gcc compile problems

michael_hoare · 10-14-2001, 12:30 AM

I am a newbie to LINUX and C generally. I am using RH71, and trying to learn C.

I am trying to compile a simple program in C, using the gcc compiler. Every time I try to use functions sqrt or pow, I get a compile error

undefined reference to "sqrt"

( or undefined refernce to "pow", if I use that function)

I tried compiling under Windows, and it works fine, so the code is probably OK

I tried compiling under the KDevelop IDE, and got the same error.

I tried commenting out the line #include <math.h>, and got the same error, which may mean that the compiler can't find the header files (but it finds stdio.h alright).

I tried directing the incude direction to /usr/include/math.h, but I get the same error.

I tried looking through the gcc documentation, which is comprehensive but hard work, and couldn't find any answers.

Do I need to set up or configure gcc in some way? What else should I check? Any help would be very much appreciated.

crabboy · 10-14-2001, 02:04 AM

I just wrote a small test program that used pow and I got the following linker error when compiling:

Code:

$ cc mathtest.c 
/tmp/ccFv4vgE.o: In function `main':
/tmp/ccFv4vgE.o(.text+0x12): undefined reference to `pow'
collect2: ld returned 1 exit status

This is a linker error stating that it can't find the library for pow.

If you go to /usr/lib and type the command: nm libm.a | grep pow

you will see that pow is in that libraty. You can get it to compile correctly by instructing the linker to use the m (math) library like this:

Code:

cc mathtest.c -lm

My test program worked well:

mathtest
pow( 5, 3 ) = [125.000000]

Gary

toxic53 · 10-15-2001, 12:30 AM

This is a common mistake when using pow and anything the math.h when compiling w/ linux:

whenever you use the math.h library do this:

gcc -o outputfile inputfile.c -lm

I dont know why you have to have the -lm its stupid but necessary thats not a one its and L in lowercase...this will work

~Jesus~

michael_hoare · 10-15-2001, 02:00 AM

Thanks for the help - I worried at this for a few hours. before posting, which is part of the learning curve. Just my luck that the first header I try to use other than stdio.h happened to be math.h with an interesting error.

linuxtard · 12-19-2001, 09:19 AM

For anyone interested in getting KDevelop to compile with the math.h library, in addition to the "#include <math.h>" in your program, I found you have to edit the "Makefile" file in your "projectname/projectname" directory and find the line that says

all_libraries =

and make it look like

all_libraries = -lm (that's -LM if it were uppercase)

cheezwhiz · 01-01-2002, 06:18 AM

Hi, I asked the same question some days ago on another board, here is the link for all interested:

http://www.cprogramming.com/cboard/s...?threadid=7808

It is explained why you have to use the -lm option

linuxtard · 02-05-2002, 08:38 AM

I just discovered what's probably easier than editing the Makefile everytime you add a file to your project....simply go to the Project->Options->Linker Options menu and check the box that says "math". I haven't tried it yet but I'm sure this is what I should have done in the first place.