[3.12] gh-123917: Fix crypt check in configure

[vstinner]

• Issue: The configure script incorrectly detects crypt() when it doesn't exist #123917

[madscientist]

Unfortunately this is not enough. Making an auto variable volatile doesn't prevent the optimizer from removing it. I tried this change first so I know it doesn't work :). That's why in my suggested change, I made x a global volatile variable instead.

[madscientist]

Just to note you can easily test this by putting this code into a sample .c file and then compile/link it without linking -lcrypt and with -O2; you will not get a linker error even though you should. That's the problem.

I am using the latest GCC 14.2 so maybe earlier versions don't remove this? Not sure but easy to test.

[vstinner]

Oh. I modified my PR to avoid volatile, but use the variable value instead.

Please check the updated PR.

[madscientist]

Just to note you can easily test this by putting this code into a sample .c file and then compile/link it without linking -lcrypt and with -O2; you will not get a linker error even though you should. That's the problem.

I am using the latest GCC 14.2 so maybe earlier versions don't remove this? Not sure but easy to test.

[madscientist]

Unfortunately, this also isn't enough to trigger the problematic behavior. I extracted the conftest.c that configure generates so I can test it:

        #ifdef HAVE_CRYPT_H
          #include <crypt.h>
        #endif
        #include <unistd.h>

int
 main (void)
 {
        #ifdef HAVE_CRYPT_R
          void *x = crypt_r;
        #else
          void *x = crypt;
        #endif
          return (x != NULL);
 }

compiling this without -lcrypt should give an error since the symbol crypt is not defined. However, it does not give an error:

$ gcc -o /tmp/crypt -O2 /tmp/crypt.c
$

If I use the version I suggested, I do get an error:

        #ifdef HAVE_CRYPT_H
          #include <crypt.h>
        #endif
        #include <unistd.h>

volatile void *x;

int
 main (void)
 {
        #ifdef HAVE_CRYPT_R
          x = crypt_r;
        #else
          x = crypt;
        #endif
          return 0;
 }

then I see:

$ gcc -o /tmp/crypt -O2 /tmp/crypt.c
/bin/ld: /tmp/ccwu2AcU.o: in function `main':
crypt.c:(.text.startup+0x7): undefined reference to `crypt'
collect2: error: ld returned 1 exit status
$

Can you reproduce this behavior in a simple test on your system?

[thesamesam]

Paul's suggested variant looks right to me.

[vstinner]

compiling this without -lcrypt should give an error since the symbol crypt is not defined. However, it does not give an error:

What's your operating system and GCC version? On Fedora 40, I get an "undefined reference" linker error using GCC 14.2.1 and clang 18.1.6.

[vstinner]

I modified again the PR to use a global volatile void *func; variable and I kept the != NULL test (to use the variable value). Does it work for you?

[madscientist]

I am using GCC 14.2.0 (I don't know of any actual 14.2.1 release... I wonder where that .1 came from?) / binutils 2.43.1 using GNU gold as the linker, on Red Hat 8.6. I also see the same behavior with GCC 9.4 / binutils 2.34 with GNU ld, on an Ubuntu 20.04 system. I'm very surprised we get such different behavior...! Just for sanity, you're sure you used -O2? As in gcc -O2 -o crypttest crypttest.c? That doesn't give an error for me with the first example code in my comment above. It also succeeds (incorrectly) if I use a volatile auto variable.

Your current latest version with the volatile global variable, does give a link error as expected.

[vstinner]

Your current latest version with the volatile global variable, does give a link error as expected.

Ok, thanks for testing. I merged my PR.

---

Building the following code with gcc -O2 x.c -o x works whereas it should fail. I think that in my previous tests, I added -lcrypt to my command line.

#ifdef HAVE_CRYPT_H
#  include <crypt.h>
#endif
#include <unistd.h>

int
main (void)
{
#ifdef HAVE_CRYPT_R
    void *x = crypt_r;
#else
    void *x = crypt;
#endif
    return (x != NULL);
}

If confirm that building the following code fails as expected with gcc -O2 x.c -o x:

#ifdef HAVE_CRYPT_H
#  include <crypt.h>
#endif
#include <unistd.h>

volatile void *x;

int
main (void)
{
#ifdef HAVE_CRYPT_R
    x = crypt_r;
#else
    x = crypt;
#endif
    return (x != NULL);
}

[madscientist]

I think that in my previous tests, I added -lcrypt to my command line.

Oh yes, that will definitely do it! :)

Thanks for the work Victor!