Finish <stdio_ext.h>.

Implement __freading and __fwriting, and clarify the documentation that was
the cause of these not being implemented for years.

Bug: http://b/17157253
Test: ran tests
Change-Id: I89542c8131b13889e2585417a024050ecf2abcb7

libc/include/stdio_ext.h

@@ -40,7 +40,9 @@ __BEGIN_DECLS
 
 size_t __fbufsize(FILE*) __INTRODUCED_IN(23);
 int __freadable(FILE*) __INTRODUCED_IN(23);
+int __freading(FILE*) __INTRODUCED_IN_FUTURE;
 int __fwritable(FILE*) __INTRODUCED_IN(23);
+int __fwriting(FILE*) __INTRODUCED_IN_FUTURE;
 int __flbf(FILE*) __INTRODUCED_IN(23);
 void __fpurge(FILE*) __INTRODUCED_IN(23);
 size_t __fpending(FILE*) __INTRODUCED_IN(23);

libc/libc.arm.map

@@ -1320,6 +1320,8 @@ LIBC_O {
 
 LIBC_P {
   global:
+    __freading; # future
+    __fwriting; # future
     getlogin_r; # future
 } LIBC_O;
 

libc/libc.arm64.map

@@ -1240,6 +1240,8 @@ LIBC_O {
 
 LIBC_P {
   global:
+    __freading; # future
+    __fwriting; # future
     getlogin_r; # future
 } LIBC_O;
 

libc/libc.map.txt

@@ -1345,6 +1345,8 @@ LIBC_O {
 
 LIBC_P {
   global:
+    __freading; # future
+    __fwriting; # future
     getlogin_r; # future
 } LIBC_O;
 

libc/libc.mips.map

@@ -1304,6 +1304,8 @@ LIBC_O {
 
 LIBC_P {
   global:
+    __freading; # future
+    __fwriting; # future
     getlogin_r; # future
 } LIBC_O;
 

libc/libc.mips64.map

@@ -1240,6 +1240,8 @@ LIBC_O {
 
 LIBC_P {
   global:
+    __freading; # future
+    __fwriting; # future
     getlogin_r; # future
 } LIBC_O;
 

libc/libc.x86.map

@@ -1302,6 +1302,8 @@ LIBC_O {
 
 LIBC_P {
   global:
+    __freading; # future
+    __fwriting; # future
     getlogin_r; # future
 } LIBC_O;
 

libc/libc.x86_64.map

@@ -1240,6 +1240,8 @@ LIBC_O {
 
 LIBC_P {
   global:
+    __freading; # future
+    __fwriting; # future
     getlogin_r; # future
 } LIBC_O;
 

libc/stdio/local.h

@@ -127,10 +127,11 @@ struct __sfileext {
 // Values for `__sFILE::_flags`.
 #define __SLBF 0x0001  // Line buffered.
 #define __SNBF 0x0002  // Unbuffered.
-// RD and WR are never simultaneously asserted: use _SRW instead.
-#define __SRD  0x0004  // OK to read.
-#define __SWR  0x0008  // OK to write.
-#define __SRW  0x0010  // Open for reading & writing.
+// __SRD and __SWR are mutually exclusive because they indicate what we did last.
+// If you want to know whether we were opened read-write, check __SRW instead.
+#define __SRD  0x0004  // Last operation was read.
+#define __SWR  0x0008  // Last operation was write.
+#define __SRW  0x0010  // Was opened for reading & writing.
 #define __SEOF 0x0020  // Found EOF.
 #define __SERR 0x0040  // Found error.
 #define __SMBF 0x0080  // `_buf` is from malloc.

libc/stdio/stdio_ext.cpp

@@ -39,17 +39,13 @@ size_t __fbufsize(FILE* fp) {
   return fp->_bf._size;
 }
 
-/* For a _SRW stream, we don't know whether we last read or wrote.
 int __freading(FILE* fp) {
-  return (fp->_flags & _SRD) != 0 || ...;
+  return (fp->_flags & __SRD) != 0;
 }
-*/
 
-/* For a _SRW stream, we don't know whether we last read or wrote.
-int __fwriting(FILE*) {
-  return (fp->_flags & _SWR) != 0 || ...;
+int __fwriting(FILE* fp) {
+  return (fp->_flags & __SWR) != 0;
 }
-*/
 
 int __freadable(FILE* fp) {
   return (fp->_flags & (__SRD|__SRW)) != 0;

tests/stdio_ext_test.cpp

@@ -116,20 +116,78 @@ TEST(stdio_ext, _flushlbf) {
 }
 
 TEST(stdio_ext, __freadable__fwritable) {
-  FILE* fp = fopen("/dev/null", "r");
+  FILE* fp;
+
+  // Read-only.
+  fp = fopen("/dev/null", "r");
   ASSERT_TRUE(__freadable(fp));
   ASSERT_FALSE(__fwritable(fp));
   fclose(fp);
 
+  // Write-only.
   fp = fopen("/dev/null", "w");
   ASSERT_FALSE(__freadable(fp));
   ASSERT_TRUE(__fwritable(fp));
   fclose(fp);
 
-  fp = fopen("/dev/null", "w+");
-  ASSERT_TRUE(__freadable(fp));
+  // Append (aka write-only).
+  fp = fopen("/dev/null", "a");
+  ASSERT_FALSE(__freadable(fp));
   ASSERT_TRUE(__fwritable(fp));
   fclose(fp);
+
+  // The three read-write modes.
+  for (auto read_write_mode : {"r+", "w+", "a+"}) {
+    fp = fopen("/dev/null", read_write_mode);
+    ASSERT_TRUE(__freadable(fp));
+    ASSERT_TRUE(__fwritable(fp));
+    fclose(fp);
+  }
+}
+
+TEST(stdio_ext, __freading__fwriting) {
+  FILE* fp;
+
+  // Append (aka write-only). Never reading. Always writing.
+  fp = fopen("/dev/zero", "a");
+  ASSERT_FALSE(__freading(fp)); // Not reading initially.
+  ASSERT_TRUE(__fwriting(fp)); // Writing initially.
+  ASSERT_TRUE(fputc('x', fp) != EOF);
+  ASSERT_FALSE(__freading(fp)); // Not reading after write.
+  ASSERT_TRUE(__fwriting(fp)); // Still writing after write.
+  fclose(fp);
+
+  // Write-only. Never reading. Always writing.
+  fp = fopen("/dev/zero", "w");
+  ASSERT_FALSE(__freading(fp)); // Not reading initially.
+  ASSERT_TRUE(__fwriting(fp)); // Writing initially.
+  ASSERT_TRUE(fputc('x', fp) != EOF);
+  ASSERT_FALSE(__freading(fp)); // Not reading after write.
+  ASSERT_TRUE(__fwriting(fp)); // Still writing after write.
+  fclose(fp);
+
+  // Read-only. Always reading. Never writing.
+  fp = fopen("/dev/zero", "r");
+  ASSERT_TRUE(__freading(fp)); // Reading initially.
+  ASSERT_FALSE(__fwriting(fp)); // Not writing initially.
+  ASSERT_TRUE(fgetc(fp) == 0);
+  ASSERT_TRUE(__freading(fp)); // Still reading after read.
+  ASSERT_FALSE(__fwriting(fp)); // Still not writing after read.
+  fclose(fp);
+
+  // The three read-write modes.
+  for (auto read_write_mode : {"r+", "w+", "a+"}) {
+    fp = fopen("/dev/zero", read_write_mode);
+    ASSERT_FALSE(__freading(fp)); // Not reading initially.
+    ASSERT_FALSE(__fwriting(fp)); // Not writing initially.
+    ASSERT_TRUE(fgetc(fp) == 0);
+    ASSERT_TRUE(__freading(fp)); // Reading after read.
+    ASSERT_FALSE(__fwriting(fp)); // Not writing after read.
+    ASSERT_TRUE(fputc('x', fp) != EOF);
+    ASSERT_FALSE(__freading(fp)); // Not reading after write.
+    ASSERT_TRUE(__fwriting(fp)); // Writing after write.
+    fclose(fp);
+  }
 }
 
 TEST(stdio_ext, __fsetlocking) {