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V1039. Character escape is used in mult…
Analyzer Diagnostics
General Analysis (C++)
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Contents

V1039. Character escape is used in multicharacter literal. This causes implementation-defined behavior.

Jun 20 2019

The analyzer has detected a multicharacter literal containing both characters and character escapes.

Multicharacter literals are implementation-defined, so different compilers handle them differently. For example, GCC and Clang evaluate them based on the order of characters in the literal, while MSVC moves the characters around depending on their type (ordinary or escape).

Consider the following example. The code below will behave differently when complied with different compilers:

#include <stdio.h>

void foo(int c)
{
  if (c == 'T\x65s\x74')                       // <= V1039
  {
    printf("Compiled with GCC or Clang.\n");
  }
  else
  {
    printf("It's another compiler (for example, MSVC).\n");
  }
}

int main(int argc, char** argv)
{
  foo('Test');
  return 0;
}

The program could output different messages depending on what compiler it has been compiled with.

This will not affect a project that uses one particular compiler, but you may encounter problems when trying to port it. For this reason, multicharacter literals should be replaced with simple numeric constants. For example, 'Test' should be changed to '0x54657374'.

The variation across compilers in how they treat multicharacter literals can be shown using sequences of 3 and 4 characters, for example, 'GHIJ' and 'GHI', and having the program output their representation in memory after compilation.

Output after compilation with Visual C++:

Hex codes are: G(47) H(48) I(49) J(4A)
              'GHIJ' : JIHG
  '\x47\x48\x49\x4A' : GHIJ
     'G\x48\x49\x4A' : HGIJ
        'GH\x49\x4A' : JIHG
        'G\x48I\x4A' : JIHG
           'GHI\x4A' : JIHG
               'GHI' : IHG
      '\x47\x48\x49' : GHI
            'GH\x49' : IHG
         '\x47H\x49' : HGI
            '\x47HI' : IHG

Output after compilation with GCC or Clang:

Hex codes are: G(47) H(48) I(49) J(4A)
              'GHIJ' : JIHG
  '\x47\x48\x49\x4A' : JIHG
     'G\x48\x49\x4A' : JIHG
        'GH\x49\x4A' : JIHG
        'G\x48I\x4A' : JIHG
           'GHI\x4A' : JIHG
               'GHI' : IHG
      '\x47\x48\x49' : IHG
            'GH\x49' : IHG
         '\x47H\x49' : IHG
            '\x47HI' : IHG
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