uncommon use of malloc

Question

I'm running into this (short) code and I'm not sure what it exactly does;

int amount = 5
int totalAllocatedMemory = 0;
totalAllocatedMemory += amount, malloc(amount);

It seems that 'malloc' has no effect here!

Thanks

Not a real question, not constructive, too localized... how to decide?! — tenfour, Jan 25 '12 at 18:01
The "totalAllocatedMemory" is a different name from "totalAllocatedMemroy" so it may not be doing what you think it's doing. Also, you are missing a semicolon after the first line. In any case, it does leak memory. — Branko Dimitrijevic, Jan 25 '12 at 18:07
I've seen embedded system malloc "implementations" which allocate linearly and have no (meaningful) free counterpart, so it could be that the `amount` here is moving the malloc linear allocator pointer to an aligned address. I can't think why else it could have been made. — Tom Whittock, Jan 25 '12 at 19:01

score 3 · Accepted Answer · edited May 23 '17 at 12:20

3

It does have an effect, in that it allocates memory. However, the code does look bizarre and the memory does get leaked.

In case you're wondering about the syntax and exact semantics, see How does the Comma Operator work

edited May 23 '17 at 12:20

Community

answered Jan 25 '12 at 17:56

NPE

except the memory never gets used, truly a bizarre piece of code – Anycorn Jan 25 '12 at 17:58

score 3 · Answer 2 · edited Jan 25 '12 at 18:05

3

It:

edited Jan 25 '12 at 18:05

Ingemar

answered Jan 25 '12 at 18:00

Drew Dormann

A nitpick: The comma operator has lower precedence than +=, so it adds 5 to `totalAllocatedMemory` and not the address. – Branko Dimitrijevic Jan 25 '12 at 18:08
1

@BrankoDimitrijevic: Agreed. I wouldn't even consider that a nitpick. – Drew Dormann Jan 25 '12 at 18:11

score 2 · Answer 3 · answered Jan 25 '12 at 18:09

Well the code looks buggy but here's what it does:

Adds amount to totalAllocatedMemory
Allocates 5 bytes and discards the result (which would be the address of the allocated memory, therefore as others said results to a memory leak)

This is because comma has the lowest precedence of all operators in C.

3 Answers3