logcat 通过读 /dev/socket/logd 套接字节点，以从 logd 进程中获取信息。
在 /logcat/logcat.cpp 的 __logcat 方法中调用 android_logger_list_read
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static int __logcat(android_logcat_context_internal* context) {
......
while (!context->stop &&
(!context->maxCount || (context->printCount < context->maxCount))) {
struct log_msg log_msg;
int ret = android_logger_list_read(logger_list, &log_msg);
if (!ret) {
logcat_panic(context, HELP_FALSE, "read: unexpected EOF!\n");
break;
}
......
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android_logger_list_read是 liblog/logger_read.c 中的接口
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LIBLOG_ABI_PUBLIC int android_logger_list_read(struct logger_list* logger_list,
struct log_msg* log_msg) {
struct android_log_transport_context* transp;
struct android_log_logger_list* logger_list_internal =
(struct android_log_logger_list*)logger_list;
int ret = init_transport_context(logger_list_internal);
if (ret < 0) {
return ret;
}
......
}
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其中调用了init_transport_context，该接口同样位于 liblog/logger_read.c ，主要实现如下
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static int init_transport_context(struct android_log_logger_list* logger_list) {
......
__android_log_lock();
/* mini __write_to_log_initialize() to populate transports */
if (list_empty(&__android_log_transport_read) &&
list_empty(&__android_log_persist_read)) {
__android_log_config_read();
}
__android_log_unlock();
......
}
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其中调用了__android_log_config_read，位于 /liblog/config_read.c，主要实现如下
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LIBLOG_HIDDEN void __android_log_config_read() {
......
#if (FAKE_LOG_DEVICE == 0)
if ((__android_log_transport == LOGGER_DEFAULT) ||
(__android_log_transport & LOGGER_LOGD)) {
extern struct android_log_transport_read logdLoggerRead;
extern struct android_log_transport_read pmsgLoggerRead;
__android_log_add_transport(&__android_log_transport_read, &logdLoggerRead);
__android_log_add_transport(&__android_log_persist_read, &pmsgLoggerRead);
}
#endif
}
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其中调用了logdLoggerRead，位于 /liblog/logd_reader.c，主要实现如下
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LIBLOG_HIDDEN struct android_log_transport_read logdLoggerRead = {
.node = { &logdLoggerRead.node, &logdLoggerRead.node },
.name = "logd",
.available = logdAvailable,
.version = logdVersion,
.read = logdRead,
.poll = logdPoll,
.close = logdClose,
.clear = logdClear,
.getSize = logdGetSize,
.setSize = logdSetSize,
.getReadableSize = logdGetReadableSize,
.getPrune = logdGetPrune,
.setPrune = logdSetPrune,
.getStats = logdGetStats,
};
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其中通过logdRead函数指针为.read字段 进行赋值，logdRead的主要实现如下
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static int logdRead(struct android_log_logger_list* logger_list,
struct android_log_transport_context* transp,
struct log_msg* log_msg) {
......
ret = logdOpen(logger_list, transp);
if (ret < 0) {
return ret;
}
......
ret = recv(ret, log_msg, LOGGER_ENTRY_MAX_LEN, 0);
......
}
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里面调用了logdOpen来打开套接字logdr，而后返回logdRead进行recv。
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static int logdOpen(struct android_log_logger_list* logger_list,
struct android_log_transport_context* transp) {
......
sock = socket_local_client("logdr", ANDROID_SOCKET_NAMESPACE_RESERVED,
SOCK_SEQPACKET);
......
return sock;
}
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