diff options
Diffstat (limited to 'libio/wfileops.c')
| -rw-r--r-- | libio/wfileops.c | 25 |
1 files changed, 24 insertions, 1 deletions
diff --git a/libio/wfileops.c b/libio/wfileops.c index 7f4c92399f..776bb4a4ba 100644 --- a/libio/wfileops.c +++ b/libio/wfileops.c @@ -602,6 +602,7 @@ static _IO_off64_t do_ftell_wide (_IO_FILE *fp) { _IO_off64_t result, offset = 0; + bool use_cached_offset = false; /* No point looking for offsets in the buffer if it hasn't even been allocated. */ @@ -702,9 +703,31 @@ do_ftell_wide (_IO_FILE *fp) is fp._offset - (_IO_read_end - new_write_ptr). */ offset -= fp->_IO_read_end - fp->_IO_write_ptr; } + + /* It is safe to use the cached offset when available if there is + unbuffered data (indicating that the file handle is active) and + the handle is not for a file open in a+ mode. The latter + condition is because there could be a scenario where there is a + switch from read mode to write mode using an fseek to an arbitrary + position. In this case, there would be unbuffered data due to be + appended to the end of the file, but the offset may not + necessarily be the end of the file. It is fine to use the cached + offset when the a+ stream is in read mode though, since the offset + is maintained correctly in that case. Note that this is not a + comprehensive set of cases when the offset is reliable. The + offset may be reliable even in some cases where there is no + unflushed input and the handle is active, but it's just that we + don't have a way to identify that condition reliably. */ + use_cached_offset = (offset != 0 && fp->_offset != _IO_pos_BAD + && ((fp->_flags & (_IO_IS_APPENDING | _IO_NO_READS)) + == (_IO_IS_APPENDING | _IO_NO_READS) + && was_writing)); } - result = get_file_offset (fp); + if (use_cached_offset) + result = fp->_offset; + else + result = get_file_offset (fp); if (result == EOF) return result; |