The abs() function is declared in <stdlib.h> which you've not included. abs()函数在<stdlib.h>声明，你没有包含它。 fabs() function is declared in <math.h>
函数abs的隐式声明 - gcc-5.1.0
Implicit declaration of function abs - gcc-5.1.0
Compiling the following code using gcc-5.1.0 produces a warning: 使用gcc-5.1.0编译以下代码会产生警告：
warning: implicit declaration of function ‘abs’ [-Wimplicit-function-declaration]
Code: 码：
#include <stdio.h>
#include <math.h>
int main (void)
{
printf ("%d\n", abs (-1));
return 0;
}
I have compiled the same code with gcc-4.9.2 and it's not producing any warning. 我用gcc-4.9.2编译了相同的代码，并没有产生任何警告。
The abs() function is declared in <stdlib.h> which you've not included. abs()函数在<stdlib.h>声明，你没有包含它。
GCC 4.9.2 didn't complain because the default compilation mode was C89/C90 ( -std=gnu89 ) and functions did not need to be declared before being used in C89 as long as they returned an int , but the default compilation mode was changed to C11 ( -stdd=gnu11 ) in GCC 5.1.0 (see the release notes ) and in C11 functions must be declared (or defined) before they are used. GCC 4.9.2没有抱怨，因为默认编译模式是C89 / C90（ -std=gnu89 ），并且只要它们返回一个int ，就不需要在C89中使用之前声明函数，但默认的编译模式是在GCC 5.1.0中更改为C11（ -stdd=gnu11 ）（参见发行说明 ），在使用之前必须声明（或定义）C11函数。
Try to include the <stdlib.h> in your code. 尝试在代码中包含<stdlib.h> 。 The abs() function is defined inside the <stdlib.h> abs()函数在<stdlib.h>定义