problem in compiling c under gcc

+undefined reference to log2 +lm

i am using ubuntu and gcc for c compiling. i have designed the following program,

#include <stdio.h>
#include <math.h>
int main(void){
float x=10.0;
float y=30.0;
printf("\n %f \t %f \t %1f",log(x),log2(x),sqrt);
return 0;
}

this is the command line result of gcc


ejaz@msiddique:~/cproj$ gcc -o math math.c
math.c: In function ‘main’:
math.c:6: warning: incompatible implicit declaration of built-in function ‘log2’/tmp/ccLMCrbz.o: In function `main':math.c.text+0x4d): undefined reference to `sqrt'
:math.c.text+0x61): undefined reference to `log2'
:math.c.text+0x6f): undefined reference to `log'
collect2: ld returned 1 exit status
ejaz@msiddique:~/cproj$


please help me in solving this trouble

You haven't included math library. Compile and link your program like this:

gcc -o math math.c -lm


and here is the output:

2.302585 3.321928 5.477226

cfant said:

You haven't included math library. Compile and link your program like this:

gcc -o math math.c -lm


and here is the output:

2.302585 3.321928 5.477226

Click to expand...

o thats good,works fine thanks for reply.but still it is giving me some warning like this

ejaz@msiddique:~/cproj$ gcc -o math math.c -lm
math.c: In function ‘main’:
math.c:6: warning: incompatible implicit declaration of built-in function ‘log2’ejaz@msiddique:~/cproj$ ./math

2.302585 3.321928 5.477226ejaz@msiddique:~/cproj$

ok also i want some explanation about this.

does math library is not in standered c library ? for which of header files we need to link them? does -lm command is sufficient to link any library?
thanx for your nice guide

Added after 1 hours 6 minutes:

also there is problem with double data types.look at the program


// programe to calculate income tax

#include<stdio.h>

int main(void){
const double TAX_RATE = 0.025;
double grossIncome ,totalTax;
printf("Enter the value of income in rupeese");
scanf("%f",&grossIncome);
totalTax = grossIncome * TAX_RATE;
printf("total tax is %f \n",totalTax);
return 0;
}

here is the output

ejaz@msiddique:~/cproj$ gcc -o tax tax.c
ejaz@msiddique:~/cproj$ ./tax
Enter the value of income in rupeese100.0
total tax is -0.000000
ejaz@msiddique:~/cproj$

Check your scanf function. You have declared f to be double, but you are using float. If you change it to:

scanf("%lf",&grossIncome);


program should run ok. As for math library -lm will link only math library.

Added after 6 minutes:

Which version of the gcc you are using? I don't see that warning. Mine is:

gcc version 3.4.3

i use gcc 4.0 .