linux shm open,shm_open()和ftruncate()的目的是什么？

O_CREAT

Create the shared memory object if it does not exist. The user and group ownership of the object are taken from the corresponding effective IDs of the calling process,and the object’s permission bits are set according to the low-order 9 bits of mode,except that those bits set in the process file mode creation mask (see umask(2)) are cleared for the new object. A set of macro constants which can be used to define mode is listed in open(2). (Symbolic definitions of these constants can be obtained by including .)

A new shared memory object initially has zero length–the size of the object can be set using ftruncate(2). The newly allocated bytes of a shared memory object are automatically initialized to 0.