On 7/13/22 01:37, John Scott via curl-library wrote:
Hi libcurl folks,
Hi John,
The CURLOPT_WRITEDATA man page says
If you are using libcurl as a win32 DLL, you MUST use a
CURLOPT_WRITEFUNCTION if you set this option or you will experience
crashes.
I was curious to investigate why this happens, what the cause of the
crashes is. However, with curl 7.84.0 I'm unable to reproduce this
issue. I've cross-built libcurl using the MinGW-w64 toolchain provided
by Debian Testing and tested the x86_64 binary under Wine, and it seems
to work just fine without setting CURLOPT_WRITEFUNCTION. I'm certain
that my test program is dynamically linked.
Is this documentation up-to-date, and if so (primarily for my
curiosity's sake) why does the crash happen?
There's an explanation for this in an old PR:
https://github.com/curl/curl/pull/1839#issuecomment-326787637
When you only set CURLOPT_WRITEDATA, you cause libcurl to perform fwrite
calls on a handle opened by YOUR application's runtime library with
LIBCURL's own and both units MAY have been linked with different runtime
libraries.
I presume underlying implementation details of FILE * may be
incompatible between runtime library versions on W$.
In short, applications MAY crash if you do this, hence it's preferable
to provide the fwrite function provided by the runtime library used to
open the file.
If your test program does not crash, both libraries are the same or at
least compatible. But that's Russian roulette !
Patrick
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