int ftruncate(int fildes, off_t length);
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 either increase the size of the file or fail. XSI-conformant systems 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, the ftruncate() function shall mark for update the st_ctime and st_mtime fields 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 shared memory object or memory mapped file and whole pages beyond the new end were previously mapped, then the whole pages beyond the new end shall be discarded.
If the Memory Protection option is supported, references to discarded pages shall result in the generation of a SIGBUS signal; otherwise, the result of such references is undefined.
If the effect of ftruncate() is to increase the size of a shared 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.
Upon successful completion, ftruncate() shall return 0; otherwise, -1 shall be returned and errno set to indicate the error.
The ftruncate() function shall fail if:
The length argument was greater than the maximum file size.
The fildes argument is not a file descriptor open for writing.
The following sections are informative.
The ftruncate() function is part of IEEE Std 1003.1-2001 as it was deemed to be more useful than truncate(). The truncate() function is provided as an XSI extension.
open() , truncate() , the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>