On Wed, 20 Aug 2025 20:15:25 GMT, Kim Barrett <kbarr...@openjdk.org> wrote:
>> This is a proposal to standardize on the use of `os::snprintf` and >> `os::snprintf`_checked across the hotspot code base, and to disallow use of >> the C library variants. (It does not touch use of `jio_printf` at all.) >> >> From: https://bugs.openjdk.org/browse/JDK-8347707 >> >> The platform `snprintf/vsnprintf` returns -1 on error, else if the buffer is >> large enough returns the number of bytes written (excluding the null byte), >> else (buffer is too small) the number of characters (excluding the >> terminating null byte) which would have been written to the final string if >> enough space had been available. Thus, a return value of size or more means >> that the output was truncated. >> >> To provide a consistent approach to error handling and truncation >> management, we provide `os::xxx` wrapper functions as described below and >> forbid the use of the library `::vsnprintf` and `::snprintf`. >> >> The potential errors are, generally speaking, not something we should >> encounter in our own well-written code: >> >> - encoding error: not applicable as we are not using extended character sets >> - invalid parameters (null buffers, specifying a limit > size of the buffer >> [Windows], things of this nature) >> - mal-formed formatting directives >> - overflow error (POSIX) if the required buffer size exceeds INT_MAX (as we >> return `int`). >> >> As these should simply never occur, we handle the checks for -1 at the >> lowest-level (`os::vsnprintf`) with an assertion, and accompanying >> precondition assertions. >> >> The potential clients of this API then fall into a number of camps: >> >> 1. Those who have sized their buffer correctly, don't need the return value >> for subsequent use, and for whom truncation (if it were possible) would be a >> programming error. >> >> For these clients we have `void os::snprintf_checked` - which returns >> nothing and asserts on truncation. >> >> 2. Those who have sized their buffer correctly, but do need the return value >> for subsequent operations (e.g. chains of `snprintf` where you advance the >> buffer pointer based on previous writes), but again for whom truncation >> should never happen. >> >> For these clients we have `os::snprintf`, but they have to add their own >> assertion for no truncation. >> >> 3. Those who present a buffer but know that truncation is a possibility, but >> don't need to do anything about it themselves, and for whom the return value >> is of no use. >> >> These clients also use `os::snprintf_checked`. The truncation assertion can >> be useful for guiding buffer sizing... > > src/hotspot/share/runtime/os.hpp line 810: > >> 808: // Performs vsnprintf and asserts the result is non-negative (so >> there was not >> 809: // an encoding error or any other kind of usage error). >> 810: [[nodiscard]] static int vsnprintf(char* buf, size_t len, const char* >> fmt, va_list args) ATTRIBUTE_PRINTF(3, 0); > > Consider moving the `ATTRIBUTE_PRINTF` to the front so all the attributes are > together? > And maybe a line break between the attributes and the signature, just to > avoid pushing the > signature way over to the right. I have done that and placed each attribute on its own line (we should have a style guide entry for this :) ). But I note that all the other ATTRIBUTE_PRINTF's are placed after the function. ------------- PR Review Comment: https://git.openjdk.org/jdk/pull/26849#discussion_r2289993291
