Please explain the use of ':' and a trailing ',' in this struct initialization C code

Question

static struct file_operations memory_fops = {
    open:       memory_open,    /* just a selector for the real open */
};

this is from mem.c file in uclinux

Answer 1

That's GNU-style initialization syntax; the open member is initialized to memory_open , the rest is left uninitialized. C99 uses a different syntax ( .open = memory_open ).

Answer 2

In C the optional trailing comma was allowed in brace-enclosed initializers since the beginning of time. It is there so that you can use uniform comma placement in initializers like

struct SomeStructType s = {
  value1,
  value2,
  value3,
};

This makes it easier, for example, to rearrange the initializers in the list, should such a need arise. Whether you want to use it or not is a matter of personal preference.

As for the : syntax, it is a GCC-specific extension as @geekosaur already explained. The corresponding functionality was standardized in C99 with a different syntax.