Doesn't actually return char *. Reset to void ?

Needs #include <string.h> for sprintf().

Oops, I meant to have it return buf.

Style: stdio.h isn't sorted correctly.

Is the intent here to create a single file per platform ID bit?

Style: extra space after sizeof.

Perhaps explicitly mask off the upper 24 bits?

resid might be a more idiomatic name.

Missing return.

sprintf(3) is prototyped in stdio.h.

Style: blank line.

It is also reasonable to check that we write as much as we asked for. E.g. on fs full you get the truncated write.

No, main() is implicitly terminated by return (0);.

but is needed for strlen

Well, a single file per blob in one ucode file, which in practice is per PlatformId bit.

In some cases there's a unique blob per PlatformId bit:

06-05-00.01
06-05-00.02
06-05-00.08

In some cases there's only one blob that supports multiple PlatformIds:

06-55-03.97

In some cases there are multiple blobs that each support multiple PlatformIds:

06-17-0a.11
06-17-0a.44
06-17-0a.a0