thad@cup.portal.com (Thad P Floryan) (10/07/90)
lath@geocub.greco-prog.fr (Laurent Lathieyre) in <279@geocub.greco-prog.fr>
writes:
here a piece of C code which provides some strange
results :
double d;
printf("%f\n",drand(48));
d=drand48();
printf("%f\n",d);
output:
0.899854
1071863078.000000
according to the manual section 3, drand48() is supposed to return a
double type value in [0.0,1.0]...
Since you have the manual (or at least section 3), look at ``INTRO(3)'' which
mentions you should ``#include <math.h>'' in which is contained a declaration
for "drand48()" as its proper double type (at least on most systems; on some,
the assertion for drand48() is missing from math.h).
Barring that, using an ANSI-compliant C compiler with forced prototypes will
cause drand48() to be properly declared.
Also, the lint library /usr/lib/llib-lc has "double drand48(){return (0.0);}"
Barring the use or presence of any of the above on your system, if you change
your program to:
double d, drand48();
printf("%f\n", drand48());
d=drand48();
printf("%f\n", d);
you'll find the expected results displayed. BTW, there is NO 'drand(48)' as
in your posted example.
As a general note, when you have the manual(s) and you see a function's
synopsis per:
``datatype functionname()''
that means that you must explicitly declare the functionname as "datatype" if
you choose not to use the appropriate *.h file or if the function is not so
defined in the *.h file.
Thad Floryan [ thad@cup.portal.com (OR) ..!sun!portal!cup.portal.com!thad ]ted@eslvcr.wimsey.bc.ca (Ted Powell) (10/07/90)
In article <279@geocub.greco-prog.fr> lath@geocub.greco-prog.fr (Laurent Lathieyre) writes: > >here a piece of C code which provides some strange >results : >double d; >printf("%f\n",drand(48)); >d=drand48(); >printf("%f\n",d); > >output: >0.899854 >1071863078.000000 > >according to the manual section 3, drand48() is supposed to return a >double type value in [0.0,1.0]... This is exactly why you get absurd results if you let the type default to int. The function returns a double, probably in a floating point register, and your program's generated code picks up garbage from wherever integer results are normally returned. To tell the compiler that drand48 returns a double value, you need a line like: double drand48(); otherwise the C compiler will helpfully (hah!) default the type to int. Some library functions will have an associated header file that contains a declaration of the function. Since this function doesn't, one needs to determine the necessary declaration from the SYNOPSIS section of the man page. drand(48) above should be drand48() -- when posting code, it's generally best to extract it from the file you actually compiled and ran. The returned range is [0.0, 1.0), not [0.0, 1.0] -- i.e. you are guaranteed not to get the value 1.0 returned. -- ted@eslvcr.wimsey.bc.ca ...!ubc-cs!van-bc!eslvcr!ted (Ted Powell)