Some __nonstring__ Turbulence

Summary

By Jonathan CorbetApril 24, 2025 New compiler releases often bring with them new warnings; those warnings are usually welcome, since they help developers find problems before they turn into nasty bugs. Adapting to new warnings can also create disruption in the development process, though, especially when an important developer upgrades to a new compiler at an unfortunate time. This is just the scenario that played out with the 6.15-rc3 kernel release and the implementation of -Wunterminated-string-initialization in GCC 15. Consider a C declaration like: char foo[8] = "bar"; The array will be initialized with the given string, including the normal trailing NUL byte indicating the end of the string. Now consider this variant: char foo[8] = "NUL-free"; This is a legal declaration, even though the declared array now lacks the room for the NUL byte. That byte will simply be omitted, creating an unterminated string. That is often not what the developer who wrote that code wants, and it can lead to unpleasant bugs that are not discovered until some later time. The -Wunterminated-string-initialization option emits a warning for this kind of initialization, with the result that, hopefully, the problem — if there is a problem — is fixed quickly. Stay on top of Linux kernel development with a one-month free trial subscription to LWN, no credit card required. The kernel community has worked to make use of this warning and, hopefully, eliminate a source of bugs. There is only one little problem with the new warning, though: sometimes the no-NUL initialization is exactly what is wanted and intended. See, for example, this declaration from fs/cachefiles/key.c: static const char cachefiles_charmap[64] = "0123456789" /* 0 - 9 */ "abcdefghijklmnopqrstuvwxyz" /* 10 - 35 */ "ABCDEFGHIJKLMNOPQRSTUVWXYZ" /* 36 - 61 */ "_-" /* 62 - 63 */ ; This char array is used as a lookup table, not as a string, so there is no need for a trailing NUL byte. GCC 15, being unaware of that usage, will emit...