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[libc] fwrite_unlocked: only return errno if an actual error occurred. #167350

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Merged
merged 4 commits into from
Nov 10, 2025
Merged

[libc] fwrite_unlocked: only return errno if an actual error occurred. #167350

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2ccd126
[libc] fwrite_unlocked: only return errno if an actual error occurred.
Sterling-Augustine Nov 10, 2025
ccb1b25
FileIOResult is not a std::expected or similar. Don't treat it that way.
Sterling-Augustine Nov 10, 2025
9b0318a
Fix size calculation.
Sterling-Augustine Nov 10, 2025
64fad09
Fix calculation to be against nmemb, not size.
Sterling-Augustine Nov 10, 2025
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10 changes: 8 additions & 2 deletions libc/src/stdio/printf_core/vfprintf_internal.h
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Original file line number Diff line number Diff line change
Expand Up @@ -51,8 +51,14 @@ LIBC_INLINE void funlockfile(::FILE *f) { ::funlockfile(f); }
LIBC_INLINE FileIOResult fwrite_unlocked(const void *ptr, size_t size,
size_t nmemb, ::FILE *f) {
// Need to use system errno in this case, as system write will set this errno
// which we need to propagate back into our code.
return {::fwrite_unlocked(ptr, size, nmemb, f), errno};
// which we need to propagate back into our code. fwrite only modifies errno
// if there was an error, and errno may have previously been nonzero. Only
// return errno if there was an error.
auto bytes = ::fwrite_unlocked(ptr, size, nmemb, f);
if (bytes == nmemb * size)
return bytes;
else
return errno;
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@vonosmas vonosmas Nov 10, 2025

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I don't think you can construct FileIOResult like that? Shouldn't you return smth. like {bytes, errno} here? Is there a way to add a test that if system errno was pre-set, but the call succeeded, this call wouldn't return system errno?

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@Sterling-Augustine Sterling-Augustine Nov 10, 2025

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done

}
#endif // LIBC_COPT_STDIO_USE_SYSTEM_FILE
} // namespace internal
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