Got it, thank you !

the code is below ,but when I use sigaction function, the compile was failed, it said
"undefined reference to `sigaction", but I have already include the signal.h , so what's the problem? Thanks very much!
...
...
#include "esp_system.h"
#include <pthread.h>
#include "esp_spi_flash.h"
#include ...

/esp/xtensa-esp32-elf/bin/../lib/gcc/xtensa-esp32-elf/8.4.0/../../../../xtensa-esp32-elf/bin/ld: esp-idf/freertos/libfreertos.a(port_common.c.obj):(.literal.main_task+0x14): undefined reference to `app_main'
/home/rehu/esp/xtensa-esp32-elf/bin/../lib/gcc/xtensa-esp32-elf/8.4.0/../../../../xtensa ...