Error Handling¶

Overview¶

Identifying and handling run-time errors is important for developing robust applications. There can be multiple kinds of run-time errors:

Recoverable errors:
- Errors indicated by functions through return values (error codes)
- C++ exceptions, thrown using throw keyword
Unrecoverable (fatal) errors:
- Failed assertions (using assert macro and equivalent methods) and abort() calls.
- CPU exceptions: access to protected regions of memory, illegal instruction, etc.
- System level checks: watchdog timeout, cache access error, stack overflow, stack smashing, heap corruption, etc.

This guide explains ESP-IDF error handling mechanisms related to recoverable errors, and provides some common error handling patterns.

For instructions on diagnosing unrecoverable errors, see Fatal Errors.

Error codes¶

The majority of ESP-IDF-specific functions use esp_err_t type to return error codes. esp_err_t is a signed integer type. Success (no error) is indicated with ESP_OK code, which is defined as zero.

Various ESP-IDF header files define possible error codes using preprocessor defines. Usually these defines start with ESP_ERR_ prefix. Common error codes for generic failures (out of memory, timeout, invalid argument, etc.) are defined in esp_err.h file. Various components in ESP-IDF may define additional error codes for specific situations.

For the complete list of error codes, see Error Code Reference.

Converting error codes to error messages¶

For each error code defined in ESP-IDF components, esp_err_t value can be converted to an error code name using esp_err_to_name() or esp_err_to_name_r() functions. For example, passing 0x101 to esp_err_to_name() will return “ESP_ERR_NO_MEM” string. Such strings can be used in log output to make it easier to understand which error has happened.

Additionally, esp_err_to_name_r() function will attempt to interpret the error code as a standard POSIX error code, if no matching ESP_ERR_ value is found. This is done using strerror_r function. POSIX error codes (such as ENOENT, ENOMEM) are defined in errno.h and are typically obtained from errno variable. In ESP-IDF this variable is thread-local: multiple FreeRTOS tasks have their own copies of errno. Functions which set errno only modify its value for the task they run in.

This feature is enabled by default, but can be disabled to reduce application binary size. See CONFIG_ESP_ERR_TO_NAME_LOOKUP. When this feature is disabled, esp_err_to_name() and esp_err_to_name_r() are still defined and can be called. In this case, esp_err_to_name() will return UNKNOWN ERROR, and esp_err_to_name_r() will return Unknown error 0xXXXX(YYYYY), where 0xXXXX and YYYYY are the hexadecimal and decimal representations of the error code, respectively.

`ESP_ERROR_CHECK` macro¶

ESP_ERROR_CHECK() macro serves similar purpose as assert, except that it checks esp_err_t value rather than a bool condition. If the argument of ESP_ERROR_CHECK() is not equal ESP_OK, then an error message is printed on the console, and abort() is called.

Error message will typically look like this:

ESP_ERROR_CHECK failed: esp_err_t 0x107 (ESP_ERR_TIMEOUT) at 0x400d1fdf

file: "/Users/user/esp/example/main/main.c" line 20
func: app_main
expression: sdmmc_card_init(host, &card)

Backtrace: 0x40086e7c:0x3ffb4ff0 0x40087328:0x3ffb5010 0x400d1fdf:0x3ffb5030 0x400d0816:0x3ffb5050

Note

If IDF monitor is used, addresses in the backtrace will be converted to file names and line numbers.

The first line mentions the error code as a hexadecimal value, and the identifier used for this error in source code. The latter depends on CONFIG_ESP_ERR_TO_NAME_LOOKUP option being set. Address in the program where error has occured is printed as well.
Subsequent lines show the location in the program where ESP_ERROR_CHECK() macro was called, and the expression which was passed to the macro as an argument.
Finally, backtrace is printed. This is part of panic handler output common to all fatal errors. See Fatal Errors for more information about the backtrace.

Error handling patterns¶

Attempt to recover. Depending on the situation, this might mean to retry the call after some time, or attempt to de-initialize the driver and re-initialize it again, or fix the error condition using an out-of-band mechanism (e.g reset an external peripheral which is not responding).

Example:
```
esp_err_t err;
do {
    err = sdio_slave_send_queue(addr, len, arg, timeout);
    // keep retrying while the sending queue is full
} while (err == ESP_ERR_TIMEOUT);
if (err != ESP_OK) {
    // handle other errors
}
```

Propagate the error to the caller. In some middleware components this means that a function must exit with the same error code, making sure any resource allocations are rolled back.

Example:

sdmmc_card_t* card = calloc(1, sizeof(sdmmc_card_t));
if (card == NULL) {
    return ESP_ERR_NO_MEM;
}
esp_err_t err = sdmmc_card_init(host, &card);
if (err != ESP_OK) {
    // Clean up
    free(card);
    // Propagate the error to the upper layer (e.g. to notify the user).
    // Alternatively, application can define and return custom error code.
    return err;
}

Convert into unrecoverable error, for example using ESP_ERROR_CHECK. See ESP_ERROR_CHECK macro section for details.

Terminating the application in case of an error is usually undesirable behaviour for middleware components, but is sometimes acceptable at application level.

Many ESP-IDF examples use ESP_ERROR_CHECK to handle errors from various APIs. This is not the best practice for applications, and is done to make example code more concise.

Example:
```
ESP_ERROR_CHECK(spi_bus_initialize(host, bus_config, dma_chan));
```

C++ Exceptions¶

Support for C++ Exceptions in ESP-IDF is disabled by default, but can be enabled using CONFIG_COMPILER_CXX_EXCEPTIONS option.

Enabling exception handling normally increases application binary size by a few kB. Additionally it may be necessary to reserve some amount of RAM for exception emergency pool. Memory from this pool will be used if it is not possible to allocate exception object from the heap. Amount of memory in the emergency pool can be set using CONFIG_COMPILER_CXX_EXCEPTIONS_EMG_POOL_SIZE variable.

If an exception is thrown, but there is no catch block, the program will be terminated by abort function, and backtrace will be printed. See Fatal Errors for more information about backtraces.

See cxx/exceptions for an example of C++ exception handling.

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