notebook/notes/c17/storage.md

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

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*);

%%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 What two functions are most prominently used for dynamic allocation? 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 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%%

Bibliography

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