c++gccstrncpygcc8

gcc-8 -Wstringop-truncation what is the good practice?

GCC 8 added a -Wstringop-truncation warning. From https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82944 :

The -Wstringop-truncation warning added in GCC 8.0 via r254630 for bug 81117 is specifically intended to highlight likely unintended uses of the strncpy function that truncate the terminating NUL charcter from the source string. An example of such a misuse given in the request is the following:

char buf[2];

void test (const char* str)
{
  strncpy (buf, str, strlen (str));
}

I get the same warning with this code. 

strncpy(this->name, name, 32);

warning: 'char* strncpy(char*, const char*, size_t)' specified bound 32 equals destination size [-Wstringop-truncation`]

Considering that this->name is char name[32] and name is a char* with a length potentially greater than 32. I would like to copy name into this->name and truncate it if it is greater than 32. Should size_t be 31 instead of 32? I'm confused. It is not mandatory for this->name to be NUL-terminated.

Solution

  • This message is trying to warn you that you're doing exactly what you're doing. A lot of the time, that's not what the programmer intended. If it is what you intended (meaning, your code will correctly handle the case where the character array will not end up containing any null character), turn off the warning.

    If you do not want to or cannot turn it off globally, you can turn it off locally as pointed out by @doron:

    #include <string.h>
char d[32];
void f(const char *s) {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wstringop-truncation"
    strncpy(d, s, 32);
#pragma GCC diagnostic pop
}