Tom Cherry a379b1ce05 liblog: document the liblog<->logd protocol format

This protocol documentation is spread out among various functions
where it is implemented.  This change makes a single
README.protocol.md file with a high level overview, referencing the
struct names where useful.

Test: n/a
Change-Id: I83c9f484352b489b4a20cce241d92413f780f9ec

2019-10-24 11:19:06 -07:00

1.7 KiB

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liblog -> logd

The data that liblog sends to logd is represented below.

struct {
    android_log_header_t header;
    union {
       struct {
            char     prio;
            char     tag[...];
            char     message[...];
        } string;
        struct {
            android_event_header_t event_header;
            android_event_*_t      payload[...];
        } binary;
    };
};

The payload, excluding the header, has a max size of LOGGER_ENTRY_MAX_PAYLOAD.

header

The header is added immediately before sending the log message to logd.

`string` payload

The string part of the union is for normal buffers (main, system, radio, etc) and consists of a single character priority, followed by a variable length null terminated string for the tag, and finally a variable length null terminated string for the message.

This payload is used for the __android_log_buf_write() family of functions.

`binary` payload

The binary part of the union is for binary buffers (events, security, etc) and consists of an android_event_header_t struct followed by a variable number of android_event_*_t (android_event_list_t, android_event_int_t, etc) structs.

If multiple android_event_*_t elements are present, then they must be in a list and the first element in payload must be an android_event_list_t.

This payload is used for the __android_log_bwrite() family of functions. It is additionally used for android_log_write_list() and the related functions that manipulate event lists.

logd -> liblog

logd sends a logger_entry struct to liblog followed by the payload. The payload is identical to the payloads defined above. The max size of the entire message from logd is LOGGER_ENTRY_MAX_LEN.

1.7 KiB Raw Blame History