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NNTP-Posting-Date: Thu, 15 Aug 2024 16:43:16 +0000
From: Mark Summerfield <mark@qtrac.eu>
Subject: Re: why does bsearch segfault on custom strcmp when qsort works fine?
Newsgroups: comp.lang.c
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On Thu, 15 Aug 2024 08:55:45 -0000 (UTC), Ike Naar wrote:

[snip]
> The elements of the words array have type pointer-to-char.
> So the first argument to bsearch should be the address of such an
> element, that is,
> a pointer-to-pointer-to-char and it should contain the adress of a
> pointer to the first character of the oscar string.
> Also, the value returned from bsearch should be interpreted as a
> pointer-to-pointer-to-char.
> 
>     char * key = "oscar";
>     char * * p = bsearch(&key, words, size, sizeof(char*), mystrcmp);
> 
>>     index = p - words[0];
>>     found = p != NULL:
> 
> Two problems here: first, if bsearch returns NULL, the subtraction is
> ill-defined.
> Second, if bsearch returns non-null the index will be p - words, not p -
> words[0];
> 
>     found = p != NULL:
>     if (found) index = p - words;

Thank you! That solved the problem and clarified my mistakes.
By changing the code along the suggested lines it works great:

    char* s = "oscar";
    char** p = bsearch(&s, words, size, sizeof(char*), mystrcmp);
    if (p) {
        index = p - words;
        found = true;
    }