New: Do not need to link against -lm for fabs*()

https://bugzilla.kernel.org/show_bug.cgi?id=60749

Bug ID: 60749
Summary: Do not need to link against -lm for fabs*()
Product: Documentation
Version: unspecified
Hardware: All
OS: Linux
Status: NEW
Severity: low
Priority: P1
Component: man-pages
Assignee: documentation_man-pages-ztI5WcYan/[email protected]
Reporter: [email protected]
Regression: No

SYNOPSIS

       #include <math.h>

       double fabs(double x);
       float fabsf(float x);
       long double fabsl(long double x);

       Link with -lm.

At least on RHEL5+ one does not appear to need to link with -lm.

# cat fabs.c
#include <math.h>
#include <stdlib.h>
#include <stdio.h>

int main(int argc, char **argv)
{
printf("%f\n",fabs(atof(argv[1]))); printf("%f\n",fabsf((float)atof(argv[1]))); printf("%Lf\n",fabsl((long double)atof(argv[1]))); return 0;
}

# gcc -g -Wall fabs.c
# ./a.out -2.5
2.500000
2.500000
2.500000
# gcc -v
Using built-in specs.
Target: x86_64-redhat-linux
Configured with: ../configure --prefix=/usr --mandir=/usr/share/man
--infodir=/usr/share/info --enable-shared --enable-threads=posix
--enable-checking=release --with-system-zlib --enable-__cxa_atexit
--disable-libunwind-exceptions --enable-libgcj-multifile
--enable-languages=c,c++,objc,obj-c++,java,fortran,ada --enable-java-awt=gtk
--disable-dssi --disable-plugin
--with-java-home=/usr/lib/jvm/java-1.4.2-gcj-1.4.2.0/jre --with-cpu=generic
--host=x86_64-redhat-linux
Thread model: posix
gcc version 4.1.2 20080704 (Red Hat 4.1.2-54)

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https://bugzilla.kernel.org/show_bug.cgi?id=60749

--- Comment #1 from Jeyaram <[email protected]> --- Following code also works fine without linking with -lm

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

int main(int argc, char **argv)
{
printf("%lf\n", pow(5,3));
return 0;
}

Actually, libc some how includes the math library
"/lib/i386-linux-gnu/libm.so.6".

If this file is not available, then following error message is displayed.

/usr/lib/gcc/i686-linux-gnu/4.7/cc1: error while loading shared libraries:
libm.so.6: cannot open shared object file: No such file or directory

I tried it by renaming libm.so.6.

Solution:

I request not to remove the line "Link with -lm". This is mandatory.

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https://bugzilla.kernel.org/show_bug.cgi?id=60749

--- Comment #2 from Jeyaram <[email protected]> --- Forgot to mention:

uname -a

Linux jeyaram-VirtualBox 3.8.0-25-generic #37-Ubuntu SMP Thu Jun 6 20:47:30 UTC 2013 i686 i686 i686 GNU/Linux

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https://bugzilla.kernel.org/show_bug.cgi?id=60749

Alan <alan-qBU/[email protected]> changed:

What |Removed |Added
----------------------------------------------------------------------------

             Status|NEW                         |RESOLVED
                 CC|                            |alan-qBU/[email protected]
         Resolution|---                         |INVALID

--- Comment #3 from Alan <alan-qBU/[email protected]> --- Use of -lm is correct even if it happens not to be needed

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https://bugzilla.kernel.org/show_bug.cgi?id=60749

Alan <alan-qBU/[email protected]> changed:

What |Removed |Added
----------------------------------------------------------------------------
Status|RESOLVED |CLOSED

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https://bugzilla.kernel.org/show_bug.cgi?id=60749

--- Comment #4 from Michael Kerrisk <[email protected]> --- Walter Harms pointed out on linux-man that what is going on here is that
gcc implements some functions (many of them are math functions) as built-ins. There is a list here: http://gcc.gnu.org/onlinedocs/gcc/Other-Builtins.html
When the function is a compiled as a gcc built-in, then it is not
necessary link against the corresponding library.

However, I'm unwilling to add that level of detail about a specific compiler to the various manual pages.

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