Undefined reference to pow linux

Cannot build simple C program in Geany on Linux — undefined reference to pow [duplicate]

I am taking an Arduino course on Coursera and was required to write a program to list the first 6 Fibonocci number. Rather a trivial assignment so I decided to go a different route and calculate the numbers using the Golden Mean. However I cannot get the program to build using the Geany IDE on Linux Mint. The code compiles without error but won’t build. It compiles without error on the Arduino also. But if I can’t build it I can’t submit it or test it.

I am a complete novice at C programming. These are the errors from Geany:

I don’t know what any of that means but I thought math.h was a built-in library. Shouldn’t Geany know what in the heck «pow» is?

3 Answers 3

You haven’t linked the math library. The math library is not part of the standard library, which gcc links by default. So you need to link it yourself with -lm .

The error you see are given by linker that it couldn’t find the definition for pow() function. Including math.h only provides the necessary prototypes/declarations. It doesn’t provide the actual library.

I don’t know what any of that means but I thought math.h was a built-in library. Shouldn’t Geany know what in the heck «pow» is?

math.h is not a library; it is a header file. Very informally, a header file lets the compiler know what the declarations of function and constants are. Kind of like saying «yo, compiler, pow is a function that takes a two arguments of type double and returns a double».

The compiler, actually the parser, needs this information to check the semantic correctness of a statement. in particular checking that the appropriate number of arguments have been provided, and that the types are correct.

Now, the compiler, actually the linker, needs to find the definition of whatever you declare and use. Since you didn’t provide a definition (i.e. the implementation of the function), the linker throws an error.

Hint: any time you see an error like «undefined reference» you have forgotten to implement a function you use, or forgotten to link the appropriate library (as l3x suggests).

N.B. I kept saying stuff like «the compiler, actually the linker» because most people refer to gcc as a compiler, but it is in fact a collection of programs that work together to transform source code to an executable (or library). At a minimum the following programs get involved:

preprocessor (on a Linux system, this is normally cpp ). This part deals with the preprocessor instructions like #include or #define . You can look at the output of the preprocessor by using the -E argument with gcc .

parser (on a Linux system this is normally cc1 generally) This part deals with checking the semantics of the program, and producing an assembly language output. You can look at the output of this phase by using the -S argument with gcc.

assembler (on a Linux system this is normally gas ). This take the output from the parser can converts it into object code. You can look at the output of this phase using the -c argument with gcc (this is the first output that is not a straight text file).

linker (on a Linux system this is normally ld ). This takes multiple object files and links them together into a single executable. It also deals with dynamic linkage and sets the executable up so that libraries can be linked at run time.

Understanding what each of these programs do, and the types of error (and reasons) will make you life much easier when troubleshooting a build.

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«undefined reference to `pow'» even with math.h and the library link -lm [duplicate]

I’m using math.h and the -lm option to compile. I have tried all of:

happens on all cases.

1 Answer 1

Put the -lm at the end of the line.

gcc processes the arguments that specify inputs to the final program in the order they appear on the command line. The -lm argument is passed to the linker, and the ssf.c argument, for example, is compiled, and the resulting object file is passed to the linker.

The linker also processes inputs in order. When it sees a library, as -lm specifies, it looks to see if that library supplies any symbols that the linker currently needs. If so, it copies the modules with those symbols from the library and builds them into the program. When the linker sees an object module, it builds that object module into the program. After bringing an object module into the program, the linker does not go back and see if it needs anything from earlier libraries.

Because you listed the library first, the linker did not see anything that it needed from the library. If you list the object module first, the linker will bring the object module into the program. In the process of doing this, the linker will make a list of all the undefined symbols that the object needs. Then, when the linker sees the library, it will see that the library supplies definitions for those symbols, and it will bring the modules with those symbols into the program.

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Undefined reference to `pow’ and `floor’

I’m trying to make a simple fibonacci calculator in C but when compiling gcc tells me that I’m missing the pow and floor functions. What’s wrong?

6 Answers 6

You need to compile with the link flag -lm , like this:

This will tell gcc to link your code against the math lib. Just be sure to put the flag after the objects you want to link.

Add -lm to your link options, since pow() and floor() are part of the math library:

For the benefit of anyone reading this later, you need to link against it as Fred said:

One good way to find out what library you need to link is by checking the man page if one exists. For example, man pow and man floor will both tell you:

An explanation for linking math library in C programming — Linking in C

In regards to the answer provided by Fuzzy:

I actually had to do something slightly different.

Project -> Properties -> C/C++ Build -> Settings -> GCC C Linker -> Libraries

Click the little green add icon, type m and hit ok. Everything in this window automatically has -l applied to it since it is a library.

To find the point where to add the -lm in Eclipse-IDE is really horrible, so it took me some time.

If someone else also uses Edlipse, here’s the way how to add the command:

Project -> Properties -> C/C++ Build -> Settings -> GCC C Linker -> Miscelleaneous -> Linker flags: in this field add the command -lm

All answers above are incomplete, the problem here lies in linker ld rather than compiler collect2: ld returned 1 exit status . When you are compiling your fib.c to object:

Where nm lists symbols from object file. You can see that this was compiled without an error, but pow , floor , and printf functions have undefined references, now if I will try to link this to executable:

Im getting similar output you get. To solve that, I need to tell linker where to look for references to pow , and floor , for this purpose I will use linker -l flag with m which comes from libm.so library.

You can now see, functions pow , floor are linked to GLIBC_2.2.5 .

Parameters order is important too, unless your system is configured to use shared librares by default, my system is not, so when I issue:

Note -lm flag before object file. So in conclusion, add -lm flag after all other flags, and parameters, to be sure.

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«Undefined reference» problem occurs when the header file uses the math library
error:

If it is referenced in VSCode, you can directly add it to the file under .vscode. The specific method is as follows:
https://blog.csdn.net/qq_42848693/article/details/87036600

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