implicit declaration of function ‘gpio_direction_output’ [-Wimplicit-function-declaration

这个警告"implicit declaration of function ‘gpio_direction_output’ [-Wimplicit-function-declaration]"意味着在当前的编译单元中没有找到名为"gpio_direction_output"的函数的声明。编译器建议你是否想使用类似的函数"gpio_direction_out"，并且默认构造了一个假设的函数"gpio_direction_out"，但返回值是int类型，因此还有一个警告说从int转换为const char *需要进行转换。

通常，当你在调用一个函数之前没有显式声明它时，编译器会报这个错误。要解决这个问题，你需要在调用函数之前提前声明函数或包含正确的头文件。<span class="em">1</span><span class="em">2</span><span class="em">3</span>
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