truncate(2)

1TRUNCATE(2)                Linux Programmer's Manual               TRUNCATE(2)
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NAME

6       truncate, ftruncate - truncate a file to a specified length
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SYNOPSIS

9       #include <unistd.h>
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11       int truncate(const char *path, off_t length);
12       int ftruncate(int fd, off_t length);
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14   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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16       truncate():
17           _XOPEN_SOURCE >= 500
18               || /* Since glibc 2.12: */ _POSIX_C_SOURCE >= 200809L
19               || /* Glibc <= 2.19: */ _BSD_SOURCE
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21       ftruncate():
22           _XOPEN_SOURCE >= 500
23               || /* Since glibc 2.3.5: */ _POSIX_C_SOURCE >= 200112L
24               || /* Glibc <= 2.19: */ _BSD_SOURCE
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DESCRIPTION

27       The  truncate()  and ftruncate() functions cause the regular file named
28       by path or referenced by fd to be truncated  to  a  size  of  precisely
29       length bytes.
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31       If  the  file  previously  was larger than this size, the extra data is
32       lost.  If the file previously was shorter, it is extended, and the  ex‐
33       tended part reads as null bytes ('\0').
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35       The file offset is not changed.
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37       If  the  size  changed,  then the st_ctime and st_mtime fields (respec‐
38       tively, time of last status change and time of last  modification;  see
39       inode(7))  for the file are updated, and the set-user-ID and set-group-
40       ID mode bits may be cleared.
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42       With ftruncate(), the file must be open for writing;  with  truncate(),
43       the file must be writable.
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RETURN VALUE

46       On  success,  zero is returned.  On error, -1 is returned, and errno is
47       set to indicate the error.
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ERRORS

50       For truncate():
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52       EACCES Search permission is denied for a component of the path  prefix,
53              or  the  named  file  is  not  writable  by the user.  (See also
54              path_resolution(7).)
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56       EFAULT The argument path points outside the process's allocated address
57              space.
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59       EFBIG  The argument length is larger than the maximum file size. (XSI)
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61       EINTR  While blocked waiting to complete, the call was interrupted by a
62              signal handler; see fcntl(2) and signal(7).
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64       EINVAL The argument length is negative or larger than the maximum  file
65              size.
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67       EIO    An I/O error occurred updating the inode.
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69       EISDIR The named file is a directory.
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71       ELOOP  Too  many  symbolic  links  were  encountered in translating the
72              pathname.
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74       ENAMETOOLONG
75              A component of a pathname exceeded 255 characters, or an  entire
76              pathname exceeded 1023 characters.
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78       ENOENT The named file does not exist.
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80       ENOTDIR
81              A component of the path prefix is not a directory.
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83       EPERM  The  underlying filesystem does not support extending a file be‐
84              yond its current size.
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86       EPERM  The operation was prevented by a file seal; see fcntl(2).
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88       EROFS  The named file resides on a read-only filesystem.
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90       ETXTBSY
91              The file is an executable file that is being executed.
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93       For ftruncate() the same errors apply, but instead of things  that  can
94       be  wrong with path, we now have things that can be wrong with the file
95       descriptor, fd:
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97       EBADF  fd is not a valid file descriptor.
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99       EBADF or EINVAL
100              fd is not open for writing.
101
102       EINVAL fd does not reference a regular file or a  POSIX  shared  memory
103              object.
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105       EINVAL or EBADF
106              The  file descriptor fd is not open for writing.  POSIX permits,
107              and portable applications should handle, either error  for  this
108              case.  (Linux produces EINVAL.)
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CONFORMING TO

111       POSIX.1-2001, POSIX.1-2008, 4.4BSD, SVr4 (these calls first appeared in
112       4.2BSD).
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NOTES

115       ftruncate() can also be used to set the size of a POSIX  shared  memory
116       object; see shm_open(3).
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118       The details in DESCRIPTION are for XSI-compliant systems.  For non-XSI-
119       compliant systems, the POSIX standard allows two behaviors  for  ftrun‐
120       cate() when length exceeds the file length (note that truncate() is not
121       specified at all in such an environment): either returning an error, or
122       extending  the file.  Like most UNIX implementations, Linux follows the
123       XSI requirement when dealing with native  filesystems.   However,  some
124       nonnative  filesystems  do  not permit truncate() and ftruncate() to be
125       used to extend a file beyond its current length: a notable  example  on
126       Linux is VFAT.
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128       The original Linux truncate() and ftruncate() system calls were not de‐
129       signed to handle large file offsets.   Consequently,  Linux  2.4  added
130       truncate64()  and  ftruncate64()  system calls that handle large files.
131       However, these details can be  ignored  by  applications  using  glibc,
132       whose  wrapper  functions  transparently  employ the more recent system
133       calls where they are available.
134
135       On some 32-bit architectures, the calling signature  for  these  system
136       calls differ, for the reasons described in syscall(2).
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BUGS

139       A  header  file  bug  in  glibc  2.12  meant  that the minimum value of
140       _POSIX_C_SOURCE required to expose the declaration of  ftruncate()  was
141       200809L  instead  of  200112L.  This has been fixed in later glibc ver‐
142       sions.
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COLOPHON

148       This page is part of release 5.12 of the Linux  man-pages  project.   A
149       description  of  the project, information about reporting bugs, and the
150       latest    version    of    this    page,    can     be     found     at
151       https://www.kernel.org/doc/man-pages/.
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155Linux                             2021-03-22                       TRUNCATE(2)