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     truncate, ftruncate — truncate or extend a file to a specified length


     Standard C Library (libc, -lc)


     #include <unistd.h>

     truncate(const char *path, off_t length);

     ftruncate(int fd, off_t length);


     The truncate() system call causes the file named by path or referenced by
     fd to be truncated or extended to length bytes in size.  If the file was
     larger than this size, the extra data is lost.  If the file was smaller
     than this size, it will be extended as if by writing bytes with the value
     zero.  With ftruncate(), the file must be open for writing.


     Upon successful completion, the value 0 is returned; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.  If the file to be modified is not a directory or a regular file,
     the truncate() call has no effect and returns the value 0.


     The truncate() system call succeeds unless:

     [ENOTDIR]          A component of the path prefix is not a directory.

     [ENAMETOOLONG]     A component of a pathname exceeded 255 characters, or
                        an entire path name exceeded 1023 characters.

     [ENOENT]           The named file does not exist.

     [EACCES]           Search permission is denied for a component of the
                        path prefix.

     [EACCES]           The named file is not writable by the user.

     [ELOOP]            Too many symbolic links were encountered in
                        translating the pathname.

     [EPERM]            The named file has its immutable or append-only flag
                        set, see the chflags(2) manual page for more

     [EISDIR]           The named file is 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.

     [EFBIG]            The length argument was greater than the maximum file

     [EINVAL]           The length argument was less than 0.

     [EIO]              An I/O error occurred updating the inode.

     [EFAULT]           The path argument points outside the process's
                        allocated address space.

     The ftruncate() system call succeeds unless:

     [EBADF]            The fd argument is not a valid descriptor.

     [EINVAL]           The fd argument references a socket, not a file.

     [EINVAL]           The fd descriptor is not open for writing.


     chflags(2), open(2)


     The truncate() system call appeared in 4.2BSD.


     These calls should be generalized to allow ranges of bytes in a file to
     be discarded.

     Use of truncate() to extend a file is not portable.