[GitHub] [qpid-dispatch] jiridanek edited a comment on pull request #853: WIP: DISPATCH-1783 testing safe_snprintf function

[GitBox <gi...@apache.org>]

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
   
   ```
   #include "qdr_doctest.h"
   #include "elfspy/SPY.h"
   #include "elfspy/Fake.h"
   
   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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