Failure to recognize siginfo_t._sifields

Failure to recognize siginfo_t._sifields

[Amittai Aviram]

This question lies somewhere between GCC and glibc, so I hope it's OK if I start here.

I have a signal handler for SIGSEGV

void handle_segfault(int signo, siginfo_t * siginfo, void * ucontext);

which changes the permissions on the page to which access has provoked the SIGSEGV as part of a copy-on-write scheme, and I am investigating a case where the address appears to be wrong.  (The access is to the heap—more below.)   My code gets the address of the access from siginfo->si_addr.  When debugging, GDB reported that siginfo_t has no such member as si_addr, so I found out, by looking at the definition of siginfo_t in my /usr/include/bits/siginfo.h (#included in /usr/include/signal.h), that the real name of the field is

siginfo_t._sifields._sigfault.si_addr

for which there is a #define statement below the definition of siginfo_t:

#define si_addr        _sifields._sigfault.si_addr

I can query GDB for the value under this name, and I get the _correct_ address.  However, if I try to change my source code so that, instead of using siginfo->si_addr, it uses siginfo->_sifields._sigfault.si_addr, I get this from GCC:

error: ‘struct <anonymous>’ has no member named ‘_sifields’

The questions pertaining to GCC are:

1.  Why doesn't GCC accept the "real" name as equivalent to the #defined alias?
2.  Why, when it refuses, does it call siginfo_t "struct <anonymous>," when siginfo.h defines the type thus?— 

typedef struct siginfo {
 /*.... */
} siginfo_t

Thanks!

Amittai

Amittai Aviram
PhD Student in Computer Science
Yale University
646 483 2639
amittai.aviram@yale.edu
http://www.amittai.com

Re: Failure to recognize siginfo_t._sifields

[Cedric Roux]

On 11/24/2010 09:20 PM, Amittai Aviram wrote:
> This question lies somewhere between GCC and glibc, so I hope it's OK if I start here.
>
> I have a signal handler for SIGSEGV
>
> void handle_segfault(int signo, siginfo_t * siginfo, void * ucontext);
>
> which changes the permissions on the page to which access has provoked the SIGSEGV as part of a copy-on-write scheme, and I am investigating a case where the address appears to be wrong.  (The access is to the heap—more below.)   My code gets the address of the access from siginfo->si_addr.  When debugging, GDB reported that siginfo_t has no such member as si_addr, so I found out, by looking at the definition of siginfo_t in my /usr/include/bits/siginfo.h (#included in /usr/include/signal.h), that the real name of the field is
>
> siginfo_t._sifields._sigfault.si_addr
>
> for which there is a #define statement below the definition of siginfo_t:
>
> #define si_addr        _sifields._sigfault.si_addr
>
> I can query GDB for the value under this name, and I get the _correct_ address.  However, if I try to change my source code so that, instead of using siginfo->si_addr, it uses siginfo->_sifields._sigfault.si_addr, I get this from GCC:
>
> error: ‘struct<anonymous>’ has no member named ‘_sifields’
>
> The questions pertaining to GCC are:
>
> 1.  Why doesn't GCC accept the "real" name as equivalent to the #defined alias?
> 2.  Why, when it refuses, does it call siginfo_t "struct<anonymous>," when siginfo.h defines the type thus?—

because "si_addr" is extended to "_sifields._sigfault.si_addr"
so you write "siginfo->_sifields._sigfault.si_addr"
and the C preprocessor spits "siginfo->_sifields._sigfault._sifields._sigfault.si_addr"

that works:
/home/cro> cat a.c
#include <signal.h>

#undef si_addr

int main(void)
{
   siginfo_t t;
   t._sifields._sigfault.si_addr = 0;
   return 0;
}

but you don't want to do that I guess.