Expose the portable "strerror_r" used by jniThrowIOException.

Also add one example caller I used to test this, and update the
libnativehelper README file.

3 files changed, 29 insertions, 22 deletions

diff --git a/libnativehelper/JNIHelp.c b/libnativehelper/JNIHelp.c
index aacecb68b..fa6b53263 100644
--- a/libnativehelper/JNIHelp.c
+++ b/libnativehelper/JNIHelp.c
@@ -59,30 +59,30 @@ int jniThrowException(JNIEnv* env, const char* className, const char* msg)
  */
 int jniThrowIOException(JNIEnv* env, int errnum)
 {
-    // note: glibc has a nonstandard
-    // strerror_r that looks like this:
-    // char *strerror_r(int errnum, char *buf, size_t n);
-
-    const char* message;
     char buffer[80];
-    char* ret;
-
-    buffer[0] = 0;
-    ret = (char*) strerror_r(errnum, buffer, sizeof(buffer));
+    const char* message = jniStrError(errnum, buffer, sizeof(buffer));
+    return jniThrowException(env, "java/io/IOException", message);
+}
 
+const char* jniStrError(int errnum, char* buf, size_t buflen)
+{
+    // note: glibc has a nonstandard strerror_r that returns char* rather
+    // than POSIX's int.
+    // char *strerror_r(int errnum, char *buf, size_t n);
+    char* ret = (char*) strerror_r(errnum, buf, buflen);
     if (((int)ret) == 0) {
         //POSIX strerror_r, success
-        message = buffer;
+        return buf;
     } else if (((int)ret) == -1) {
         //POSIX strerror_r, failure
-
-        snprintf (buffer, sizeof(buffer), "errno %d", errnum);
-        message = buffer;
+        // (Strictly, POSIX only guarantees a value other than 0. The safest
+        // way to implement this function is to use C++ and overload on the
+        // type of strerror_r to accurately distinguish GNU from POSIX. But
+        // realistic implementations will always return -1.)
+        snprintf(buf, buflen, "errno %d", errnum);
+        return buf;
     } else {
         //glibc strerror_r returning a string
-        message = ret;
+        return ret;
     }
-
-    return jniThrowException(env, "java/io/IOException", message);
 }
-
diff --git a/libnativehelper/README b/libnativehelper/README
index 56ec17839..fa757c073 100644
--- a/libnativehelper/README
+++ b/libnativehelper/README
@@ -4,10 +4,9 @@ Support functions for Android's class libraries
 These are VM-agnostic native functions that implement methods for system
 class libraries.  All code here:
 
- - MUST be associated with a standard java.* class (no Android-specific stuff)
- - MUST use JNI
- - SHOULD be written in C rather than C++ (it's usually smaller and all of
-   our VMs are written in C)
+ - MUST not be associated with an android.* class (that code lives in
+   frameworks/base/).
+ - SHOULD be written in C rather than C++ where possible.
 
-Some helper functions are defined in ../include/nativehelper/JNIHelp.h.
+Some helper functions are defined in include/nativehelper/JNIHelp.h.
 
diff --git a/libnativehelper/include/nativehelper/JNIHelp.h b/libnativehelper/include/nativehelper/JNIHelp.h
index 3982797c8..1e268f845 100644
--- a/libnativehelper/include/nativehelper/JNIHelp.h
+++ b/libnativehelper/include/nativehelper/JNIHelp.h
@@ -58,6 +58,14 @@ int jniThrowException(C_JNIEnv* env, const char* className, const char* msg);
 int jniThrowIOException(C_JNIEnv* env, int errnum);
 
 /*
+ * Return a pointer to a locale-dependent error string explaining errno
+ * value 'errnum'. The returned pointer may or may not be equal to 'buf'.
+ * This function is thread-safe (unlike strerror) and portable (unlike
+ * strerror_r).
+ */
+const char* jniStrError(int errnum, char* buf, size_t buflen);
+
+/*
  * Create a java.io.FileDescriptor given an integer fd
  */
 jobject jniCreateFileDescriptor(C_JNIEnv* env, int fd);