Should I cast the result of malloc (in C)?

In this question, someone suggested in a comment that I should not cast the result of malloc. i.e., I should do this:

int *sieve = malloc(sizeof(*sieve) * length);

rather than:

int *sieve = (int *) malloc(sizeof(*sieve) * length);

Why would this be the case?

Solution

No; you shouldn’t cast the result, since:

It is unnecessary, as void * is automatically and safely promoted to any other pointer type in this case.
It adds clutter to the code, casts are not very easy to read (especially if the pointer type is long).
It makes you repeat yourself, which is generally bad.
It can hide an error if you forgot to include <stdlib.h>. This can cause crashes (or, worse, not cause a crash until way later in some totally different part of the code). Consider what happens if pointers and integers are differently sized; then you're hiding a warning by casting and might lose bits of your returned address. Note: as of C99 implicit functions are gone from C, and this point is no longer relevant since there's no automatic assumption that undeclared functions return int.

As a clarification, note that I said “you shouldn’t cast”, not “you don't need to cast”. In my opinion, it's a blunder to include the cast, even if you got it right. There are simply no benefits to doing it, but a bunch of potential risks, and including the cast indicates that you don't know about the risks.

Also note, as commentators point out, that the above talks about straight C, not C++. I very firmly believe in C and C++ as separate languages.

To add further, your code needlessly repeats the type information (int) which can cause errors. It's better to de-reference the pointer being used to store the return value, to "lock" the two together:

int *sieve = malloc(length * sizeof *sieve);

Some may say: “Well, previously the type was repeated, and now the variable name is repeated; isn’t this just as repetitive as before? How is that any better?” The difference is that if you one day change the type of the variable and forget to change the type under the sizeof to match, you will silently get an allocation of the wrong size and no warning about it; but if you change the name of the variable, but forget to change the name under sizeof to match, it is more probable that the old name no longer resolves to anything, so your code will stop compiling, prompting you to fix the mistake.

This also moves the length to the front for increased visibility, and drops the redundant parentheses with sizeof; they are only needed when the argument is a type name. Many people seem to not know (or ignore) this, which makes their code more verbose. Remember: sizeof is not a function! :)

While moving length to the front may increase visibility in some rare cases, one should also pay attention that in the general case, it should be better to write the expression as:

int *sieve = malloc(sizeof *sieve * length);

Since keeping the sizeof first, in this case, ensures multiplication is done with at least size_t math.

Compare: malloc(sizeof *sieve * length * width) vs. malloc(length * width * sizeof *sieve) the second may overflow the length * width when width and length are smaller types than size_t.