AIX compilation fix for io classes (#3275)

Summary:
in AIX OS ,as fileno is defined as C macro,   we get the compilation error during preprocessing step.

In file included from /ranjit/Faiss/faiss/faiss/invlists/InvertedListsIOHook.h:10:
/ranjit/Faiss/faiss/faiss/impl/io.h:35:17: error: expected member name or ';' after declaration specifiers
   35 |     virtual int fileno();
      |     ~~~~~~~~~~~ ^
/usr/include/stdio.h:517:30: note: expanded from macro 'fileno'
  517 | #define fileno(__p)     ((__p)->_file)

Pull Request resolved: #3275

Reviewed By: algoriddle

Differential Revision: D54944388

Pulled By: mdouze

fbshipit-source-id: 40c4314de93547778ac274281245ff59e3a18b6d

faiss/impl/io.cpp

@@ -20,11 +20,11 @@ namespace faiss {
  * IO functions
  ***********************************************************************/
 
-int IOReader::fileno() {
+int IOReader::filedescriptor() {
     FAISS_THROW_MSG("IOReader does not support memory mapping");
 }
 
-int IOWriter::fileno() {
+int IOWriter::filedescriptor() {
     FAISS_THROW_MSG("IOWriter does not support memory mapping");
 }
 
@@ -85,8 +85,12 @@ size_t FileIOReader::operator()(void* ptr, size_t size, size_t nitems) {
     return fread(ptr, size, nitems, f);
 }
 
-int FileIOReader::fileno() {
+int FileIOReader::filedescriptor() {
+#ifdef _AIX
+    return fileno(f);
+#else
     return ::fileno(f);
+#endif
 }
 
 FileIOWriter::FileIOWriter(FILE* wf) : f(wf) {}
@@ -116,8 +120,12 @@ size_t FileIOWriter::operator()(const void* ptr, size_t size, size_t nitems) {
     return fwrite(ptr, size, nitems, f);
 }
 
-int FileIOWriter::fileno() {
+int FileIOWriter::filedescriptor() {
+#ifdef _AIX
+    return fileno(f);
+#else
     return ::fileno(f);
+#endif
 }
 
 /***********************************************************************

faiss/impl/io.h

@@ -32,7 +32,7 @@ struct IOReader {
     virtual size_t operator()(void* ptr, size_t size, size_t nitems) = 0;
 
     // return a file number that can be memory-mapped
-    virtual int fileno();
+    virtual int filedescriptor();
 
     virtual ~IOReader() {}
 };
@@ -45,7 +45,7 @@ struct IOWriter {
     virtual size_t operator()(const void* ptr, size_t size, size_t nitems) = 0;
 
     // return a file number that can be memory-mapped
-    virtual int fileno();
+    virtual int filedescriptor();
 
     virtual ~IOWriter() noexcept(false) {}
 };
@@ -73,7 +73,7 @@ struct FileIOReader : IOReader {
 
     size_t operator()(void* ptr, size_t size, size_t nitems) override;
 
-    int fileno() override;
+    int filedescriptor() override;
 };
 
 struct FileIOWriter : IOWriter {
@@ -88,7 +88,7 @@ struct FileIOWriter : IOWriter {
 
     size_t operator()(const void* ptr, size_t size, size_t nitems) override;
 
-    int fileno() override;
+    int filedescriptor() override;
 };
 
 /*******************************************************