C99 to C89

Reality is, many of these C language features are cosmetic extensions introduced in C99 that are trivially emulated using classic C89 syntax. Consider designated initializers:

struct {
    int a, b;
} var = { .b = 1, };

This can be trivially emulated in C89 by using the following syntax:

struct {
    int a, b;
} var = { 0, 1 };

For unions, you can change the initialization (as long as the size of the first field is large enough to hold the contents of any other field in the union) to do a binary translation of the initialized field type to the first field type:

union {
    unsigned int a;
    float b;
} var = { .b = 1.0, };

becomes:

union {
    unsigned int a;
    float b;
} var = { 0x3f800000, };

Here, 0x3f800000 is the binary representation of the floating point number 1.0. If the value to be converted is not static, the assignment can simply become a statement on its own:

union {
    unsigned int a;
    float b;
} var;
var.b = 1.0;

Other C99 language features (e.g. compound literals) can be translated in a similar manner:

struct {
    int *list;
} var = { (int *) { 0, 1 } };

becomes:

int *list = { 0, 1 };
struct {
    int *list;
} var = { list };