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Date: Wed, 20 Jun 2012 12:33:16 -0400
From: <ohaya@cox.net>
To: modules-dev@httpd.apache.org
Subject: Re: Best (safest) way to edit char string (from envvars)?
Cc: Daniel Gruno <rumble@cord.dk>
In-Reply-To: <4FE1F8C5.9020800@cord.dk>
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---- Daniel Gruno <rumble@cord.dk> wrote: 
> On 06/20/2012 06:17 PM, ohaya@cox.net wrote:
> > 
> > Hi Ben and Daniel,
> > 
> > I must be doing wrong.  After I added code to strip the begin and end lines, I get a segfault when I test.
> > 
> > I notice that the apr_cpystrn() doesn't include a pool parameter, so I guess it's just moving the char strings in memory that already exists in the pool?
> > 
> > Here's the code I have thus far:
> > 
> >     const char * ssl_client_cert;
> >     const char * ssl_client_cert_after_remove_begin;
> >     const char * ssl_client_cert_after_remove_end;
> >     int end_of_cert;
> >     
> >     printf("SSL_CLIENT_CERT=[%s]\n", apr_table_get(r->subprocess_env, "SSL_CLIENT_CERT"));
> >     ssl_client_cert = apr_table_get(r->subprocess_env, "SSL_CLIENT_CERT");
> > 
> >     printf("printf ssl_client_cert=[%s]\n", ssl_client_cert);
> >     printf("ssl_client_cert + 27=[%s]\n", ssl_client_cert+27);
> > 
> > // SEGFAULT after here...
> > 
> >     end_of_cert = strlen(ssl_client_cert);
> >     apr_cpystrn(ssl_client_cert_after_remove_begin, ssl_client_cert+27, end_of_cert-27  );
> >     end_of_cert = strlen(ssl_client_cert);
> > 
> >     printf("printf ssl_client_cert_after_remove_begin=[%d]-[%s]\n", end_of_cert, ssl_client_cert_after_remove_begin);
> >     end_of_cert = strlen(ssl_client_cert_after_remove_begin);
> >     apr_cpystrn(ssl_client_cert_after_remove_end, ssl_client_cert_after_remove_begin+21, end_of_cert-21);
> > 
> >     printf("printf ssl_client_cert_after_remove_end=[%d]-[%s]\n", end_of_cert, ssl_client_cert_after_remove_end);
> > 
> > Thanks,
> > Jim
> > 
> > 
> apr_cpystrn expects the destination to be already allocated space of the
> correct size.
> 
> You need to either allocate the space for your char* yourself via
> apr_palloc or apr_pcalloc, or use apr_pstrndup instead, if you expect
> httpd to handle the memory management.
> 
> With regards,
> Daniel.


Hi,

I'll try the palloc, but yourself and I think Ben mentioned apr_pstrndup(), and I don't quite understand that.  It looks like pstrndup() just creates a new string in the pool that's the same size as the original string, but I don't quite understand why having that duplicated char string in the pool helps?  

Or, are you all suggesting using apr_pstrndup() to create a copy of string I want to manipulate, and then use something like:

 apr_cpystrn(newly_allocated_string, old_string+27, old_string_lgh-27)

Thanks again!

Jim