#include <unistd.h> int ftruncate(int fildes, off_t length);
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.
The Base Definitions volume of POSIX.1-2008, <unistd.h>
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