When can I omit curly braces in C?

Question

I\'m almost certain this has been asked before, but I can\'t find it being answered anywhere.

When can I omit curly braces in C? I\'ve seen brace-less return

Answer 1

But this doesn't always seem to work for all statements.

Specifically? When a single statement is expected, then it's perfectly valid. Loops, the if statement, etc. all expect a statement and that's either a block, or, well, a single statement without being enclosed in a block.

Answer 2

If you look at the C syntax, there are a number of contexts that require a statement, a term that's defined by the grammar itself.

In any of those contexts, one of forms of statement you can use is a compound-statement, which consists of an opening brace {, a sequence of zero or more declarations and/or statements, and a closing brace }. (In C90, all declarations in a compound-statement must precede all statements; C99 removed that restriction.)

A function-definition specifically requires a compound-statement, not just any kind of statement. (I think that's the only case where a compound-statement is the only kind of statement you can use). If not for that restriction, you'd be able to write:

void say_hello(void) printf("Hello, world\n");

But since most function definitions contain multiple declarations and/or statements, there wouldn't be much advantage in permitting that.

There's a separate question: when should you omit braces. In my personal opinion, the answer is "hardly ever". This:

if (condition)
     statement;

is perfectly legal, but this:

if (condition) {
    statement;
}

IMHO reads better and is easier to maintain (if I want to add a second statement, the braces are already there). It's a habit I picked up from Perl, which requires braces in all such cases.

The only time I'll omit the braces is when an entire if statement or something similar fits on a single line, and doing so makes the code easier to read, and I'm unlikely to want to add more statements to each if:

if (cond1) puts("cond1");
if (cond2) puts("cond2");
if (cond3) puts("cond3");
/* ... */

I find such cases are fairly rare. And even then, I'd still consider adding the braces anyway:

if (cond1) { puts("cond1"); }
if (cond2) { puts("cond2"); }
if (cond3) { puts("cond3"); }
/* ... */

Answer 3

You mainly need curly braces when you want to combine multiple statements or expressions into one, e.g.:

{
  x = 1;
  y = 2;
}

So if you put the above under if or else the whole thing in the braces will be executed as a whole, whereas if you omit the braces, only the first one (x = 1; in this case) will be used as part of if or else.

You also typically use them with switch():

switch (x)
{
case 1:
  // do smth
  break;
case 2:
  // do smth else
  break;
}

You typically need them with the do-while statement:

do
{
  printf("%d\n", x);
  x++;
} while (x < 10);

You need them with C89 compilers when you want to define and use a temporary variable in the middle of the code:

int main(void)
{
  puts("Hello World!");
  {
    int x;
    for (x = 0; x < 10; x++)
      printf("%d\n", x);
  }
  return 0;
}

You use them to begin and end the body of a function, a structure/union definition, an enumeration definition, initialization of a structure/union/array, e.g.:

void blah(void)
{
}

enum e
{
  e1 = 1,
  e2 = 2
};

struct s
{
  int foo;
  int bar;
} s = { 1, 2 };

int a[3] = { 1, 2, 3 };

You may omit them sometimes in initializations, e.g.:

char str2[] = { "abc" };
char str1[] = "abc";