How to initialize a struct in accordance with C programming language standards

In (ANSI) C99, you can use a designated initializer to initialize a structure:

MY_TYPE a = { .flag = true, .value = 123, .stuff = 0.456 };

Other members are initialized as zero: "Omitted field members are implicitly initialized the same as objects that have static storage duration." (https://gcc.gnu.org/onlinedocs/gcc/Designated-Inits.html)

You can do it with a compound literal. According to that page, it works in C99 (which also counts as ANSI C).

MY_TYPE a;

a = (MY_TYPE) { .flag = true, .value = 123, .stuff = 0.456 };
...
a = (MY_TYPE) { .value = 234, .stuff = 1.234, .flag = false };

The designations in the initializers are optional; you could also write:

a = (MY_TYPE) { true,  123, 0.456 };
...
a = (MY_TYPE) { false, 234, 1.234 };

I see you've already received an answer about ANSI C 99, so I'll throw a bone about ANSI C 89. ANSI C 89 allows you to initialize a struct this way:

typedef struct Item {
    int a;
    float b;
    char* name;
} Item;

int main(void) {
    Item item = { 5, 2.2, "George" };
    return 0;
}

An important thing to remember, at the moment you initialize even one object/ variable in the struct, all of its other variables will be initialized to default value.

If you don't initialize the values in your struct, all variables will contain "garbage values".

Good luck!