path: root/src
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authorRich Felker <>2014-02-07 00:57:50 -0500
committerRich Felker <>2014-02-07 00:57:50 -0500
commit3af2edee150484940916eba1984f78c3b965dd05 (patch)
tree74ef92b1862aa0eb7b33d081684f1d1fad43d0ba /src
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')
3 files changed, 5 insertions, 1 deletions
diff --git a/src/internal/stdio_impl.h b/src/internal/stdio_impl.h
index 2083b2fe..79be9fdb 100644
--- a/src/internal/stdio_impl.h
+++ b/src/internal/stdio_impl.h
@@ -17,6 +17,7 @@
#define F_EOF 16
#define F_ERR 32
#define F_SVB 64
+#define F_APP 128
struct _IO_FILE {
unsigned flags;
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;
diff --git a/src/stdio/ftell.c b/src/stdio/ftell.c
index 82371e37..bb62897a 100644
--- a/src/stdio/ftell.c
+++ b/src/stdio/ftell.c
@@ -4,7 +4,9 @@
off_t __ftello_unlocked(FILE *f)
- off_t pos = f->seek(f, 0, SEEK_CUR);
+ off_t pos = f->seek(f, 0,
+ (f->flags & F_APP) && f->wpos > f->wbase
if (pos < 0) return pos;
/* Adjust for data in buffer. */