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[pdclib] / functions / stdio / ftell.c

diff --git a/functions/stdio/ftell.c b/functions/stdio/ftell.c
index 09dd17fe189113739befd74e293b4229b57b8738..63c7f75e1d5de1ebf786a1d9e38e1c9b592a1492 100644 (file)
--- a/functions/stdio/ftell.c
+++ b/functions/stdio/ftell.c
@@ -13,8 +13,10 @@
 
 long int ftell( struct _PDCLIB_file_t * stream )
 {
-    /* FIXME: A bit too fuzzy in the head now. stream->ungetidx should be in here
-             somewhere.
+    /* If offset is too large for return type, report error instead of wrong
+       offset value.
+       FIXME: A bit too fuzzy in the head right now; stream->ungetidx should be
+       in here somewhere.
     */
     if ( stream->pos.offset > ( LONG_MAX - stream->bufidx ) )
     {
@@ -22,11 +24,22 @@ long int ftell( struct _PDCLIB_file_t * stream )
         _PDCLIB_errno = _PDCLIB_EINVAL;
         return -1;
     }
-    /* Position of start-of-buffer, plus:
-       - buffered, unwritten content (for output streams), or
-       - already-parsed content from buffer (for input streams)
+    /* ftell() must take into account:
+       - the actual *physical* offset of the file, i.e. the offset as recognized
+         by the operating system (and stored in stream->pos.offset); and
+       - any buffers held by PDCLib, which
+         - in case of unwritten buffers, count in *addition* to the offset; or
+         - in case of unprocessed pre-read buffers, count in *substraction* to
+           the offset. (Remember to count ungetidx into this number.)
+       Conveniently, the calculation ( ( bufend - bufidx ) + ungetidx ) results
+       in just the right number in both cases:
+         - in case of unwritten buffers, ( ( 0 - unwritten ) + 0 )
+           i.e. unwritten bytes as negative number
+         - in case of unprocessed pre-read, ( ( preread - processed ) + unget )
+           i.e. unprocessed bytes as positive number.
+       That is how the somewhat obscure return-value calculation works.
     */
-    return (long int)( stream->pos.offset + stream->bufidx - stream->ungetidx );
+    return (long int)( stream->pos.offset - ( ( stream->bufend - stream->bufidx ) + stream->ungetidx ) );
 }
 
 #endif
@@ -48,8 +61,7 @@ int main( void )
     */
     char * buffer = (char*)malloc( 4 );
     FILE * fh;
-    remove( testfile );
-    TESTCASE( ( fh = fopen( testfile, "w+" ) ) != NULL );
+    TESTCASE( ( fh = tmpfile() ) != NULL );
     TESTCASE( setvbuf( fh, buffer, _IOLBF, 4 ) == 0 );
     TESTCASE( fputc( '1', fh ) == '1' );
     TESTCASE( fputc( '2', fh ) == '2' );
@@ -75,9 +87,8 @@ int main( void )
     TESTCASE_NOREG( fh->bufidx == 0 );
     /* Reading back first character after rewind for basic read check */
     TESTCASE( fgetc( fh ) == '1' );
-    TESTCASE( fclose( fh ) == 0 );
     /* TODO: t.b.c. */
-    remove( testfile );
+    TESTCASE( fclose( fh ) == 0 );
     return TEST_RESULTS;
 }