HiLog is the log system of OpenHarmony that provides logging for the system framework, services, and applications to record information on user operations and system running status.
> **NOTE**
> This development guide is applicable only when you use Native APIs for application development. For details about the APIs, see [HiLog Native API Reference](https://gitee.com/openharmony-sig/interface_native_header/blob/master/en/native_sdk/dfx/log.h).
>
> This development guide is applicable only when you use Native APIs for application development. For details about the APIs, see [HiLog Native API Reference](../reference/native-apis/_hi_log.md).
## Available APIs
| API/Macro| Description|
| -------- | -------- |
| int OH_LOG_Print(LogType type, LogLevel level, unsigned int domain, const char *tag, const char *fmt, ...) | Outputs logs based on the specified log type, log level, service domain, log tag, and variable parameters determined by the format specifier and privacy identifier in the printf format.|
| int OH_LOG_Print(LogType type, LogLevel level, unsigned int domain, const char *tag, const char *fmt, ...) | Outputs logs based on the specified log type, log level, service domain, log tag, and variable parameters determined by the format specifier and privacy identifier in the printf format.<br>Input parameters: See [Parameter Description](#parameter-description).<br>Output parameters: None<br>Return value: total number of bytes if log printing is successful; **-1** otherwise.|
| #define OH_LOG_DEBUG(type, ...) ((void)OH_LOG_Print((type), LOG_DEBUG, LOG_DOMAIN, LOG_TAG, \_*VA*ARGS__))| Outputs DEBUG logs. This is a function-like macro.|
| #define OH_LOG_INFO(type, ...) ((void)OH_LOG_Print((type), LOG_INFO, LOG_DOMAIN, LOG_TAG, \_*VA*ARGS__)) | Outputs INFO logs. This is a function-like macro.|
| #define OH_LOG_WARN(type, ...) ((void)OH_LOG_Print((type), LOG_WARN, LOG_DOMAIN, LOG_TAG, \_*VA*ARGS__)) | Outputs WARN logs. This is a function-like macro.|
| #define OH_LOG_ERROR(type, ...) ((void)OH_LOG_Print((type), LOG_ERROR, LOG_DOMAIN, LOG_TAG, \_*VA*ARGS__)) | Outputs ERROR logs. This is a function-like macro.|
| #define OH_LOG_FATAL(type, ...) ((void)OH_LOG_Print((type), LOG_FATAL, LOG_DOMAIN, LOG_TAG, \_*VA*ARGS__)) | Outputs FATAL logs. This is a function-like macro.|
| bool OH_LOG_IsLoggable(unsigned int domain, const char *tag, LogLevel level) | Checks whether logs of the specified service domain, tag, and level can be printed.<br>Input arguments:<br>- **domain**: service domain.<br>- **tag**: log tag.<br>- **level**: log level.<br>Output arguments: none<br>Return value: Returns **true** if the specified logs can be printed; returns **false** otherwise.|
| bool OH_LOG_IsLoggable(unsigned int domain, const char *tag, LogLevel level) | Checks whether logs of the specified service domain, tag, and level can be printed.<br>Input parameters: See [Parameter Description](#parameter-description).<br>Output arguments: none<br>Return value: **true** if the specified logs can be printed; **false** otherwise.|
| type | enum | Yes | Enum of log printing types. The default value is **LOG_APP** for application logs.|
| level | enum | Yes | Log printing level. For details, see [Log Level](#loglevel).|
| domain | number | Yes | Service domain of logs. The value ranges from **0x0** to **0xFFFF**.<br>You can define the value as required. |
| tag | string | Yes | Log tag in the string format. You are advised to use this parameter to identify a particular service behavior or the class holding the ongoing method.|
| fmt | string | Yes | Format string used to output logs in a specified format. It can contain several parameters, where the parameter type and privacy identifier are mandatory.<br>Parameters labeled **{public}** are public data and are displayed in plaintext; parameters labeled **{private}** (default value) are private data and are filtered by **\<private>**.|
| args | any[] | Yes | Variable-length parameter list corresponding to the format string. The number and type of parameters must map to the identifier in the format string.|
| DEBUG | 3 | Log level used to record more detailed process information than INFO logs to help developers analyze service processes and locate faults.|
| INFO | 4 | Log level used to record key service process nodes and exceptions that occur during service running,<br>Log level used to record information about unexpected exceptions, such as network signal loss and login failure.<br>These logs should be recorded by the dominant module in the service to avoid repeated logging conducted by multiple invoked modules or low-level functions.|
| WARN | 5 | Log level used to record severe, unexpected faults that have little impact on users and can be rectified by the programs themselves or through simple operations.|
| ERROR | 6 | Log level used to record program or functional errors that affect the normal running or use of the functionality and can be fixed at a high cost, for example, by resetting data.|
| FATAL | 7 | Log level used to record program or functionality crashes that cannot be rectified.
## Development Examples
1. Include the **hilog** header file in the source file.
1. Add the link of **libhilog_ndk.z.so** to **CMakeLists.txt**:
```
target_link_libraries(entry PUBLIC libhilog_ndk.z.so)
```
2. Include the **hilog** header file in the source file, and define the **domain** and **tag** macros.
```c++
#include "hilog/log.h"
```
2. Define the **domain** and **tag** macros.
```c++
#undef LOG_DOMAIN
#undef LOG_TAG
#define LOG_DOMAIN 0x3200 // Service domain. The value ranges from 0xD0000 to 0xDFFFF.
#define LOG_TAG "MY_TAG"
#define LOG_DOMAIN 0x3200 // Global domain, which identifies the service domain.
#define LOG_TAG "MY_TAG" // Global tag, which identifies the module log tag.
```
3. Print logs. For example, to print INFO logs, use the following code:
3. Print logs. For example, to print ERROR logs, use the following code:
```c++
OH_LOG_INFO(LOG_APP,"Failed to visit %{private}s, reason:%{public}d.",url,errno);
OH_LOG_ERROR(LOG_APP,"Failed to visit %{private}s, reason:%{public}d.",url,errno);
```
4. View the output log information.
```
12-11 12:21:47.579 2695 2695 I A03200/MY_TAG: Failed to visit <private>, reason:11.
12-11 12:21:47.579 2695 2695 E A03200/MY_TAG: Failed to visit <private>, reason:11.
