[libc-commits] [libc] [libc] fwrite_unlocked: only return errno if an actual error occurred. (PR #167350)

[Jackson Stogel via libc-commits]

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@@ -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.
+  auto bytes = ::fwrite_unlocked(ptr, size, nmemb, f);
+  return {bytes, bytes == nmemb * size ? 0 : errno};
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jtstogel wrote:

Docs for `fwrite` say say that it returns the number of elements written, not bytes (https://man7.org/linux/man-pages/man3/fwrite.3p.html#RETURN_VALUE). Is fwrite_unlocked different?

https://github.com/llvm/llvm-project/pull/167350