Difference between malloc() and simple declaration in fixed sizes

Question

Let assume that I want to get space for an array of x double, x being an integer.

Can someone explain me the difference between

double myArray[x];

and

malloc(x*sizeof(double));

excepted the fact that malloc() returns a void pointer ?

Thank you in advance for your answer.

Nishant · Accepted Answer · 2016-06-07T11:16:43.953

1

double myArray[x]; Here memory is

declared on stack
Declared at compile-time, hence faster
Accessible only in the scope of the declaring function,( globally if declared in global scope)
Will be freed if the declaring function returns
generally used when the size of array is known at compile time

myArray = malloc(x*sizeof(double)); Here memory is

declared on heap (except for variable length arrays(C99) which are allocated on stack as pointed by Malina, read more here)
Declared at run-time, hence slower
Accessible wherever the myArray variable is accessible
Will be freed when free(myArray) is called or the program exits
generally used if the size of array is unknown at compile time

edited Jun 07 '16 at 11:16

answered May 13 '15 at 11:49

Nishant

2

Declared on stack and Accessible only locally. Not true if is declared global or static. – LPs May 13 '15 at 11:50
2

Declared on stack only if constant-sized array. In C99, variable-sized arrays are on the program's internal automatic memory. – Imobilis May 13 '15 at 11:54
@Malina can you explain further. I am hearing this term - "program's internal automatic memory" for the first time – Nishant May 13 '15 at 11:59
@Malina , VLAs are also allocated on the stack, aren't they? – Spikatrix May 13 '15 at 12:07
https://gcc.gnu.org/onlinedocs/gcc/Variable-Length.html a good reading imo. – Imobilis May 13 '15 at 12:27

1 Answers1