[Bug 2012076] Re: printf function outputs different results for same arguments

[Simon Chopin]

Note that the issue is actually not with the glibc printf
implementation, but directly in the compiler, which knows about the
details of printf specification, notably in order to give you proper
warnings, and chooses not to add the problematic argument.

I'm guessing that if you were to use a "dumber" compiler for your code
but still linked against glibc it could result in the same thing as
you're seeing with MSVC.

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https://bugs.launchpad.net/bugs/2012076

Title:
  printf function outputs different results for same arguments

Status in glibc package in Ubuntu:
  Invalid

Bug description:
  A union is defined for storing different datatype values; and all
  values' data lengths are the same.

  Since the data lengths of the union members are the same, i suppose
  that printf function can have specified output format with specifying
  any union member.

  The scenario can be seen in the following sample code. Variable v is
  assigned with a floating number in the beginning; to output this
  value, the output format is specified as "%f" in the printf function
  format string, but the value is given by v.p, which is a void * type,
  as next argument. See the output results of the printf("v.p_f = %f\n",
  v.p); statement, there are 3 different result.

  //===========================
  #include <stdio.h>
  #include <stdlib.h>

  typedef union 
  {
      double f;
      long int i;
      void *p;
  } type_t;

  void func2(int a, int b, type_t v);

  int main(void)
  {
      type_t v;
      unsigned char *p;

  
      printf("sizeof(double)   = %ld\n", sizeof(double));
      printf("sizeof(long int) = %ld\n", sizeof(long int));
      printf("sizeof(void *)   = %ld\n", sizeof(void *));
      printf("sizeof(type_t)   = %ld\n", sizeof(type_t));

      v.f = 123.456789;

      printf("\nIn %s()\n", __FUNCTION__);
      printf("v.p_f = %f\n",  v.p);
      printf("v.i_x = %lX\n", v.i);

      printf("cont. = ");
      p = (unsigned char *) &v;
      for (int i = 7; i >= 0; --i)
          printf("%02X", p[i]);
      printf("\n");

      printf("v.p_p = %p\n",  v.p);
      printf("v.i_d = %ld\n", v.i);
      printf("v.f_f = %f\n",  v.f);
      printf("v.p_f = %f\n",  v.p);

      func2(1, 2, v);

      printf("\nIn %s()\n", __FUNCTION__);
      printf("v.i_d = %ld\n", v.i);
      printf("v.p_f = %f\n",  v.p);
      printf("v.f_f = %f\n",  v.f);
      printf("v.p_f = %f\n",  v.p);
      printf("\n");

  #ifdef __GNUC__
      printf("__GNUC__\n");
      printf("__VERSION__ = %s\n", __VERSION__);
  #endif

      fflush(stdout);

  #ifdef __linux__
      system("`find /usr/lib -executable -name *libc.so*` --version");
      printf("\n");
  #endif
      return (0);
  }

  void func2(int a, int b, type_t v)
  {
      unsigned char *p;

      printf("\nIn %s()\n", __FUNCTION__);
      printf("v.p_f = %f\n",  v.p);
      printf("v.i_x = %lX\n", v.i);

      printf("cont. = ");
      p = (unsigned char *) &v;
      for (int i = 7; i >= 0; --i)
          printf("%02X", p[i]);
      printf("\n");

      printf("v.p_p = %p\n",  v.p);
      printf("v.i_d = %ld\n", v.i);
      printf("v.f_f = %f\n",  v.f);
      printf("v.p_f = %f\n",  v.p);
  }

  
  //==================================
  sizeof(double)   = 8
  sizeof(long int) = 8
  sizeof(void *)   = 8
  sizeof(type_t)   = 8

  In main()
  v.p_f = 0.000000
  v.i_x = 405EDD3C07EE0B0B
  cont. = 405EDD3C07EE0B0B
  v.p_p = 0x405edd3c07ee0b0b
  v.i_d = 4638387916139006731
  v.f_f = 123.456789
  v.p_f = 123.456789

  In func2()
  v.p_f = 0.000000
  v.i_x = 405EDD3C07EE0B0B
  cont. = 405EDD3C07EE0B0B
  v.p_p = 0x405edd3c07ee0b0b
  v.i_d = 4638387916139006731
  v.f_f = 123.456789
  v.p_f = 123.456789

  In main()
  v.i_d = 4638387916139006731
  v.p_f = -5486124068793688683255936251187209270074392635932332070112001988456197381759672947165175699536362793613284725337872111744958183862744647903224103718245670299614498700710006264535590197791934024641512541262359795191593953928908168990292758500391456212260452596575509589842140073806143686060649302051520512.000000
  v.f_f = 123.456789
  v.p_f = 123.456789

  __GNUC__
  __VERSION__ = 9.4.0
  GNU C Library (Ubuntu GLIBC 2.31-0ubuntu9.9) stable release version 2.31.
  Copyright (C) 2020 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.
  Compiled by GNU CC version 9.4.0.
  libc ABIs: UNIQUE IFUNC ABSOLUTE
  For bug reporting instructions, please see:
  <https://bugs.launchpad.net/ubuntu/+source/glibc/+bugs>.

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