When you get the error: implicit declaration of function it should also list the offending function. Often this error happens because of a forgotten or missing header file, so at the shell prompt you can type man 2 functionname and look at the SYNOPSIS section at the top, as this section will list any header files that need to be included. Or try http://linux.die.net/man/ This is the online man pages they are hyperlinked and easy to search. Functions are often defined in the header files, including any required header files is often the answer. Like cnicutar said,

您正在使用的函数的编译器没有看到 声明(“原型”)。

When you get the error: implicit declaration of function it should also list the offending function. Often this error happens because of a forgotten or missing header file, so at the shell prompt you can type man 2 functionname and look at the SYNOPSIS section at the top, as this section will list any header files that need to be included. Or try http://linux.die.net/man/ This is the online man pages they are hyperlinked and easy to search. Functions are often defined in the header files, including any required header files is often the answer. Like cnicutar said,

您正在使用的函数的编译器没有看到 声明(“原型”)。

如果你定义了正确的头文件并且使用了非GlibC库(比如Musl C)，当遇到malloc_trim这样的GNU扩展时，gcc也会抛出error:函数的隐式声明。

解决方案是包装扩展和头:

#if defined (__GLIBC__)
  malloc_trim(0);
#endif

不要忘记，如果函数中调用了任何函数，它们的原型必须位于代码中函数的上方。否则，编译器在尝试编译函数之前可能找不到它们。这将产生问题中的错误。

在main.c中执行#includes时，将#include引用放在包含被引用函数的文件的include列表顶部。 例如，这是main.c，你引用的函数在“SSD1306_LCD.h”中

#include "SSD1306_LCD.h"    
#include "system.h"        #include <stdio.h>
#include <stdlib.h>
#include <xc.h>
#include <string.h>
#include <math.h>
#include <libpic30.h>       // http://microchip.wikidot.com/faq:74
#include <stdint.h>
#include <stdbool.h>
#include "GenericTypeDefs.h"  // This has the 'BYTE' type definition

上面的代码不会生成“函数隐式声明”错误，但是下面的代码会生成-

#include "system.h"        
#include <stdio.h>
#include <stdlib.h>
#include <xc.h>
#include <string.h>
#include <math.h>
#include <libpic30.h>       // http://microchip.wikidot.com/faq:74
#include <stdint.h>
#include <stdbool.h>
#include "GenericTypeDefs.h"     // This has the 'BYTE' type definition
#include "SSD1306_LCD.h"    

完全相同的#include列表，只是顺序不同。

对我来说确实如此。

发生此错误是因为您正在尝试使用编译器无法理解的函数。如果要使用的函数是用C语言预定义的，只需包含与隐式函数关联的头文件。 如果它不是一个预定义的函数，那么在主函数之前声明它总是一个很好的做法。