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# read

[[glibc]]'s `read` is in `glibc/sysdeps/mach/hurd/read.c:__libc_read`.

A buffer (and its size) to store the to-be-read data in is supplied by the
caller of `read`.

> `__libc_read` calls `glibc/hurd/fd-read.c:_hurd_fd_read`.

>> `_hurd_fd_read` calls `__io_read`, which is an [[RPC]]:
>> `hurd/hurd/io.defs:io_read`.

>>> Enter user-side RPC stub `glibc.obj/hurd/RPC_io_read.c:__io_read`.  Process
>>> stuff, switch to kernel, etc.

(For example) [[translator/hello]] server, [[libtrivfs]]-based.  Enter
server-side RPC stub `hurd.obj/libtrivfs/ioServer.c:_Xio_read`.  Process stuff,
call `hurd/trans/hello.c:trivfs_S_io_read`.

A 2048 byte buffer is provided.

> `trivfs_S_io_read`.  Depending on the internatl state, either a new memory
> region is set-up (and returned as out-of-line data), or the desired amount of
> data is returned in-line.

Back in `_Xio_read`.

If the 2048 byte buffer is not decided to be used (out-of-line case or bigger
than 2048 bytes case; server decides to instead provide a new memory region),
the [[`dealloc`|microkernel/mach/mig/dealloc]] flag is being set, which causes
Mach to unmap that memory region from the server's address space, i.e., doing a
memory *move* from the server to the client.

Leave server-side RPC stub `_Xio_read`.

>>> Return from kernel, continue client-side RPC stub `io_read`.  Have to copy
>>> data.  Three cases: out-of-line data (pass pointer to memory area);
>>> returned more data than fits into the originally supplied buffer (allocate
>>> new buffer, copy all data into it, pass pointer of new buffer); otherwise
>>> copy as much data as is available into the originally supplied buffer.
>>> I.e., in all cases *all* data which was provided by the server is made
>>> available to the caller.

>> Back in `_hurd_fd_read`.  If a new buffer has been allocated previously, or
>> the out-of-line mechanism has been used, the returned data now has to be
>> copied into the originally supplied buffer.  If the server returned more
>> data than requested, this is a [[protocol_violation|EGRATUITOUS]].

> Back in `__libc_read`.


---

Samuel:

(For example) [[translator/ext2fs]] server, enter server-side RPC stub
`hurd.obj/libdiskfs/ioServer.c:_Xio_read`.  Process stuff, call
`hurd/libdiskfs/io-read.c:diskfs_S_io_read`.

A 2048 byte buffer is provided.

> `diskfs_S_io_read` calls `_diskfs_rdwr_internal`.

>> That calls `hurd/libpager/pager-memcpy.c:pager_memcpy`, which usually
>> basically just tells the kernel to virtually project the memory object
>> corresponding to the file in the caller process's memory.  No read is
>> actually done.

  * Then, when the process actually reads the data, the kernel gets the user
    page fault (`gnumach/i386/i386/trap.c:user_trap`), which calls `vm_fault`,
    etc., until actually getting to `gnumach/vm/vm_fault/vm_fault_page` which
    eventually calls `memory_object_data_request`, which is an [[RPC]], i.e.,
    that actually results into the [[translator/ext2fs]] server calling
    `hurd/libpager/data-request.c:_pager_seqnos_memory_object_data_request`.

  * That calls `hurd/ext2fs/pager.c:pager_read_page`, which looks for where the
    data is on the disk, and eventually calls
    `hurd/libstore/rdwr.c:store_read`, which eventually calls `device_read`,
    which is an [[RPC]], i.e., that actually gets into the kernel calling
    `gnumach/linux/dev/glue/block.c:device_read`.