Minimal step-by-step example
Compile GCC from source. Condensed commands:
sudo apt-get build-dep gcc
git clone git://gcc.gnu.org/git/gcc.git
cd gcc
git checkout gcc-6_4_0-release
./contrib/download_prerequisites
mkdir build
cd build
../configure --enable-languages=c,c++ --prefix="$(pwd)/install"
make -j`nproc`
Wait from 30-minutes to two hours. Now let's use this test program a.cpp
:
#include <cassert>
#include <queue>
int main() {
std::priority_queue<int> q;
q.emplace(2);
q.emplace(1);
q.emplace(3);
assert(q.top() == 3);
q.pop();
assert(q.top() == 2);
q.pop();
assert(q.top() == 1);
q.pop();
}
First compile and run it to ensure that the initial compilation worked:
gcc/build/install/bin/g++ -g -std=c++11 -O0 -o a.out ./a.cpp
./a.out
Now let's hack up the priority_queue
constructor.
First, you can find the actual constructor easily with GDB as explained at: When should I use make_heap vs. Priority Queue?
So we hack it up with this patch:
diff --git a/libstdc++-v3/include/bits/stl_queue.h b/libstdc++-v3/include/bits/stl_queue.h
index 5d255e7300b..deec7bc4d99 100644
--- a/libstdc++-v3/include/bits/stl_queue.h
+++ b/libstdc++-v3/include/bits/stl_queue.h
@@ -61,6 +61,7 @@
#if __cplusplus >= 201103L
# include <bits/uses_allocator.h>
#endif
+#include <iostream>
namespace std _GLIBCXX_VISIBILITY(default)
{
@@ -444,7 +445,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
priority_queue(const _Compare& __x = _Compare(),
_Sequence&& __s = _Sequence())
: c(std::move(__s)), comp(__x)
- { std::make_heap(c.begin(), c.end(), comp); }
+ {
+ std::cout << "hacked" << std::endl;
+ std::make_heap(c.begin(), c.end(), comp);
+ }
template<typename _Alloc, typename _Requires = _Uses<_Alloc>>
explicit
Then rebuild and re-install just libstdc++ to save a lot of time:
cd gcc/build/x86_64-pc-linux-gnu/libstdc++-v3
make -j`nproc`
make install
and now the next build and run:
gcc/build/install/bin/g++ -g -std=c++11 -O0 -o a.out ./a.cpp
./a.out
outputs:
hacked
Tested on Ubuntu 16.04.
glibc
As a bonus, if you are also interested in C: Multiple glibc libraries on a single host