CODEWITHSUNDEEP
cc-preprocessor
santa
santa

Reputation: 1003

Is this a plain Bug or a clever precompiler Trick?

scrolling through http://git.suckless.org/st/plain/st.c I stumbled upon

#define LEN(a) (sizeof(a) / sizeof(a)[0])

I know precompiler magic is hard, and my C aint the best

but it makes me wonder if the parens at the end make sense - shouldn't it be something like

#define LEN(a) (sizeof(a) / sizeof((a)[0]))

Upvotes: 1

Views: 130

Answers (1)

mcleod_ideafix
mcleod_ideafix

Reputation: 11438

The sizeof operator doesn't really need parenthesis when the operand is a left-side expression instead of a type, so your parenthesis at the second sizeof are not needed, as this one doesn't expect a type (because of the trailing [0]). This LEN macro seems to be used to get the size of a statically defined vector.

For example:

int v[10];

LEN(v) will expand to (sizeof(v)/sizeof(v)[0])

And (v)[0] is the same as v[0] so the second sizeof is actually something like sizeof v[0] which is valid use of sizeof (no parenthesis needed)

A well-known (at least here at Stack Overflow) example of a sizeof that doesn't needing parenthesis is:

int *v;

v=malloc(10*sizeof *v);

To get a dynamically defined 10 element vector. Note that the previous LEN macro won't work here.

Upvotes: 2

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