On Mon, Mar 22, 2004 at 09:28:24AM +0800, Didier Casse wrote: > On 21/03/04, at 18:36 -0600, stucklenp <stucklenp@xxxxxxxxxxx> wrote: > > > > I routinely program using C and compile with gcc. My problem is that I need > > some math log functions--ln, log10, and log2--and these functions are not > > included in math.h from what I can determine. Can anyone suggest how I can > > get these functions? > > All the functions that you're saying are included. Have a look here for > math.h fucntions: > > http://www.opengroup.org/onlinepubs/007908799/xsh/math.h.html > > Do a "man function" to find out more about the individual functions. e.g > "man log". > > My guess is that you haven't linked your source code to the math > library. (-lm option) > > i.e > > gcc -lm myfile.c -o myfile Yes... -lm is needed by the link editor (ld). $ cc t.c /tmp/ccISAzDN.o(.text+0x23): In function `main': : undefined reference to `log' collect2: ld returned 1 exit status and $ cc t.c -lm returns success and a.out does the expected. $ a.out log(10.0) is 2.302585 $ grep 2.302585 /usr/include/math.h # define M_LN10 2.30258509299404568402 /* log_e 10 */ I wondered how one could discover this if it was unknown... The "man log" command found the man page for log()/ EXP(3) The 3 tells me it is in section three of the man pages. Someone once told me that the "intro" man pages were worth reading and full of nifty stuff. I tried "man 3 intro" and found a clue. As I scanned down (3M) These functions are contained in the arithmetic library libm. They are used by the f77(1) FORTRAN compiler by default, but not by the cc(1) C compiler, which needs the option -lm. So for a "C" program -lm is the magic compile line addition. And that is good to know because, I really do not want to resort to the silly and brutal: (for i in /usr/lib/*.so* # might need to do a find... do echo ================================ echo $i nm $i | grep log$ echo ---------------------- done)| less to find the symbol the hard way in a nasty long list of junk. ================================ /usr/lib/libm.so 0000b020 W clog 0000b020 t __clog 00007850 t __ieee754_log 00009fa0 W log <-----found it. 00009fa0 t __log ---------------------- I guess the intro man pages are worth a second/third look. ;-) Summary: Good question, the answer is not obvious if you don't know it. -- T o m M i t c h e l l /dev/null the ultimate in secure storage.
