Failure to recognize siginfo_t._sifields

[Amittai Aviram <amittai dot aviram at yale dot edu>]

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