I'm working on an XS interface to an external C library. My current code builds and works fine on 32-bit x86 Linux (Fedora Core 8, Perl 5.8.8) and on Solaris 10, but when I build it for 64-bit Linux (FC8 on x86_64) I get this compiler warning and the program segfaults when this routine is called:

Cap5.xs: In function ‘printf_handler’:
Cap5.xs:1013: warning: passing argument 4 of ‘Perl_sv_vsetpvf’ from
    incompatible pointer type

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The relevant lines of Cap5.xs are:

static
int printf_handler(const char *format, va_list args)
{
    SV *printf_str;
    dSP;
    
    ENTER;
    SAVETMPS;
    
    printf_str = NEWSV(0, strlen(format) + 32);
    sv_vsetpvf(printf_str, format, &args);

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Line 1013 is the last one shown above.

Perl's proto.h file declares Perl_sv_vsetpvf as:

PERL_CALLCONV void Perl_sv_vsetpvf(pTHX_ SV* sv, const char* pat,
    va_list* args);

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The final argument there is a va_list*, which is exactly what my code is giving it. I'm not sure it really should have been a va_list* given that vprintf et al. take a va_list, but that's what the header says and it works fine on the other systems.

Interestingly, if I drop the & and just pass args as the final argument the code seems to work fine on 64-bit Linux (although the compiler still warns), but then it segfaults on the other systems.

Any advice? I'm guessing this may be a bug in gcc and not really a Perl problem at all...

Update: It appears that taking the address of a va_list is non-conforming C, so Perl's API is broken. See slide 19 of this PDF slideshow for details. I looked in RT and only found one bug report from 2002 that could be about this issue; should I report it?

2nd Update: On amd64 (and apparently on PowerPC too) va_list is defined by the ABI as an array:

typedef struct
    unsigned int gp_offset;
    unsigned int fp_offset;
    void *overflow_arg_area;
    void *reg_save_area;
} va_list[1];

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Thus within printf_handler() the args variable is already a pointer to the structure. This explains why dropping the & works as I described in my original question. Unfortunately that's no use on other architectures though.

Using va_copy() (or its equivalent) to copy args into an auto variable allows me to take the address of the copy and pass that pointer into sv_vsetpvf().

In reply to XS on 64-bit: Warning from sv_vsetpvf call by songmaster

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