trunc function returns a double? :confused:

zahadumy · 12-03-2005, 08:02 PM

The question is I need to use trunc function, but i saw in the manual page this:

Code:

NAME
       trunc, truncf, truncl - round to interger, towards zero

SYNOPSIS
       #include <math.h>

       double trunc(double x);

How come it returns a double?

Shouldn't it return a int? I tried to make a conversion to int (something like this)

Code:

(int)trunc(sqrt(n))

but it doesn't work. Thank you.

Ygrex · 12-03-2005, 08:17 PM

Quote:

Originally Posted by zahadumy

Code:

(int)trunc(sqrt(n))

but it doesn't work. Thank you.

It must work

What did you mean?
Did you "#define __USE_ISOC99" before "#include math.h"? It will help you if you see some warnings about implicit declarations.

zahadumy · 12-03-2005, 08:40 PM

Quote:

Originally Posted by Ygrex

Did you "#define __USE_ISOC99" before "#include math.h"?

What do I have to include before that define? I used these functions a lot, but I've never got these error messages, they worked fine until now.

This is a simple program and the error messages I get when I try to compile it.

Code:

[cosmin@localhost fara sincronizari]$ cat test.c
#include <stdio.h>
#include <math.h>

int
main ()
{
  int n = 5, x;
  x = (int) trunc (sqrt (n));
  printf ("Radical din n: %d\n", x);
}
[cosmin@localhost fara sincronizari]$ gcc -o test test.c
/tmp/cc8bYcrs.o(.text+0x31): In function `main':
: undefined reference to `sqrt'
/tmp/cc8bYcrs.o(.text+0x43): In function `main':
: undefined reference to `trunc'
collect2: ld returned 1 exit status

Thank you.

Ygrex · 12-03-2005, 08:47 PM

Ok, you have no need in 'define', but:

Code:

gcc -o test test.c -lm

zahadumy · 12-03-2005, 08:58 PM

Quote:

Originally Posted by Ygrex

Code:

gcc -o test test.c -lm

It works. Thanks.
But why? What is this? Are you sure this is what I need? I told you I used these a lot, but I think I have never used the functions in math.h in C (only C++), and there they work. Is it possible to put some code instead of "-lm" option? Thank you.

Ygrex · 12-03-2005, 09:35 PM

Well, you tried to use function trunc. You know its name. But do you know its code? What the algorithm does it execute whenevre you use 'trunc()'?
You can divide the compilation process into two steps. The first - creating of the object file. It contains your code and etry-points - the functions you have not written by yourself but you use them, they are declared in the included headers.
The second stage - linking your object file with different libraries, which contain all necessary functions (e.g. trunc() or sqrt()). It defines where entry-points should find their according function codes (exact algorithms). These functions are stored in the binary state in the shared libraries. I know that anything from math.h is in the libm.so library. So you should point the linker (an application, which executes the second stage of the compiling) to the appropriate library (libm.so in you case). You can do it with the '-l' option followed by the library name.
Library name could be either the explicit path (/usr/lib/libm.so) or implict, in the last case linker will add 'lib' prefix and '.so' extension to the name you specify, for any library begins with 'lib' and ends with '.so' (and, maybe a version number as well).
The most important thing. Where can one find which library contains the function one find in any header? One should read man-page abit more attentively. In your case see trunc(3) manpage, the SYNOPSIS finishes with:

Quote:

Link with -lm.

zahadumy · 12-03-2005, 10:20 PM

Quote:

Originally Posted by Ygrex

The most important thing. Where can one find which library contains the function one find in any header? One should read man-page abit more attentively. In your case see trunc(3) manpage, the SYNOPSIS finishes with:

Quote:

Link with -lm.

On Fedora Core 3 and Slackware 10.1, if you type "man trunc" you will see this. Thank you.