Strange behaviour of std::cout not printing numbers

Question

The following code (compiled as part of a big project) does not print numbers (not just one-byte integers, but any of them: std::int32_t, std::size_t, double, etc). Of course, this code compiled on its own works perfectly, and therefore it is impossible to provide a minimal example which reproduces this problem: something in this big project causes it to happen, but I can't put my finger on what exactly causes it - hence this question.

The code:

std::int32_t n = 42;
std::cout << "test 1" << std::endl; // prints
std::cout << 3.14 << std::endl;     // doesn't print
std::cout << 456464 << std::endl;   // doesn't print
std::cout << n << std::endl;        // doesn't print
std::cout << "test 2" << std::endl; // still doesn't print
printf("printf: %d\n", n);          // prints
std::cout.clear();
std::cout << "test 3" << std::endl; // prints
std::cout << 42 << std::endl;       // doesn't print

Output:

test 1
printf: 1
test 3

So, number literals 3.14 and 456464 and variable n are not printed as well as string test 2 (I attempt to print a string after failed printing of an integer). The printf function works. I use std::endl so it should not be any kind of buffering problem.

Using std::cout.clear() seems to fix the problem (we can print strings again), but only until another number is printed.

What kind of manipulation with output streams may potentially cause this? Yes there is using namespace std;.

The code is compiled with -std=c++11 using GCC toolchains (I tried various versions including 5.3.1, 6.3.1, and 8.2.0 - the result is always the same).

Compilation command line:

g++ c -MMD -pipe -std=c++11 -fPIC -O3 \\
-fmax-errors=3 -msse4.1 -mavx2 source.cpp -o target/objects/source.o

Linking command line:

g++ <object files> -o executable -s \\
-Wl,--build-id=uuid -static-libstdc++ -pthread -Wl,--no-undefined

Answer 1

After some debugging I managed to find the culprit. This is not a complete answer because it doesn't explain the observed symptoms (not printing particularly numbers and why using std::cout.clear() solves problem at least partially), but this answer will provide a minimal example that can be used to reproduce the problem. Hopefully it will save some time for someone.

The reason for strange behaviour is problem with memory usage, namely with running out of thread local storage (TLS). The issue can be reproduced in any way which involves heavy usage of TLS. The example below uses OpenCV and linking statically to libstdc++ for this. To speed up things we will use Python to avoid writing too much code.

C++ part:

#include <iostream>
#pragma GCC visibility push(default)
extern "C" void fun() {
    std::cout << "hello 0" << std::endl;
    std::cout << 42 << std::endl;
    std::cout << "hello 2" << std::endl;
}
#pragma GCC visibility pop

Compile and link it with (the -static-libstdc++ is important bit here as this library is known for heavy usage of TLS):

g++ -std=c++11 -fPIC test.cpp -s -static-libstdc++ -pthread -shared -o libtest.so

Now use Python and ctypes module to load our library:

import numpy as np
lib = np.ctypeslib.load_library('libtest', '.')
lib.fun()

If run like this, it will print (as expected):

hello 0
42
hello 1

Now let's add another library with heavy usage of TLS:

import numpy as np
import cv2  # <-----
lib = np.ctypeslib.load_library('libtest', '.')
lib.fun()

This will print:

hello 0

Please note that particular reproducibility may depend on your OS's settings for TLS limits.