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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)):


|| /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
|| /* glibc <= 2.19: */ _BSD_SOURCE


|| /* 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():

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).)
The argument path points outside the process's allocated address space.
The argument length is larger than the maximum file size. (XSI)
While blocked waiting to complete, the call was interrupted by a signal handler; see fcntl(2) and signal(7).
The argument length is negative or larger than the maximum file size.
An I/O error occurred updating the inode.
The named file is a directory.
Too many symbolic links were encountered in translating the pathname.
A component of a pathname exceeded 255 characters, or an entire pathname exceeded 1023 characters.
The named file does not exist.
A component of the path prefix is not a directory.
The underlying filesystem does not support extending a file beyond its current size.
The operation was prevented by a file seal; see fcntl(2).
The named file resides on a read-only filesystem.
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:

fd is not a valid file descriptor.
fd is not open for writing.
fd does not reference a regular file or a POSIX shared memory object.
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.1-2001, 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)

2024-06-15 Linux man-pages (unreleased)