tune the buffer size for reading file

KxSMBProvider.m

@@ -2158,31 +2158,38 @@ - (id)readDataOfLength:(NSUInteger)length
         NSError *error = [self openFile];
         if (error) return error;        
     }
-
-    Byte buffer[32768];
+
+    // it seems 512 kb is an optimal buffer size
+    const size_t bufferSize = MIN(length, 512*1024);
+    Byte *buffer = malloc(bufferSize);
+    if (!buffer) {
+        return mkKxSMBError(KxSMBErrorOutOfMemory, nil);
+    }
 
     smbc_read_fn readFn = smbc_getFunctionRead(_context);
     NSMutableData *md = [NSMutableData data];
     NSInteger bytesToRead = length;
 
     while (bytesToRead > 0) {
 
-        ssize_t r = readFn(_context, _file, buffer, MIN(bytesToRead, sizeof(buffer)));
+        ssize_t r = readFn(_context, _file, buffer, MIN(bytesToRead, bufferSize));
 
         if (r == 0)
             break;
 
         if (r < 0) {
 
             const int err = errno;
+            free(buffer);
             return mkKxSMBError(errnoToSMBErr(err),
                                 NSLocalizedString(@"Unable read file:%@ (errno:%d)", nil), _path, err);
         }
 
         [md appendBytes:buffer length:r];
         bytesToRead -= r;
     }
-
+
+    free(buffer);
     return md;
 }
 

KxSMBSample/FileViewController.m

@@ -301,7 +301,7 @@ -(void) updateDownloadStatus: (id) result
 - (void) download
 {    
     __weak __typeof(self) weakSelf = self;
-    [_smbFile readDataOfLength:32768
+    [_smbFile readDataOfLength:1024*1024
                          block:^(id result)
     {
         FileViewController *p = weakSelf;