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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | STANDARDS | HISTORY | NOTES | BUGS | SEE ALSO | COLOPHON |
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truncate(2) System Calls Manual truncate(2)
truncate, ftruncate - truncate a file to a specified length
Standard C library (libc, -lc)
#include <unistd.h>
int truncate(const char *path, off_t length);
int ftruncate(int fd, off_t length);
Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):
truncate():
_XOPEN_SOURCE >= 500
|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
|| /* glibc <= 2.19: */ _BSD_SOURCE
ftruncate():
_XOPEN_SOURCE >= 500
|| /* Since glibc 2.3.5: */ _POSIX_C_SOURCE >= 200112L
|| /* glibc <= 2.19: */ _BSD_SOURCE
The truncate() and ftruncate() functions cause the regular file
named by path or referenced by fd to be truncated to a size of
precisely length bytes.
If the file previously was larger than this size, the extra data
is lost. If the file previously was shorter, it is extended, and
the extended part reads as null bytes ('\0').
The file offset is not changed.
If the size changed, then the st_ctime and st_mtime fields
(respectively, time of last status change and time of last
modification; see inode(7)) for the file are updated, and the set-
user-ID and set-group-ID mode bits may be cleared.
With ftruncate(), the file must be open for writing; with
truncate(), the file must be writable.
On success, zero is returned. On error, -1 is returned, and errno
is set to indicate the error.
For truncate():
EACCES Search permission is denied for a component of the path
prefix, or the named file is not writable by the user.
(See also path_resolution(7).)
EFAULT The argument path points outside the process's allocated
address space.
EFBIG The argument length is larger than the maximum file size.
(XSI)
EINTR While blocked waiting to complete, the call was interrupted
by a signal handler; see fcntl(2) and signal(7).
EINVAL The argument length is negative or larger than the maximum
file size.
EIO An I/O error occurred updating the inode.
EISDIR The named file is a directory.
ELOOP Too many symbolic links were encountered in translating the
pathname.
ENAMETOOLONG
A component of a pathname exceeded 255 characters, or an
entire pathname exceeded 1023 characters.
ENOENT The named file does not exist.
ENOTDIR
A component of the path prefix is not a directory.
EPERM The underlying filesystem does not support extending a file
beyond its current size.
EPERM The operation was prevented by a file seal; see fcntl(2).
EROFS The named file resides on a read-only filesystem.
ETXTBSY
The file is an executable file that is being executed.
For ftruncate() the same errors apply, but instead of things that
can be wrong with path, we now have things that can be wrong with
the file descriptor, fd:
EBADF fd is not a valid file descriptor.
EBADF or EINVAL
fd is not open for writing.
EINVAL fd does not reference a regular file or a POSIX shared
memory object.
EINVAL or EBADF
The file descriptor fd is not open for writing. POSIX
permits, and portable applications should handle, either
error for this case. (Linux produces EINVAL.)
The details in DESCRIPTION are for XSI-compliant systems. For
non-XSI-compliant systems, the POSIX standard allows two behaviors
for ftruncate() when length exceeds the file length (note that
truncate() is not specified at all in such an environment): either
returning an error, or extending the file. Like most UNIX
implementations, Linux follows the XSI requirement when dealing
with native filesystems. However, some nonnative filesystems do
not permit truncate() and ftruncate() to be used to extend a file
beyond its current length: a notable example on Linux is VFAT.
On some 32-bit architectures, the calling signature for these
system calls differ, for the reasons described in syscall(2).
POSIX.1-2008.
POSIX.1-2001, 4.4BSD, SVr4 (first appeared in 4.2BSD).
The original Linux truncate() and ftruncate() system calls were
not designed to handle large file offsets. Consequently, Linux
2.4 added truncate64() and ftruncate64() system calls that handle
large files. However, these details can be ignored by
applications using glibc, whose wrapper functions transparently
employ the more recent system calls where they are available.
ftruncate() can also be used to set the size of a POSIX shared
memory object; see shm_open(3).
A header file bug in glibc 2.12 meant that the minimum value of
_POSIX_C_SOURCE required to expose the declaration of ftruncate()
was 200809L instead of 200112L. This has been fixed in later
glibc versions.
truncate(1), open(2), stat(2), path_resolution(7)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-17 truncate(2)
Pages that refer to this page: truncate(1), fallocate(2), F_GETLEASE(2const), F_GET_SEALS(2const), F_NOTIFY(2const), fsync(2), getrlimit(2), io_uring_enter2(2), io_uring_enter(2), memfd_create(2), memfd_secret(2), mmap(2), syscall(2), syscalls(2), io_uring_prep_ftruncate(3), off_t(3type), shm_open(3), inode(7), inotify(7), landlock(7), shm_overview(7), signal-safety(7), xfs_io(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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