Debian Issues

now

-- 
Amit Kumar Saha
[URL]:https://amitsaha.in.googlepages.com

>
> 2. GCC reports linker error while using "sin", "cos", etc. However it
> is solved when i append a "-lm" during compilation
>

> Hi Amit,
>
> On 17/08/07, Amit Kumar Saha <amitsaha.in@gmail.com> wrote:
> >
> > 2. GCC reports linker error while using "sin", "cos", etc. However it
> > is solved when i append a "-lm" during compilation
> >
>
> You are including the maths libraries when you compile your program
> with "-lm" (think Link Maths)
>
> You can include this line in your source file file to do the same
> (omit the -lm flag in this case)
>
> #include <math.h>

> More can be found here: https://aplawrence.com/Linux/c_compiling_linux.html

#include <stdio.h>
#include <math.h>
main ()
{
printf("pi = %.5f\n", 4 * atan(1.0));
}

$ gcc -o pi pi.c
/tmp/ccLmoMxc.o(.text+0x33): In function `main':
: undefined reference to `atan'
collect2: ld returned 1 exit status

gcc -o pi -lm pi.c

> [...]
> On reflection, I included the information needed to access the
> libraries - the header file, but not told the linker to include them
> at compilation. Doh!

CC =3D colorgcc
CFLAGS =3D -I. -I/usr/local/pgsql/include -DDEBUG -g
LDFLAGS=3D -L/usr/local/pgsql/lib -lpq -lpcre

# For Intel ICC and Core 2 Duo use:
# /opt/intel/cc/9.1.042/bin/icc -I. -xP -DDEBUG -o dailywtl dailywtl.c

DEPS =3D dailywtlpg.h
OBJDAILYWTL =3D dailywtlpg.o

%.s: %.c $(DEPS)
	$(CC) -S -o $@ $< $(CFLAGS)

%.o: %.c $(DEPS)
	$(CC) -c -o $@ $< $(CFLAGS)

dailywtlpg: $(OBJDAILYWTL)
	$(CC) -o $@ $^ $(CFLAGS) $(LDFLAGS)

all: dailywtlpg

> The first lines define CC, the compiler, and the flags being used for
> the compiler and linker runs. Put all desirable include paths into
> CFLAGS and put the paths for the corresponding libraries into LDFLAGS.
> Both will be used when calling the compiler.
>
> DEPS lists all dependencies, OBJDAILYWTL lists all object files. "%.s"
> defines a rule to create .s files (assembler listings, I am curious ;),
> "%.o" defines a rule for creating object files. The last two rules
> explain how make should create the dailywtlpg binary and what needs to
> be done when "make all" is executed.
>

> Hi Jim,
>
> thanks for the clarification, I should really have read more of the
> link I mentioned.
>
> It's been years since I did anything in C and I thought I knew the answer.
>
> On reflection, I included the information needed to access the
> libraries - the header file, but not told the linker to include them
> at compilation. Doh!

> On Fri, 17 Aug 2007, Steve Brown wrote:

> > On reflection, I included the information needed to access the
> > libraries - the header file, but not told the linker to include them
> > at compilation. Doh!
>
> It's a very common misconception. Actually including the header files
> doesn't really "access" the libraries. It generally simply includes the
> library function prototypes, and any macros/defines that the library
> writer documents or requires as useful for calling the library functions.
> These files are simple C code text files in /usr/include (usually) - take
> a look at say /usr/include/math.h - there is no C code - just a bunch of
> (admittedly complicated) #defines and typedefs.
>
> The functions are in the library object files and the c compiler has to be
> told which library to link into the program with the '-lxxx' compiler
> flag. The precompiled library will be found in one of a series of
> "prefined" library directories, usually /lib /usr/lib , as the file
> "libxxx.so" or for non-shared libraries "libxxx.a".
>

> Er... you can and maybe should put that include in your source, but it
> is NOT a substitute for linking with the correct library.
>
> as this page ...
>
> > More can be found here: https://aplawrence.com/Linux/c_compiling_linux.html
>
> actually mentions...
>
> "
> ... And there may even be more to do. Look at this:
>
> #include <stdio.h>
> #include <math.h>
> main ()
> {
> printf("pi = %.5f\n", 4 * atan(1.0));
> }
>
> We looked up the atan function and found it needs math.h. We've told the
> compliler to include it. Trust me, the program is right. But..
>
> $ gcc -o pi pi.c
> /tmp/ccLmoMxc.o(.text+0x33): In function `main':
> : undefined reference to `atan'
> collect2: ld returned 1 exit status
>
> What we need here is a special compiler flag:
>
> gcc -o pi -lm pi.c
>
> "lm" tells it to link in the math libraries. Math libraries? Sure, you
> didn't think C by itself knows how to do arc tangents, did you? C by by
> itself doesn't know much. Most of its functions - the things you can use -
> come from external libraries that get linked in. How would you know when
> you need to use a special library? It's just something you have to learn.
> Usually, if there is a special linker flag, the man page will have told
> you. In the case of atan, it does not, so you just need to remember that
> you need "-lm" when you see "#include <math.h>" mentioned."

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

int main(){
printf("%d\n",pow(3,2));
return 0;
}

OUTPUT:

amit@etch-desktop:~/junk$ gcc MathDemo.c
amit@etch-desktop:~/junk$ ./a.out
0

-- 
Amit Kumar Saha
[URL]:https://amitsaha.in.googlepages.com

> [...]
> Consider the following program :
> #include <stdio.h>
> #include <math.h>
> int main(){
> printf("%d\n",pow(3,2));
> return 0;
> }
> OUTPUT:
> amit@etch-desktop:~/junk$ gcc MathDemo.c
> amit@etch-desktop:~/junk$ ./a.out
> 0
> Could some one please explain the above output?

lynx@agamemnon:/tmp $ gcc -Wall math.c=20
math.c: In function =E2=80=98main=E2=80=99:
math.c:5: warning: format =E2=80=98%d=E2=80=99 expects type =E2=80=98int=E2=
=80=99, but argument 2 has
type =E2=80=98double=E2=80=99
lynx@agamemnon:/tmp=20

> lynx@agamemnon:/tmp $ gcc -Wall math.c
> math.c: In function 'main':
> math.c:5: warning: format '%d' expects type 'int', but argument 2 has
> type 'double'
> lynx@agamemnon:/tmp
>
> pow() returns floating point data, but your format string expects an
> integer type, so I suspect that the conversion inside printf() fails
> because it isn't considering all the bits. If you change the format
> string to "%f\n" everything's fine.

-- 
Amit Kumar Saha
[URL]:https://amitsaha.in.googlepages.com

> [...]
> My C knowledge so far would make me believe, that in a situation where
> "%d" is used instead of "%f" the numbers after the "." would be
> truncated, and only the integral portion displayed.

> However as it has turned out, I can never call myself equipped with
> adequate C knowledge. As a matter of fact, I always felt C is too
> mysterious to be mastered fully by any one, we can only try to master
> some part of it 

> Knowing the assembly language of the CPU where the C program runs helps,
> too. ;)

-- 
Amit Kumar Saha
[URL]:https://amitsaha.in.googlepages.com

> Someday, when I have lots of time to play around and the only thing in
> the world to do is C programming, I swear I shall decipher each
> instruction upto assembly language level that are executed even when I
> say "Hello World" using C statements!

	gcc -S hello.c
	cat hello.s

> Hello,
>
> On Sat, 18 Aug 2007, Amit Kumar Saha wrote:
> > Someday, when I have lots of time to play around and the only thing in
> > the world to do is C programming, I swear I shall decipher each
> > instruction upto assembly language level that are executed even when I
> > say "Hello World" using C statements!
>
>         gcc -S hello.c
>         cat hello.s
>

-- 
Amit Kumar Saha
[URL]:https://amitsaha.in.googlepages.com

> Funny it is, I have been programming C using GCC for past 4 years or
> so, this is for the first time I am using a trigonometric function and
> I learn that I have to link the math library "-lm" to be able to
> actually use them. {Moron me  }

> By the way, is "pow ()" also defined in "math.h" ? If yes, then it
> compiles correctly  for me without adding the "-lm".

pwr.c.text+0x50): undefined reference to `pow'

> Consider the following program :
>
> #include <stdio.h>
> #include <math.h>
>
> int main(){
> printf("%d\n",pow(3,2));
> return 0;
> }
>

> OUTPUT:
>
> amit@etch-desktop:~/junk$ gcc MathDemo.c
> amit@etch-desktop:~/junk$ ./a.out
> 0
>
> Could some one please explain the above output?
>
> Regards
> --
> Amit Kumar Saha
> [URL]:https://amitsaha.in.googlepages.com
>
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>
>
> I checked the man page on my (slightly old) debian box, and while it
> mentions including math.h, it makes no mention of linking with libm
> which I would expect it to do.

-- 
Amit Kumar Saha
[URL]:https://amitsaha.in.googlepages.com

>
> You are including the maths libraries when you compile your program
> with "-lm" (think Link Maths)
>
> You can include this line in your source file file to do the same
> (omit the -lm flag in this case)
>
> #include <math.h>

-- 
Amit Kumar Saha
[URL]:https://amitsaha.in.googlepages.com

> Actually I had already included the header file before I reported the error.

> Hi Amit,
>
> On 17/08/07, Amit Kumar Saha <amitsaha.in@gmail.com> wrote:
> > 2. GCC reports linker error while using "sin", "cos", etc. However it
> > is solved when i append a "-lm" during compilation
>
> You are including the maths libraries when you compile your program
> with "-lm" (think Link Maths)
>
> You can include this line in your source file file to do the same
> (omit the -lm flag in this case)
>
> #include <math.h>