What is "..." in switch-case in C code

Question

Here is a piece of code in /usr/src/linux-3.10.10-1-ARCH/include/linux/printk.h:

static inline int printk_get_level(const char *buffer)
{
  if (buffer[0] == KERN_SOH_ASCII && buffer[1]) {
    switch (buffer[1]) {
    case '0' ... '7':
    case 'd':  /* KERN_DEFAULT */
      return buffer[1];
    }
  }
}

Is it a kind of operator? Why does "The C Programming Language" not mention it?

Limelight

gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/… and read also Strange initializer expression? – Holy

@GrijeshChauhan The questions and the answers to the dup listed do not even remotely match this question. This questions asks what is this feature the dup asks how to convert from this feature. It is silly to list it as a dup. – Gluteal

@ShafikYaghmour Is it? Indeed the answers posted here are different. Through question is answer nicely (and accepted) and I don't see any scope to add new one..I voting for reopen. Thanks! – Holy

@ShafikYaghmour Ok I have voted to reopen we have to wait till more users vote it for reopen either OP or you or me have to rise a flag to reopen... no other choice. – Holy

Answer 1

This is a gcc extension called case ranges, this is how it is explained in the document:

You can specify a range of consecutive values in a single case label, like this:

case low ... high:

You can find a complete list of gcc extensions here. It seems like clang also supports this to try and stay compatible with gcc. Using the -pedantic flag in either gcc or clang will warn you that this is non-standard, for example:

warning: range expressions in switch statements are non-standard [-Wpedantic]

It is interesting to note that Linux kernel uses a lot of gcc extensions one of the extensions not covered in the article is statement expressions.

Gluteal

FAQ/DoWhile0 covers statement expressions and Linux kernel. – Gluteal

Answer 2

It is gcc compiler extension allowing to combine several case statement in one line.

Mailand

Answer 3

Beware, it is not standard C and therefore not portable. It is a shorthand devised for case statements. It's well-defined since in C you can only switch on integral types.

In standard C, ... is only used in variable length argument lists.

Stollings

Answer 4

case '0'...'7': is case ranges Speciacation in gcc.

Range specification for case statement.

Write spaces around the ..., for otherwise it may be parsed wrong when you use it with integer values

case '0' or case '1' or case '3' and so on case '7':
or case 'b' :
just return buffer[1]; 

Sibship