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

[via libc-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-libc

Author: None (Sterling-Augustine)

<details>
<summary>Changes</summary>

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.

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


1 Files Affected:

- (modified) libc/src/stdio/printf_core/vfprintf_internal.h (+8-2) 


``````````diff
diff --git a/libc/src/stdio/printf_core/vfprintf_internal.h b/libc/src/stdio/printf_core/vfprintf_internal.h
index 564441d3bf51a..a14ddee5f62fd 100644
--- a/libc/src/stdio/printf_core/vfprintf_internal.h
+++ b/libc/src/stdio/printf_core/vfprintf_internal.h
@@ -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;
 }
 #endif // LIBC_COPT_STDIO_USE_SYSTEM_FILE
 } // namespace internal

``````````

</details>


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