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.
