обрезать файл до указанной длины (truncate a file to a specified length)
Пролог (Prolog)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
Имя (Name)
ftruncate — truncate a file to a specified length
Синопсис (Synopsis)
#include <unistd.h>
int ftruncate(int fildes, off_t length);
Описание (Description)
If fildes is not a valid file descriptor open for writing, the
ftruncate() function shall fail.
If fildes refers to a regular file, the ftruncate() function
shall cause the size of the file to be truncated to length. If
the size of the file previously exceeded length, the extra data
shall no longer be available to reads on the file. If the file
previously was smaller than this size, ftruncate() shall increase
the size of the file. If the file size is increased, the extended
area shall appear as if it were zero-filled. The value of the
seek pointer shall not be modified by a call to ftruncate().
Upon successful completion, if fildes refers to a regular file,
ftruncate() shall mark for update the last data modification and
last file status change timestamps of the file and the S_ISUID
and S_ISGID bits of the file mode may be cleared. If the
ftruncate() function is unsuccessful, the file is unaffected.
If the request would cause the file size to exceed the soft file
size limit for the process, the request shall fail and the
implementation shall generate the SIGXFSZ signal for the thread.
If fildes refers to a directory, ftruncate() shall fail.
If fildes refers to any other file type, except a shared memory
object, the result is unspecified.
If fildes refers to a shared memory object, ftruncate() shall set
the size of the shared memory object to length.
If the effect of ftruncate() is to decrease the size of a memory
mapped file or a shared memory object and whole pages beyond the
new end were previously mapped, then the whole pages beyond the
new end shall be discarded.
References to discarded pages shall result in the generation of a
SIGBUS signal.
If the effect of ftruncate() is to increase the size of a memory
object, it is unspecified whether the contents of any mapped
pages between the old end-of-file and the new are flushed to the
underlying object.
Возвращаемое значение (Return value)
Upon successful completion, ftruncate() shall return 0;
otherwise, -1 shall be returned and errno set to indicate the
error.
Ошибки (Error)
The ftruncate() function shall fail if:
EINTR A signal was caught during execution.
EINVAL The length argument was less than 0.
EFBIG or EINVAL
The length argument was greater than the maximum file
size.
EFBIG The file is a regular file and length is greater than the
offset maximum established in the open file description
associated with fildes.
EIO An I/O error occurred while reading from or writing to a
file system.
EBADF or EINVAL
The fildes argument is not a file descriptor open for
writing.
The following sections are informative.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
None.
Обоснование (Rationale)
None.
Будущие направления (Future directions)
None.
Смотри также (See also)
open(3p), truncate(3p)
The Base Definitions volume of POSIX.1‐2017, unistd.h(0p)
