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From: Richard Harnden <richard.nospam@gmail.invalid>
Newsgroups: comp.lang.c
Subject: Re: why does bsearch segfault on custom strcmp when qsort works fine?
Date: Thu, 15 Aug 2024 10:06:53 +0100
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On 15/08/2024 06:56, Mark Summerfield wrote:
> I have a program (complete source at the end) which correctly outputs this:
> 
> ["charlie" "foxtrot" "oscar" "echo" "alpha" "golf" "tango" "delta" "bravo"]
> ["alpha" "bravo" "charlie" "delta" "echo" "foxtrot" "golf" "oscar" "tango"]
> check_array OK
> check_index_found true OK
> check_index_found false OK
> 
> However, if you uncomment the "//#define BUG" line, the output (in gdb) is this:
> 
> (gdb) run
> Starting program: /home/mark/tmp/mycmpstr/mycmpstr
> [Thread debugging using libthread_db enabled]
> Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
> ["charlie" "foxtrot" "oscar" "echo" "alpha" "golf" "tango" "delta" "bravo"]
> ["alpha" "bravo" "charlie" "delta" "echo" "foxtrot" "golf" "oscar" "tango"]
> check_array OK
> 
> Program received signal SIGSEGV, Segmentation fault.
> __strcmp_avx2 () at ../sysdeps/x86_64/multiarch/strcmp-avx2.S:283
> 283	../sysdeps/x86_64/multiarch/strcmp-avx2.S: No such file or directory.
> (gdb) bt
> #0  __strcmp_avx2 () at ../sysdeps/x86_64/multiarch/strcmp-avx2.S:283
> #1  0x00005555555553e0 in mystrcmp (s=0x555555556030, t=0x7fffffffde10) at mycmpstr.c:50
> #2  0x00007ffff7e0a53c in __GI_bsearch (__key=0x555555556030, __base=0x7fffffffddf0,
>      __nmemb=<optimized out>, __size=8, __compar=0x5555555553b7 <mystrcmp>)
>      at ../bits/stdlib-bsearch.h:33
> #3  0x0000555555555317 in main () at mycmpstr.c:30
> 
> The difference is that without BUG defined I use my own binary search,
> but with BUG defined I use bsearch.
> 
> [...]
> 
> int mystrcmp(const void* s, const void* t) {
>      return strcmp(*(const char**)s, *(const char**)t);

You don't need to cast void*s, change that to:
     return strcmp(s, t);

> }
>