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https://issues.apache.org/jira/browse/DISPATCH-1783?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17412768#comment-17412768
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ASF GitHub Bot commented on DISPATCH-1783:
------------------------------------------
jiridanek edited a comment on pull request #853:
URL: https://github.com/apache/qpid-dispatch/pull/853#issuecomment-698563800
There is multiple options how to do this. I've been writing a more complete
list, but I only have an example for two.
## preprocessor magic
See the PR
## elfspy
https://github.com/mollismerx/elfspy
The code below, plus need to add a library
```c++
#include "qdr_doctest.h"
#include "elfspy/SPY.h"
#include "elfspy/Fake.h"
int fake_vsnprintf(char * s, size_t n, const char * format, va_list arg) {
return -1;
}
TEST_CASE("safe_snprintf_vsnprintf_failed") {
const int OUTPUT_SIZE = 128;
const char *TEST_MESSAGE = "something";
const int LEN = strlen(TEST_MESSAGE);
size_t len;
char output[OUTPUT_SIZE];
// weird elfspy boilerplate
char* argv[2] = {(char*)"c_unittests", nullptr};
spy::initialise(0, argv);
// setup the fake, it will be unset when the variables go out of scope
// it changes dynamic linking in the running process, to replace call to
vsnprintf in glibc with the fake
auto vsnprintf_ = SPY(&vsnprintf);
auto vsnprintf_fake = spy::fake(vsnprintf_, &fake_vsnprintf);
// alternative way for previous line:
// auto vsnprintf_fake = spy::fake(vsnprintf_, [](auto ...) { return -1;
}); // I can haz lambdaz !!!
// run the test, simulating a failed vsnprintf
output[0] = 'a';
len = safe_snprintf(output, LEN+10, TEST_MESSAGE);
CHECK(0 == len);
CHECK('\0' == output[0]);
CHECK("" == output);
}
```
## more possibilities
* use the `__wrap` feature in gnu linker
* libraries similar to elfspy, such as mimick,
https://github.com/Snaipe/Mimick
* linking order, just define vsnprintf in the same translation unit as the
test and things work out just right
* LD_PRELOAD
* ...
## general consideration
In any case, static functions cannot be mocked in C, and also inlining, when
compiler decides to optimize this way, destroys the ability to replace a
function. That implies that this kind of tests can be either only run in a
debug build, or it has to build a second, `-testing` version of the dispatch
library, for this testing. Third option is not to link to the dispatch library
and `#include` the needed .c files. That also works, if the surface area tested
is small. It encourages creating many smaller ctest test targets, instead of
one large one.
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> Add ability to use test doubles (mocks, fakes, ...) in C tests
> --------------------------------------------------------------
>
> Key: DISPATCH-1783
> URL: https://issues.apache.org/jira/browse/DISPATCH-1783
> Project: Qpid Dispatch
> Issue Type: Improvement
> Components: Tests
> Affects Versions: 1.14.0
> Reporter: Jiri Daněk
> Priority: Major
>
> During the work on https://github.com/apache/qpid-dispatch/pull/684, there
> was at one point a version of the PR, which tested what happens if
> {{vsnprintf}} called from {{safe_snprinf}} returns {{-1}}.
> This was done by replacing the {{vsnprintf}} with a mock (technically it's
> probably a stub, or maybe fake, but let's ignore the proper terminology).
> There are various possibilities how to approach mocking in C. They are less
> nice than in more dynamic languages such as Java, or even C++ (where it's
> possible to subclass things). Nevertheless, it can be useful technique, and
> it should be explored.
> CC [~cans]
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