storage duration. What must the struct or union contain? Back: A member with array type. Reference: “ISO: Programming Languages - C17,” April 2017, https://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf.

END%%

Dynamic Allocation

The <stdlib.h> header provides the two most prominent functions used for managing dynamic memory: malloc and free. The former is used to allocate new memory whereas the latter is used to annihilate it.

void* malloc(size_t size);
void free(void*);

Dynamically allocated objects have allocated storage duration. Their lifetime spans allocation to explicit destruction.

%%ANKI Basic Which standard library header exposes malloc? Back: <stdlib.h> Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Which standard library header exposes free? Back: <stdlib.h> Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Why is malloc named the way it is? Back: It stands for memory allocate. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Which two functions are most prominently used for managing dynamically allocated memory? Back: malloc and free. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Cloze Generally speaking, memory created with {malloc} should be deleted with {free}. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What three specialized variants of malloc are exposed by <stdlib.h>? Back: calloc, realloc, and aligned_alloc. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic On success, what does a call to malloc return? Back: A valid void* pointer. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic On failure, what does a call to malloc return? Back: A null pointer value. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What are the argument(s) to malloc? Back: The size (in bytes) of the block of memory to allocate. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What is wrong with the following code snippet?

double *d = (double*)malloc(sizeof *d);

Back: The return type of malloc should not be cast. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What is wrong with the following code snippet?

double *d = malloc(sizeof *d);

Back: N/A. This is valid. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What possible error can occur by explicitly casting the return type of malloc? Back: If <stdlib.h> isn't imported, older C compilers assume malloc returns an int. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Without <stdlib.h>, an older C compiler might assume what malloc declaration? Back:

int malloc();

Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Within <stdlib.h>, what function prototype does malloc have? Back:

void* malloc(size_t size);

Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Cloze Storage allocated through malloc is {uninitialized} and has {no type}. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Does the following invoke undefined behavior? Why or why not?

free(0);

Back: No. free ignores null pointer values. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Does the following invoke undefined behavior? Why or why not?

double d = 0.0;
free(&d);

Back: Yes. d was not dynamically allocated. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Does the following invoke undefined behavior? Why or why not?

double* d = malloc(sizeof *d);
free(&d);

Back: Yes. d was not dynamically allocated. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Does the following invoke undefined behavior? Why or why not?

double* d = malloc(sizeof *d);
free(d);

Back: No. The address d pointed to was dynamically allocated. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Does the following invoke undefined behavior? Why or why not?

double* d = malloc(sizeof *d);
free(d);
d = 0;
free(d);

Back: No. free ignores null pointer values. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic Does the following invoke undefined behavior? Why or why not?

double* d = malloc(sizeof *d);
free(d);
free(d);

Back: Yes. free should not be invoked on already free'd data. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic malloc and its variants all have the size parameter in what position? Back: The last position. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What storage duration is associated with malloc and free? Back: Allocated. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What is the lifetime of an allocated object? Back: From its allocation to when it is explicitly deallocated. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

%%ANKI Basic What two functions are typically used to deallocate an allocated object? Back: free and realloc. Reference: Jens Gustedt, Modern C (Shelter Island, NY: Manning Publications Co, 2020).

END%%

Bibliography