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

fwrite and friends don't modify errno if no error occurred. Therefore
frite_unlocked's return value shouldn't be constructed from errno
without checking if an error actually occurred.

This fixes an error introduced by
https://github.com/llvm/llvm-project/commit/9e2f73fe9052a4fbf382a06e30b2441c6d99fb7e

1 files changed, 5 insertions, 2 deletions

diff --git a/libc/src/stdio/printf_core/vfprintf_internal.h b/libc/src/stdio/printf_core/vfprintf_internal.h
index 564441d3bf51..c47a03d741f9 100644
--- a/libc/src/stdio/printf_core/vfprintf_internal.h
+++ b/libc/src/stdio/printf_core/vfprintf_internal.h
@@ -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 members_written = ::fwrite_unlocked(ptr, size, nmemb, f);
+  return {members_written, members_written == nmemb ? 0 : errno};
 }
 #endif // LIBC_COPT_STDIO_USE_SYSTEM_FILE
 } // namespace internal