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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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7 changes: 5 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 +51,11 @@ 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.
size_t bytes = ::fwrite_unlocked(ptr, size, nmemb, f);
return {bytes, bytes == size ? 0 : errno};
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@jtstogel jtstogel Nov 10, 2025

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The fwrite() function shall return the number of elements successfully written, which may be less than nitems if a write error is encountered.

https://man7.org/linux/man-pages/man3/fwrite.3p.html#RETURN_VALUE

I think that instead of bytes == size ? 0, it should be bytes == nmemb ? 0. Could we also update the name bytes to reflect that it's the number of elements rather than number of bytes?

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@Sterling-Augustine Sterling-Augustine Nov 10, 2025

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cleaned up.

}
#endif // LIBC_COPT_STDIO_USE_SYSTEM_FILE
} // namespace internal
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