[Bug c++/85700] New: Spurious -Wstringop-truncation warning with strncat

[lopresti at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85700

            Bug ID: 85700
           Summary: Spurious -Wstringop-truncation warning with strncat
           Product: gcc
           Version: 8.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lopresti at gmail dot com
  Target Milestone: ---

Test program:

---
#include <cstring>
#include <cstdio>

char buf1[256];

void append(const char *s)
{
  std::strncat(buf1, s, sizeof(buf1) - strlen(buf1) - 1);
}

void doit(int err)
{
  char tmp[256];
  std::snprintf(tmp, sizeof(tmp), " errno %d (%s)",
                err, std::strerror(err));
  append(tmp);
}
---

Compile with "g++ -O3 -Wall -c test.cc".


Expected results:

No warning, as the documentation says for this idiomatic use of strncat().


Actual results:

In function ‘void append(const char*)’,
    inlined from ‘void doit(int)’ at test.cc:16:9:
test.cc:8:15: warning: ‘char* strncat(char*, const char*, size_t)’ output may
b\
e truncated copying between 0 and 255 bytes from a string of length 255
[-Wstri\
ngop-truncation]
   std::strncat(buf1, s, sizeof(buf1) - strlen(buf1) - 1);
   ~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


Note that a similar program in C does not trigger the warning.