truncate, ftruncate - truncate a file to a specified length


   #include <unistd.h>
   #include <sys/types.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)):

       _XOPEN_SOURCE >= 500
           || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
           || /* Glibc versions <= 2.19: */ _BSD_SOURCE

       _XOPEN_SOURCE >= 500
           || /* Since glibc 2.3.5: */ _POSIX_C_SOURCE >= 200112L
           || /* Glibc versions <= 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 stat(2)) 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 appropriately.


   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

   EFAULT The argument path points outside the process's allocated address

   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

   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

          A component of a pathname exceeded 255 characters, or an  entire
          pathname exceeded 1023 characters.

   ENOENT The named file does not exist.

          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.

          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.

          fd is not open for writing.

   EINVAL fd does not reference a regular file.

          The file descriptor fd is not open for writing.  POSIX  permits,
          and  portable  applications should handle, either error for this
          case.  (Linux produces EINVAL.)


   POSIX.1-2001, POSIX.1-2008, 4.4BSD, SVr4 (these calls first appeared in


   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.

   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.

   On some 32-bit architectures, the calling signature  for  these  system
   calls differ, for the reasons described in syscall(2).


   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


   truncate(1), open(2), stat(2), path_resolution(7)


   This page is part of release 4.09 of the Linux  man-pages  project.   A
   description  of  the project, information about reporting bugs, and the
   latest    version    of    this    page,    can     be     found     at


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