Standard C90 requires the elements of an initializer to appear in a fixed order, the same as the order of the elements in the array or structure being initialized.
In ISO C99 you can give the elements in any order, specifying the array indices or structure field names they apply to, and GNU C allows this as an extension in C90 mode as well. This extension is not implemented in GNU C++.
To specify an array index, write ‘[index] =’ before the element value. For example,
int a[6] = { [4] = 29, [2] = 15 };
is equivalent to
int a[6] = { 0, 0, 15, 0, 29, 0 };
The index values must be constant expressions, even if the array being initialized is automatic.
An alternative syntax for this that has been obsolete since GCC 2.5 but GCC still accepts is to write ‘[index]’ before the element value, with no ‘=’.
To initialize a range of elements to the same value, write ‘[first ... last] = value’. This is a GNU extension. For example,
int widths[] = { [0 ... 9] = 1, [10 ... 99] = 2, [100] = 3 };
If the value in it has side-effects, the side-effects happen only once, not for each initialized field by the range initializer.
Note that the length of the array is the highest value specified plus one.
In a structure initializer, specify the name of a field to initialize with ‘.fieldname =’ before the element value. For example, given the following structure,
struct point { int x, y; };
the following initialization
struct point p = { .y = yvalue, .x = xvalue };
is equivalent to
struct point p = { xvalue, yvalue };
Another syntax that has the same meaning, obsolete since GCC 2.5, is ‘fieldname:’, as shown here:
struct point p = { y: yvalue, x: xvalue };
Omitted field members are implicitly initialized the same as objects that have static storage duration.
The ‘[index]’ or ‘.fieldname’ is known as a designator. You can also use a designator (or the obsolete colon syntax) when initializing a union, to specify which element of the union should be used. For example,
union foo { int i; double d; };
union foo f = { .d = 4 };
converts 4 to a double to store it in the union using
the second element. By contrast, casting 4 to type union foo
stores it into the union as the integer i, since it is
an integer. (See Cast to Union.)
You can combine this technique of naming elements with ordinary C initialization of successive elements. Each initializer element that does not have a designator applies to the next consecutive element of the array or structure. For example,
int a[6] = { [1] = v1, v2, [4] = v4 };
is equivalent to
int a[6] = { 0, v1, v2, 0, v4, 0 };
Labeling the elements of an array initializer is especially useful
when the indices are characters or belong to an enum type.
For example:
int whitespace[256]
= { [' '] = 1, ['\t'] = 1, ['\h'] = 1,
['\f'] = 1, ['\n'] = 1, ['\r'] = 1 };
You can also write a series of ‘.fieldname’ and ‘[index]’ designators before an ‘=’ to specify a nested subobject to initialize; the list is taken relative to the subobject corresponding to the closest surrounding brace pair. For example, with the ‘struct point’ declaration above:
struct point ptarray[10] = { [2].y = yv2, [2].x = xv2, [0].x = xv0 };
If the same field is initialized multiple times, it has the value from the last initialization. If any such overridden initialization has side-effect, it is unspecified whether the side-effect happens or not. Currently, GCC discards them and issues a warning.