Hello:
        I can't seem to get math functions referenced on my gcc 2.7.2
compiler running on Linux. Since I was the person who installed the
software, I wonder if my compiler is broken or the problem is more my
inexperience in C. When I compile the attached demo program, I get the
following:
/root/math.c:13: undefined reference to `log'
/root/math.c:15: undefined reference to `exp'
My command line was: gcc -g -lm -o math.o math.c
I thought the math library would be linked by using -lm above. Any help
for a novice would be appreciated. The libraries have been ldconfig-ed.
________________________________________________________________________________
        Larry W. Roberts                                SeaTac, WA.

        http://www.*-*-*.com/ ~lroberts     http://www.*-*-*.com/

________________________________________________________________________________

[ math.c < 1K ]
#include <stdio.h>
#include <math.h>
main()
{      
double exp(double x);

double log(double y);

double x, y, c;      

x = 10;
y = 100;
c = log(y);
printf("The natural log of 100 is %lf", c);
c = exp(x);
printf("\nThe value of e^10 is %lf", c);

(BTW, please eschew the quoting of a multi-line sig when posting a
followup).

--
=============================================================================

     linux for fun, M$ for $$$...and the NFL for what really counts!
=============================================================================

This may seem silly but perhaps you forgot:
#include <math.h>

Your linker is telling you that it cannot find the code to use those
functions.

I renamed math.c to arith.c just to avoid a possible confusion suggested
by someone else with the include file math.h . Then:
lroberts:~# gcc -g  -o arith.o arith.c -lm
/tmp/cca001781.o: In function `main':
/root/arith.c:13: undefined reference to `log'
/root/arith.c:15: undefined reference to `exp'

As you see, there isn't any difference in the outcome even with the -lm at
the end. Thanks for the FAQ suggestion. I'll see if I can find it.
________________________________________________________________________________
        Larry W. Roberts                                SeaTac, WA.

        http://www.oz.net/~lroberts     http://lroberts.oz.net

________________________________________________________________________________

In any case, you should have read the comp.lang.c FAQ list; I'm sure
this is in there.

|>   This message is in MIME format.  The first part should be readable text,
|>   while the remaining parts are likely unreadable without MIME-aware tools.

|>
|> --1036932765-789891942-846583923=:269
|> Content-Type: TEXT/PLAIN; CHARSET=US-ASCII

|>
|> Hello:
|>   I can't seem to get math functions referenced on my gcc 2.7.2
|> compiler running on Linux. Since I was the person who installed the
|> software, I wonder if my compiler is broken or the problem is more my
|> inexperience in C. When I compile the attached demo program, I get the
|> following:
|> /root/math.c:13: undefined reference to `log'
|> /root/math.c:15: undefined reference to `exp'
|> My command line was: gcc -g -lm -o math.o math.c
|> I thought the math library would be linked by using -lm above. Any help
|> for a novice would be appreciated. The libraries have been ldconfig-ed.

Close, but you're putting -lm in the wrong place.  Gcc searches libraries
in the order they're indicated on the command line, so the way you've
typed the command, gcc searches libm.so BEFORE math.c is compiled,
so no external references are resolved, then math.c is compiled and there's
no libm around to resolve the references (gcc doesn't "remember" the -lm
before math.c).  What you want to do is put the switch *after* the code
you're compiling, i.e.

gcc -g math.c -lm

kent

(P.S., why are you naming the executable math.o?  That name implies
that math.o is the object file corresponding to math.c, but what you're
doing is compiling AND linking math.c.  If you want to generate the object
file for math.c, you can do gcc -g -c math.c,  here -lm is irrelevant because
math.c is not linked.)

|> ________________________________________________________________________________
|>   Larry W. Roberts                                SeaTac, WA.

|>   http://www.oz.net/~lroberts     http://lroberts.oz.net

|> ________________________________________________________________________________
|>
|> --1036932765-789891942-846583923=:269
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|> Content-Transfer-Encoding: BASE64

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|> --1036932765-789891942-846583923=:269--

:
:       I can't seem to get math functions referenced on my gcc 2.7.2
: compiler running on Linux. Since I was the person who installed the
: software, I wonder if my compiler is broken or the problem is more my
: inexperience in C. When I compile the attached demo program, I get the
: following:
: /root/math.c:13: undefined reference to `log'
: /root/math.c:15: undefined reference to `exp'
: My command line was: gcc -g -lm -o math.o math.c
: I thought the math library would be linked by using -lm above. Any help
: for a novice would be appreciated. The libraries have been ldconfig-ed.

You are right that -lm links the math library. Unfortunately it is
important where in your command line it appears. Command line options
for the linker have to be placed at the very end of the command line.
Simply try "gcc -g -o math.o math.c -lm". Calling your executable file
"math.o" does not seem to be a very good idea, because UNIX (and Linux)
uses the .o suffix for object files which are generated during compiling.
Generating an executable "math.o" from an object file "math.o" might
result in some trouble.

- Jochen

--
Jochen Schoof   (http://www-info2.informatik.uni-wuerzburg.de/staff/joscho)
___________________________________________________________________________
\_Address __/  Informatik II, Uni Wuerzburg, Am Hubland, D-97074 Wuerzburg

    ues12:~/tmp 58> uname -a
    Linux ues12 1.2.13 #4 Sat Oct 7 22:32:36 MET 1995 i486
    ues12:~/tmp 59> cat test.c
    #include <stdio.h>
    #include <math.h>

    main()
    {
        printf("%f\n", log(1.0) + exp(0.0));
        return 0;
    }
    ues12:~/tmp 60> gcc test.c -lm
    ues12:~/tmp 61> ./a.out
    1.000000

My suggestion: go to linux support newsgroup.  Your problem has nothing
to do with the topic of this newsgroup: the C programming language.

Dan
--
Dan Pop
CERN, CN Division

Mail:  CERN - PPE, Bat. 31 1-014, CH-1211 Geneve 23, Switzerland