path: root/src/stdio/__fdopen.c
diff options
authorRich Felker <>2014-02-07 00:57:50 -0500
committerRich Felker <>2014-02-07 00:57:50 -0500
commit3af2edee150484940916eba1984f78c3b965dd05 (patch)
tree74ef92b1862aa0eb7b33d081684f1d1fad43d0ba /src/stdio/__fdopen.c
parent89511cd943d807b7fe31b61c06d4fe09888b8e9a (diff)
fix ftello result for append streams with unflushed output
when there is unflushed output, ftello (and ftell) compute the logical stream position as the underlying file descriptor's offset plus an adjustment for the amount of buffered data. however, this can give the wrong result for append-mode streams where the unflushed writes should adjust the logical position to be at the end of the file, as if a seek to end-of-file takes place before the write. the solution turns out to be a simple trick: when ftello (indirectly) calls lseek to determine the current file offset, use SEEK_END instead of SEEK_CUR if the stream is append-mode and there's unwritten buffered data. the ISO C rules regarding switching between reading and writing for a stream opened in an update mode, along with the POSIX rules regarding switching "active handles", conveniently leave undefined the hypothetical usage cases where this fix might lead to observably incorrect offsets. the bug being fixed was discovered via the test case for glibc issue
Diffstat (limited to 'src/stdio/__fdopen.c')
1 files changed, 1 insertions, 0 deletions
diff --git a/src/stdio/__fdopen.c b/src/stdio/__fdopen.c
index 59690f6d..a2ca62b1 100644
--- a/src/stdio/__fdopen.c
+++ b/src/stdio/__fdopen.c
@@ -33,6 +33,7 @@ FILE *__fdopen(int fd, const char *mode)
if (*mode == 'a') {
int flags = __syscall(SYS_fcntl, fd, F_GETFL);
__syscall(SYS_fcntl, fd, F_SETFL, flags | O_APPEND);
+ f->flags |= F_APP;
f->fd = fd;