CODEWITHSUNDEEP
c
rpsml
rpsml

Reputation: 1498

Proper argument for malloc

I have always used the malloc function as, for exemple,

int size = 10000;
int *a;
a = malloc(size * sizeof(int));

I recently run into a piece of code that discards the sizeof(int) part, i.e.

int size = 10000;
int *a;
a = malloc(size);

This second code seems to be working fine.

My question is then, which form is correct? If the second form is, am I allocating needless space with the first form.

Upvotes: 2

Views: 3205

Answers (3)

user3657941
user3657941

Reputation:

From the man page:

The malloc() function allocates size bytes and returns a pointer to the allocated memory.

The first form is correct.

Even if the sizeof(int) on the machine you are targeting is one (which is sometimes true on 8-bit microcontrollers) you still want your code to be readable.

The reason the "second code seems to be working fine" is that you are lucky.

The version of malloc you are using might be returning a pointer to an area of memory that is larger than what you requested. No matter what is happening behind the scenes, the behavior may change if you switch to a different compiler, so you do not want to rely on it.

Upvotes: 2

Kerrek SB
Kerrek SB

Reputation: 476990

malloc allocates a given number of bytes worth of memory, suitably aligned for any type. If you want to store N elements of type T, you need N * sizeof(T) bytes of aligned storage. Typically, T * p = malloc(N * sizeof(T)) provides that and lets you index the elements as p[i] for i in [0, N).

Upvotes: 3

Fred Foo
Fred Foo

Reputation: 363547

The argument to malloc is the number of bytes to be allocated. If you need space for an array of n elements of type T, call malloc(n * sizeof(T)). malloc does not know about types, it only cares about bytes.

The only exception is that when you allocate space for (byte/char) strings, the sizeof can be omitted because sizeof(char) == 1 per definition in C. Doing something like

int *a = malloc(10000);
a[9000] = 0;

may seem to work now, but actually exploits undefined behavior.

Upvotes: 6

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