Read a message from a channel.


#include <zircon/syscalls.h>

zx_status_t zx_channel_read(zx_handle_t handle,
                            uint32_t options,
                            void* bytes,
                            zx_handle_t* handles,
                            uint32_t num_bytes,
                            uint32_t num_handles,
                            uint32_t* actual_bytes,
                            uint32_t* actual_handles);


zx_channel_read() attempts to read the first message from the channel specified by handle into the provided bytes and/or handles buffers.

The parameters num_bytes and num_handles are used to specify the size of the respective read buffers. num_bytes is a count of bytes, and num_handles is a count of elements of type zx_handle_t.

The length of bytes, in bytes, is stored in the location pointed to by actual_bytes. The number of handles is stored in the location pointed to by actual_handles. Either actual_bytes or actual_handles may be NULL, in which case they will be ignored.

Channel messages may contain both byte data and handle payloads and may only be read in their entirety. Partial reads are not possible.

The bytes buffer is written before the handles buffer. In the event of overlap between these two buffers, the contents written to handles will overwrite the portion of bytes it overlaps.

When communicating to an untrusted party over a channel, it is recommended that the zx_channel_read_etc() form is used and each handle type and rights are validated against the expected values.


handle must be of type ZX_OBJ_TYPE_CHANNEL and have ZX_RIGHT_READ.

Return value

Returns ZX_OK on success. If non-NULL, the locations pointed to by actual_bytes and actual_handles contain the exact number of bytes and count of handles read.


ZX_ERR_BAD_HANDLE handle is not a valid handle.

ZX_ERR_WRONG_TYPE handle is not a channel handle.

ZX_ERR_INVALID_ARGS If any of bytes, handles, actual_bytes, or actual_handles are non-NULL and an invalid pointer.

ZX_ERR_ACCESS_DENIED handle does not have ZX_RIGHT_READ.

ZX_ERR_SHOULD_WAIT The channel contained no messages to read and the other side of the channel is open.

ZX_ERR_PEER_CLOSED The channel contained no messages to read and the other side of the channel is closed.

ZX_ERR_NO_MEMORY Failure due to lack of memory. There is no good way for userspace to handle this (unlikely) error. In a future build this error will no longer occur.

ZX_ERR_BUFFER_TOO_SMALL The provided bytes or handles buffers are too small (in which case, the minimum sizes necessary to receive the message will be written to actual_bytes and actual_handles, provided they are non-NULL). If options has ZX_CHANNEL_READ_MAY_DISCARD set, then the message is discarded.

See also