C语言 C 编译失败：找不到 math.h 函数
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C Failing to compile: Can't find math.h functions
提问by Aviator45003
I'm writing a prime number finder. Mathematically, it is faster to, instead of doing for (unsigned long i = 2; i < number/2; i++)it's a lot faster, and still just as effective, to do for (unsigned long i = 2; i < sqrt(number); i++)
我正在写一个质数查找器。从数学上讲，这样做更快，而不是for (unsigned long i = 2; i < number/2; i++)更快，而且仍然同样有效for (unsigned long i = 2; i < sqrt(number); i++)
But it's not working. The below is my code.
但它不起作用。下面是我的代码。
// Stuff goes up here, including a function prototype and:
#include <math.h>
char isPrime (unsigned long number)
{
if (number <= 1) {
return 0;
}
long double sqrtOfNumber = sqrt(number); // Calculate once.
for (unsigned long i = 2; i < sqrtOfNumber; i++) {
if (number%i == 0) { // It has a divisor.
return 0;
}
}
// Nothing broke us yet....
return 1;
}
And then below is the error I get from GCC.
然后下面是我从 GCC 得到的错误。
/tmp/ccFBlUz5.o: In function `isPrime':
main.c:(.text+0xb3): undefined reference to `sqrt'
collect2: error: ld returned 1 exit status
Changing the type of "number" to a double causes problems with the % operator. And casting it to a double for the sqrt() call doesn't change anything.
将“数字”的类型更改为双精度会导致 % 运算符出现问题。并且为 sqrt() 调用将其转换为 double 不会改变任何内容。
Any advice?
有什么建议吗？
Oh, and my math.h IS being imported, if I comment out that line, I get warned that there is an implicit declaration there.
哦，我的 math.h 正在导入，如果我注释掉那行，我会收到警告说那里有一个隐式声明。
main.c: In function 'isPrime':
main.c:28:2: warning: implicit declaration of function 'sqrt' [-Wimplicit-function-declaration]
long double sqrtOfNumber = sqrt(number); // Calculate once.
^
main.c:28:29: warning: incompatible implicit declaration of built-in function 'sqrt' [enabled by default]
long double sqrtOfNumber = sqrt(number); // Calculate once.
^
plus the other warning under that.
加上其他警告。
回答by Shafik Yaghmour
-lmneeds to added to the command line after the file that requires that library for example if main.crequired the math library then you would use:
-lm需要在需要该库的文件之后添加到命令行，例如，如果main.c需要数学库，那么您将使用：
gcc -x c main.c -lm
You can see a live example here, there are three command lines available in the example. One without -lm, one with -lmbefore the file that needs it one after the files that needs it.
您可以在此处查看实时示例，示例中提供了三个命令行。一个没有-lm，一个-lm在需要它的文件之前，一个在需要它的文件之后。
For completeness sake if we refer to the gcc docs on options for Linkingit says the following for -l:
为了完整起见，如果我们参考gcc docs on options for Linking它说以下内容-l：
[...]It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, ‘foo.o -lz bar.o' searches library ‘z' after file foo.o but before bar.o. If bar.o refers to functions in ‘z', those functions may not be loaded. [...]
[...]在命令中编写此选项的位置有所不同；链接器按照指定的顺序搜索和处理库和目标文件。因此，'foo.o -lz bar.o' 在文件 foo.o 之后但在 bar.o 之前搜索库 'z'。如果 bar.o 引用 'z' 中的函数，则可能不会加载这些函数。[...]
回答by Karl Bielefeldt
You need to link the math library. Use the -lmoption on the command line.
您需要链接数学库。使用-lm命令行上的选项。
