examples/block-ip: show how to block IP addresses

[jay]

• Show how CURLOPT_OPENSOCKETFUNCTION can be used to block IP addresses.

This is a new example that demonstrates IP blocking.

Ref: #15710 (reply in thread)

Closes #xxxx

[jay]

Is -Werror=missing-prototypes really needed for examples?

[vszakats]

Is -Werror=missing-prototypes really needed for examples?

It adds complexity to apply and remember different rules for
different parts of the codebase. Examples can be copied and
compiled by users outside of curl and may have this warning
enabled there.

Isn't it simpler to add static to the 4 local functions?

[bagder]

Isn't it simpler to add static to the 4 local functions?

I believe so. We encourage use of static for local functions anyway.

[jay]

@vszakats do you have any suggestion for the failing amiga CI job? Does it not have a working inet ntop/pton? I see in your commit that recently created the job that: inet_ntop, inet_pton: fix using it for AmigaOS. cmake detects them, and they did not compile with AmigaOS. Should this example be excluded from that platform?

[vszakats]

@vszakats do you have any suggestion for the failing amiga CI job? Does it not have a working inet ntop/pton? I see in your commit that recently created the job that: inet_ntop, inet_pton: fix using it for AmigaOS. cmake detects them, and they did not compile with AmigaOS. Should this example be excluded from that platform?

These functions exist on AmigaOS3, but they possibly require proto/bsdsocket.h (can't double-check ATM). Looking at lib/curl_setup.h, it would have to be detected, alongside some "magic" conditions for including it. Then they have slightly different signatures, needing casts.

Sounds overkill for a standalone example. I think excluding it is a reasonable solution.

[jay]

I disabled it for Amiga by defining an alternate main in the example for __AMIGA__ that says amiga is not supported.

I had considered instead removing block_ip, when amiga host, from the check_PROGRAMS list after it's imported by CMakeLists.txt and Makefile.am. In cmake it would be easy list(REMOVE_ITEM check_PROGRAMS "block_ip"), however in Makefile.am the code would need to be POSIX compliant and there's no good way to do it without string comparison ($(host_os) = amigaos) and something like a GNU automake text function filter-out. Though technically there are some workarounds like use the shell to filter the variable and then re-import it, it would be messy. Also there's POSIX substitution but it only works with suffixes, eg $(check_PROGRAMS:block_ip=)

[vszakats]

I disabled it for Amiga by defining an alternate main in the example for __AMIGA__ that says amiga is not supported.

I had considered instead removing block_ip, when amiga host, from the check_PROGRAMS list after it's imported by CMakeLists.txt and Makefile.am. In cmake it would be easy list(REMOVE_ITEM check_PROGRAMS "block_ip"), however in Makefile.am the code would need to be POSIX compliant and there's no good way to do it without string comparison ($(host_os) = amigaos) and something like a GNU automake text function filter-out. Though technically there are some workarounds like use the shell to filter the variable and then re-import it, it would be messy. Also there's POSIX substitution but it only works with suffixes, eg $(check_PROGRAMS:block_ip=)

Agreed, it sounds messy on the build-level.

I think it's perfectly fine to handle it in C. Somebody may compile
it as-is as a C source (outside cmake or autotools), and this is now
handled too.