I don't understant the problem here,

if ttyname_r() return 0; it mean success, then:

 return ttyname_r(0, buf, 64) ? 1 : 0;

return 0, action-if-true is evaluated. seems correct to me.

Your logic is right, however the test still returns false.

"checking for working ttyname_r() implementation... no, posix_ttyname() will be thread-unsafe"

configure.log show this:
"configure:79083: checking for working ttyname_r() implementation
configure:79104: x86_64-pc-linux-gnu-gcc -o conftest -I/usr/include -O2 -march=native -pipe -pthread  -D_REENTRANT -L/usr/lib64 -Wl,-O1 -Wl,--as-needed conftest.c -liodbc -lmcrypt -lltdl -lonig -lstdc++ -lcrypto -lssl -lcrypto -lcrypt -lpam -lgmp -lt1 -lX11 -lXpm -lpng -lz -ljpeg -lvpx -lcrypto -lssl -lcrypto -lenchant -ldb-5.1 -lgdbm -lcurl -lbz2 -lz -lpcre -lcrypto -lssl -lcrypto -lrt -lm -ldl -lnsl  -lxml2 -lz -lm -ldl -lcurl -lnghttp2 -lidn -lrtmp -lz -lgmp -lgnutls -lhogweed -lnettle -lssh2 -lssh2 -lssl3 -lsmime3 -lnss3 -lnssutil3 -lplds4 -lplc4 -lnspr4 -lz -lxml2 -lz -lm -ldl -lfreetype -licui18n -licuuc -licudata -licuio -liodbc -liodbcinst -ldl -lodbc >&5
configure:79104: $? = 0
configure:79104: ./conftest
configure:79104: $? = 1
configure: program exited with status 1
configure: failed program was:
| /* confdefs.h */
| #define PACKAGE_NAME ""
| #define PACKAGE_TARNAME ""
| #define PACKAGE_VERSION ""
--
| #define HAVE_GETPWUID_R 1
| #define HAVE_GETGRGID_R 1
| /* end confdefs.h.  */
|
| #include <unistd.h>
|
| int main(int argc, char *argv[])
| {
|       char buf[64];
|
|       return ttyname_r(0, buf, 64) ? 1 : 0;
| }
|
configure:79114: result: no, posix_ttyname() will be thread-unsafe"

I modified the check to output the return value, which was turned out to be: 25.
Errno #25 means: ENOTTY.

I modified the test program like this:
"
#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>

int main(int argc, char *argv[])
{
        char buf[64];
        int retval;

        retval = ttyname_r(0, buf, 64);
        printf("\nretval 0: %s, fd path 0: %s.\n", strerror(retval), buf);
        retval = ttyname_r(1, buf, 64);
        printf("retval 1: %s, fd path 1: %s.\n", strerror(retval), buf);
        retval = ttyname_r(2, buf, 64);
        printf("retval 2: %s, fd path 2: %s.\n", strerror(retval), buf);

        return ttyname_r(0, buf, 64) ? 1 : 0;
}
"

This is the output I got:
checking for working ttyname_r() implementation...
retval 0: Inappropriate ioctl for device, fd path 0: H1gB?.
retval 1: Success, fd path 1: /dev/pts/2.
retval 2: Inappropriate ioctl for device, fd path 2: /dev/pts/2.

So my logic was wrong, but checking for fd 0 is inappropriate.
I guess ti would be better to check for fd 1, instead!

What's your opinion?

Sorry, I misunderstood the discussion and applied the incorrect fix. Reverted now.

However the other proposed fix, using fd 1 instead of 0, does not work for me either.

Might it be sufficient to just check for the availability of the ttyname_r symbol?

I believe it's quite possible, that the given file descriptor specified in the fix works well only for Gentoo sandbox. Other environments can have another fd wining.
Therefore the simple configure code snippet would not fit for all circumstances.
One possibility is to test only for the availability of the symbol - as you've suggested. Otherwise the configure code must be more complex for this test.
Please also note I'm just a power user and doesn't want to act like a smartass...

Someone messed the repo again, reopening.