logarithm of a variable in C

[James P]

Hey, I have a problem with following code:


#include <stdio.h>
#include <math.h>

int main() {
int x;
x = 10;
printf("%.2f\n", log(x));
}

when I try to compile it in GCC it shows an error message:
In function `main':
logarithm.c:(.text+0x19): undefined reference to `log'

how can I fix it?

Thans James

[Banfa]

It compiles for me (using MinGW) which suggests a problem either with your compiler command line or with missing standard libraries or with the path to the libraries being wrong.

[donbock]

Are you using C or C++?
Change the definition of x so that it is a double.

C++ supports three different functions all named log. It uses magic to determine which of the three to use based on the type of the argument. If you're using C++ then perhaps passing an int argument fooled the magic into looking for a nonexistent fourth version of log.

[weaknessforcats]

C++ would automatically convert the int to a float so I suspect Banfa is correct and a library has been left out of the build.

[James P]

I'm using C, probably there's problem with compiler, when I try to compile and run the program in codeblocks everything was good, no errors

Thanks alot for your responses

[donbock]

Try adding the -lm switch to the compiler command line. This tells it to link in library m, which is actually a file named libm.a This is where the math library is typically located.

[James P]

It doesn't work :(, if it helps I've got Ubuntu 11
I don't know what's wrong

[Banfa]

Have you installed the build-essential package ?

sudo apt-get install build-essential

[James P]

I haven't, thank you, I'll try and see if it will help

[James P]

it didn't help :(, but thank you very much for your responses

[nubera]

I don't know exactly why it helps, but it does: use an optimization flag (-O1..O3) when compiling:

gcc -O1 myprogram.c

Hope that helps in your case too. Cheers.

[johny10151981]

compile with -lm parmater
gcc -lm myprogram.c

somebody did the advice already :)