[all-commits] [llvm/llvm-project] 98fb5d: [libc] Fix EOF handling in fscanf (#211982)

[Alexey Samsonov via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 98fb5dae73c3887e6659ee126b131b004d12592d
      https://github.com/llvm/llvm-project/commit/98fb5dae73c3887e6659ee126b131b004d12592d
  Author: Alexey Samsonov <vonosmas at gmail.com>
  Date:   2026-07-28 (Tue, 28 Jul 2026)

  Changed paths:
    M libc/src/stdio/scanf_core/CMakeLists.txt
    M libc/src/stdio/scanf_core/vfscanf_internal.h
    M libc/test/src/stdio/CMakeLists.txt
    M libc/test/src/stdio/fscanf_test.cpp

  Log Message:
  -----------
  [libc] Fix EOF handling in fscanf (#211982)

`fscanf` family of functions should return EOF if:
- end of input is reached before either the first successful conversion
or a matching failure occurs;
- a read error occurs.

This wasn't handled correctly before - in "system FILE" mode (in overlay
build) `EOF` return value from `getc` was passed through to parser,
violating `Reader` interface, which triggered infinite loops on
"while-not-EOF" `fscanf` loops. In llvm-libc-FILE mode `fscanf` simply
returned zero instead of EOF, because the code only checked error
indicator on a stream.

This PR removes _any_ lookups of eof/error indicators on an input stream
- instead we can simply rely on the `getc` (system or internal) function
behavior, which returns `EOF` on unsuccessful reads.

Extend the test cases for `fscanf` to cover various return values in
cases when `EOF` is reached (before or after conversions / matching
errors).



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