Sistemi operativi - Syscall shmat

shmat - shared memory attach operation

The shmat() function attaches the shared memory segment associated with the shared memory identifier specified by shmid to the address space of the calling process. The segment is attached at the address specified by one of the following criteria:

• If shmaddr is a null pointer, the segment is attached at the first available address as selected by the system.
• If shmaddr is not a null pointer and (shmflg & SHM_RND) is non-zero, the segment is attached at the address given by (shmaddr-((uintptr_t)shmaddr%SHMLBA)). The character % is the C-language remainder operator.
• If shmaddr is not a null pointer and (shmflg & SHM_RND) is 0, the segment is attached at the address given by shmaddr.

The segment is attached for

reading if (shmflg&SHM_RDONLY) is non-zero and the calling process has read permission; otherwise, if it is 0 and the calling process has read and write permission, the segment is attached for reading and writing.

RETURN VALUE

Upon successful completion, shmat() increments the value of shm_nattch in the data structure associated with the shared memory ID of the attached shared memory segment and returns the segment's start address.

Otherwise, the shared memory segment is not attached, shmat() returns -1 and errno is set to indicate the error.

ERRORS

The shmat() function will fail if:

• [EACCES] Operation permission is denied to the calling process, see IPC.
• [EINVAL] The value of shmid is not a valid shared memory identifier; the shmaddr is not a null pointer and the value of (shmaddr-((ptrdiff_t)shmaddr%SHMLBA)) is an illegal address for attaching shared memory; or the shmaddr is not a null pointer.