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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;
    };
};

where the embedded structs are defined as:

struct android_log_header_t {
    uint8_t id;
    uint16_t tid;
    log_time realtime;
};

struct log_time {
    uint32_t tv_sec = 0;
    uint32_t tv_nsec = 0;
}

struct android_event_header_t {
    int32_t tag;
};

struct android_event_list_t {
    int8_t type;  // EVENT_TYPE_LIST
    int8_t element_count;
};

struct android_event_float_t {
    int8_t type;  // EVENT_TYPE_FLOAT
    float data;
};

struct android_event_int_t {
    int8_t type;   // EVENT_TYPE_INT
    int32_t data;
} android_event_int_t;

struct android_event_long_t {
    int8_t type;   // EVENT_TYPE_LONG
    int64_t data;
};

struct android_event_string_t {
    int8_t type;     // EVENT_TYPE_STRING;
    int32_t length;
    char data[];
};

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.

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