1# zx_channel_read  - zx_channel_read_etc
2
3## NAME
4
5channel_read - read a message from a channel
6
7## SYNOPSIS
8
9```
10#include <zircon/syscalls.h>
11
12zx_status_t zx_channel_read(zx_handle_t handle, uint32_t options,
13                            void* bytes, zx_handle_t* handles,
14                            uint32_t num_bytes, uint32_t num_handles,
15                            uint32_t* actual_bytes, uint32_t* actual_handles);
16
17
18zx_status_t zx_channel_read_etc(zx_handle_t handle, uint32_t options,
19                                void* bytes, zx_handle_info_t* handles,
20                                uint32_t num_bytes, uint32_t num_handles,
21                                uint32_t* actual_bytes, uint32_t* actual_handles);
22```
23
24## DESCRIPTION
25
26**channel_read**() and **channel_read_etc**() attempts to read the first
27message from the channel specified by *handle* into the provided *bytes*
28and/or *handles* buffers.
29
30The parameters *num_bytes* and *num_handles* are used to specify the
31size of the respective read buffers.
32
33Channel messages may contain both byte data and handle payloads and may
34only be read in their entirety.  Partial reads are not possible.
35
36The *bytes* buffer is written before the *handles* buffer. In the event of
37overlap between these two buffers, the contents written to *handles*
38will overwrite the portion of *bytes* it overlaps.
39
40Both forms of read behave the same except that **channel_read**() returns an
41array of raw ``zx_handle_t`` handle values while **channel_read_etc**() returns
42an array of ``zx_handle_info_t`` structures of the form:
43
44```
45typedef struct {
46    zx_handle_t handle;     // handle value
47    zx_obj_type_t type;     // type of object, see ZX_OBJ_TYPE_
48    zx_rights_t rights;     // handle rights
49    uint32_t unused;        // set to zero
50} zx_handle_info_t;
51```
52
53When communicating to an untrusted party over a channel, it is recommended
54that the **channel_read_etc**() form is used and each handle type and rights
55are validated against the expected values.
56
57## RIGHTS
58
59*handle* must have **ZX_RIGHT_READ**.
60
61## RETURN VALUE
62
63Both forms of read returns **ZX_OK** on success, if *actual_bytes*
64and *actual_handles* (if non-NULL), contain the exact number of bytes
65and count of handles read.
66
67## ERRORS
68
69**ZX_ERR_BAD_HANDLE**  *handle* is not a valid handle.
70
71**ZX_ERR_WRONG_TYPE**  *handle* is not a channel handle.
72
73**ZX_ERR_INVALID_ARGS**  If any of *bytes*, *handles*, *actual_bytes*, or
74*actual_handles* are non-NULL and an invalid pointer.
75
76**ZX_ERR_ACCESS_DENIED**  *handle* does not have **ZX_RIGHT_READ**.
77
78**ZX_ERR_SHOULD_WAIT**  The channel contained no messages to read.
79
80**ZX_ERR_PEER_CLOSED**  The other side of the channel is closed.
81
82**ZX_ERR_NO_MEMORY**  Failure due to lack of memory.
83There is no good way for userspace to handle this (unlikely) error.
84In a future build this error will no longer occur.
85
86**ZX_ERR_BUFFER_TOO_SMALL**  The provided *bytes* or *handles* buffers
87are too small (in which case, the minimum sizes necessary to receive
88the message will be written to *actual_bytes* and *actual_handles*,
89provided they are non-NULL). If *options* has **ZX_CHANNEL_READ_MAY_DISCARD**
90set, then the message is discarded.
91
92## NOTES
93
94*num_handles* and *actual_handles* are counts of the number of elements
95in the *handles* array, not its size in bytes.
96
97## SEE ALSO
98
99[handle_close](handle_close.md),
100[handle_duplicate](handle_duplicate.md),
101[handle_replace](handle_replace.md),
102[object_wait_one](object_wait_one.md),
103[object_wait_many](object_wait_many.md),
104[channel_call](channel_call.md),
105[channel_create](channel_create.md),
106[channel_write](channel_write.md).
107