Merge "Add __freadahead."

docs/status.md

@@ -54,6 +54,7 @@ Current libc symbols: https://android.googlesource.com/platform/bionic/+/master/
 New libc functions in U (API level 34):
   * `close_range` and `copy_file_range` (Linux-specific GNU extensions).
   * `memset_explicit` in <string.h> (C23 addition).
+  * `__freadahead` in <stdio_ext.h> (in musl but not glibc).
 
 New libc behavior in U (API level 34):
   * Support for `%b` and `%B` in the printf/wprintf family, `%b` in the

libc/include/stdio_ext.h

@@ -96,12 +96,20 @@ void __fpurge(FILE* __fp) __INTRODUCED_IN(23);
 
 /**
  * [__fpending(3)](http://man7.org/linux/man-pages/man3/__fpending.3.html) returns the number of
- * bytes in the output buffer.
+ * bytes in the output buffer. See __freadahead() for the input buffer.
  *
  * Available since API level 23.
  */
 size_t __fpending(FILE* __fp) __INTRODUCED_IN(23);
 
+/**
+ * __freadahead(3) returns the number of bytes in the input buffer.
+ * See __fpending() for the output buffer.
+ *
+ * Available since API level 34.
+ */
+size_t __freadahead(FILE* __fp) __INTRODUCED_IN(34);
+
 /**
  * [_flushlbf(3)](http://man7.org/linux/man-pages/man3/_flushlbf.3.html) flushes all
  * line-buffered streams.

libc/libc.map.txt

@@ -1578,6 +1578,7 @@ LIBC_T { # introduced=Tiramisu
 
 LIBC_U { # introduced=UpsideDownCake
   global:
+    __freadahead;
     close_range;
     copy_file_range;
     memset_explicit;

libc/stdio/stdio_ext.cpp

@@ -67,6 +67,12 @@ size_t __fpending(FILE* fp) {
   return fp->_p - fp->_bf._base;
 }
 
+size_t __freadahead(FILE* fp) {
+  // Normally _r is the amount of input already available.
+  // When there's ungetc() data, _r counts that and _ur is the previous _r.
+  return fp->_r + (HASUB(fp) ? fp->_ur : 0);
+}
+
 void _flushlbf() {
   // If we flush all streams, we know we've flushed all the line-buffered streams.
   fflush(nullptr);

tests/stdio_ext_test.cpp

@@ -78,6 +78,24 @@ TEST(stdio_ext, __fpending) {
   fclose(fp);
 }
 
+TEST(stdio_ext, __freadahead) {
+#if defined(__GLIBC__)
+  GTEST_SKIP() << "glibc doesn't have __freadahead";
+#else
+  FILE* fp = tmpfile();
+  ASSERT_NE(EOF, fputs("hello", fp));
+  rewind(fp);
+
+  ASSERT_EQ('h', fgetc(fp));
+  ASSERT_EQ(4u, __freadahead(fp));
+
+  ASSERT_EQ('H', ungetc('H', fp));
+  ASSERT_EQ(5u, __freadahead(fp));
+
+  fclose(fp);
+#endif
+}
+
 TEST(stdio_ext, __fpurge) {
   FILE* fp = tmpfile();