Home/ Questions/Q 8908537
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-06-15T03:12:20+00:00

Can someone explain why the following code produces different output for the test4 variable

  • 0

Can someone explain why the following code produces different output for the test4 variable compared to the other 3? I have checked this with gcc version 4.2.1 and 4.5.3 (and others in between).

Maybe I am missing something obvious, but it looks pretty straightforward…

#include <stdio.h>
#include <complex.h>

main()
{
    double complex test1, test2, test3, test4;

    test1 = 81141117.0;
    test2 = 81141117.0 + I * 0;
    test3 = 81141117 + I * 0.0;
    test4 = 81141117 + I * 0;
    printf("%ld + %ld I, %ld + %ld I, ", (long)creal(test1), (long)cimag(test1), (long)creal(test2), (long)cimag(test2));
    printf("%ld + %ld I, %ld + %ld I\n", (long)creal(test3), (long)cimag(test3), (long)creal(test4), (long)cimag(test4));
}

Output:

81141117 + 0 I, 81141117 + 0 I
81141117 + 0 I, 81141120 + 0 I

It appears that test4, having integer terms only, is being promoted to a float instead of the declared double, and round-off is coming into play.

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    I’ve tested GCC 4.7.1 on Mac OS X 10.7.5 (home-built GCC), and also the system provided /usr/bin/gcc and /usr/bin/clang, and they all produce the same result you report.

    I agree with your analysis that there must be a conversion to float instead of double when both terms of the complex number are expressed as integers. The C 2011 standard includes the example:

    ISO/IEC 9899:2011 §6.7.9 Initialization

    ¶24 EXAMPLE 1 Provided that <complex.h> has been #included, the declarations

    int i = 3.5;
double complex c = 5 + 3 * I;

    define and initialize i with the value 3 and c with the value 5. 0 + i3. 0.

    This clearly indicates that you should be able to write integer expressions and obtain a valid double complex (though examples are non-normative). However, it does not address whether the integer value should be converted to float before being converted to double, but there’s nowhere else in the language that would do that automatically (obviously, you can force it), so it is unlikely to be the intended interpretation.

    On the whole, I think this is likely to be a bug that could be reported to the GCC team (and that’s not something I advocate doing lightly). If you run gcc --help, the output ends with the message:

    For bug reporting instructions, please see:
<http://gcc.gnu.org/bugs.html>

    I would extend your example as shown below (or, rather, I did extend your example as shown below):

    #include <stdio.h>
#include <complex.h>

int main(void)
{
    double complex test1, test2, test3, test4, test5, test6, test7, test8;

    test1 = 81141117.0;
    test2 = 81141117.0 + I * 0;
    test3 = 81141117 + I * 0.0;
    test4 = 81141117 + I * 0;
    test5 = (float)81141117 + I * 0;
    test6 = 81141117 + I * (float)0;
    test7 = 81141117.F + I * 0;
    test8 = 81141117 + I * 0.F;
    printf("%ld + %ld I, %ld + %ld I\n", (long)creal(test1), (long)cimag(test1), (long)creal(test2), (long)cimag(test2));
    printf("%ld + %ld I, %ld + %ld I\n", (long)creal(test3), (long)cimag(test3), (long)creal(test4), (long)cimag(test4));
    printf("%ld + %ld I, %ld + %ld I\n", (long)creal(test5), (long)cimag(test5), (long)creal(test6), (long)cimag(test6));
    printf("%ld + %ld I, %ld + %ld I\n", (long)creal(test7), (long)cimag(test7), (long)creal(test8), (long)cimag(test8));
}

    The output I got was:

    81141117 + 0 I, 81141117 + 0 I
81141117 + 0 I, 81141120 + 0 I
81141120 + 0 I, 81141120 + 0 I
81141120 + 0 I, 81141120 + 0 I

    As you can see, the last two lines, where there was an explicit (float) cast or explicit float constants, produce the same results as your problematic line (but the values are legitimate).

    You might (or might not) want to experiment with the CMPLX, CMPLXF (and maybe CMPLXL) macros; I’d only add them to the reported example if they produced ‘interesting’ values for some definition of ‘interesting’.

    Compiler version numbers:

    $ /usr/gcc/v4.7.1/bin/gcc --version
gcc (GCC) 4.7.1
Copyright (C) 2012 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ /usr/bin/gcc --version
i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.11.00)
Copyright (C) 2007 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

$ clang --version
Apple clang version 4.1 (tags/Apple/clang-421.11.65) (based on LLVM 3.1svn)
Target: x86_64-apple-darwin11.4.2
Thread model: posix
$
    • 0