ftruncate(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>
10       #include <sys/types.h>
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12       int truncate(const char *path, off_t length);
13       int ftruncate(int fd, off_t length);
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DESCRIPTION

16       The  truncate()  and ftruncate() functions cause the regular file named
17       by path or referenced by fd to be truncated  to  a  size  of  precisely
18       length bytes.
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20       If  the  file  previously  was larger than this size, the extra data is
21       lost.  If the file previously was shorter,  it  is  extended,  and  the
22       extended part reads as null bytes ('\0').
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24       The file offset is not changed.
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26       If  the  size  changed,  then the st_ctime and st_mtime fields (respec‐
27       tively, time of last status change and time of last  modification;  see
28       stat(2)) for the file are updated, and the set-user-ID and set-group-ID
29       permission bits may be cleared.
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31       With ftruncate(), the file must be open for writing;  with  truncate(),
32       the file must be writable.
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RETURN VALUE

35       On  success,  zero is returned.  On error, -1 is returned, and errno is
36       set appropriately.
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ERRORS

39       For truncate():
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41       EACCES Search permission is denied for a component of the path  prefix,
42              or  the  named  file  is  not  writable  by the user.  (See also
43              path_resolution(2).)
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45       EFAULT Path points outside the process's allocated address space.
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47       EFBIG  The argument length is larger than the maximum file size. (XSI)
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49       EINTR  A signal was caught during execution.
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51       EINVAL The argument length is negative or larger than the maximum  file
52              size.
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54       EIO    An I/O error occurred updating the inode.
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56       EISDIR The named file is a directory.
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58       ELOOP  Too  many  symbolic  links  were  encountered in translating the
59              pathname.
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61       ENAMETOOLONG
62              A component of a pathname exceeded 255 characters, or an  entire
63              pathname exceeded 1023 characters.
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65       ENOENT The named file does not exist.
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67       ENOTDIR
68              A component of the path prefix is not a directory.
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70       EPERM  The  underlying  file  system  does not support extending a file
71              beyond its current size.
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73       EROFS  The named file resides on a read-only file system.
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75       ETXTBSY
76              The file is a pure procedure (shared text) file  that  is  being
77              executed.
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79       For  ftruncate()  the same errors apply, but instead of things that can
80       be wrong with path, we now have things that can be wrong with fd:
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82       EBADF  The fd is not a valid descriptor.
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84       EBADF or EINVAL
85              The fd is not open for writing.
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87       EINVAL The fd does not reference a regular file.
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CONFORMING TO

90       4.4BSD, SVr4, POSIX.1-2001 (these calls first appeared in 4.2BSD).
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NOTES

93       The above description is for XSI-compliant systems.   For  non-XSI-com‐
94       pliant  systems,  the  POSIX  standard allows two behaviours for ftrun‐
95       cate() when length exceeds the file length (note that truncate() is not
96       specified at all in such an environment): either returning an error, or
97       extending the file.  Like most Unix implementations, Linux follows  the
98       XSI  requirement  when dealing with native file systems.  However, some
99       non-native file systems do not permit truncate() and ftruncate() to  be
100       used  to  extend a file beyond its current length: a notable example on
101       Linux is VFAT.
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