in C
and with shell tool
delete the part at the end of a file, use "truncate" or "ftruncate"
(not sure if it is support by C99)
NAME
truncate, ftruncate - truncate a file to a specified length
SYNOPSIS
#include <unistd.h>
#include <sys/types.h>
int truncate(const char *path, off_t length);
int ftruncate(int fd, off_t length);
DESCRIPTION
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 zero bytes.
The file pointer is not changed.
If the size changed, then the ctime and mtime fields for the file are
updated, and suid and sgid mode bits may be cleared.
With ftruncate, the file must be open for writing; with truncate, the
file must be writable.
RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is
set appropriately.
ERRORS
For truncate:
EACCES Search permission is denied for a component of the path prefix,
or the named file is not writable by the user.
EFAULT Path points outside the process’s allocated address space.
EFBIG The argument length is larger than the maximum file size. (XSI)
EINTR A signal was caught during execution.
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
path name exceeded 1023 characters.
ENOENT The named file does not exist.
ENOTDIR
A component of the path prefix is not a directory.
EROFS The named file resides on a read-only file system.
ETXTBSY
The file is a pure procedure (shared text) 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 fd:
EBADF The fd is not a valid descriptor.
EBADF or EINVAL
The fd is not open for writing.
EINVAL The fd does not reference a regular file.
CONFORMING TO
4.4BSD, SVr4 (these function calls first appeared in BSD 4.2). POSIX
1003.1-1996 has ftruncate. POSIX 1003.1-2001 also has truncate, as an
XSI extension.
SVr4 documents additional truncate error conditions EMFILE, EMULTIHP,
ENFILE, ENOLINK. SVr4 documents for ftruncate an additional EAGAIN
error condition.
NOTES
The above description is for XSI-compliant systems. For non-XSI-com-
pliant systems, the POSIX standard allows two behaviours for ftruncate
when length exceeds the file length (note that truncate is not speci-
fied at all in such an environment): either returning an error, or
extending the file. (Most Unices follow the XSI requirement.)
SEE ALSO
open(2)
1998-12-21 TRUNCATE(2)
delete the beginning part, use temporary file
if delete operation works on line mode, shell command csplit may helpful.
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