Different behavior of linkage between Clang and GCC.

Different behavior of linkage for C and C++#

For the following program:

#include <math.h>
#include <stdio.h>

int main() {

    printf("e^{1} = %f\n", exp(1));
    return 0;
}

If we compile the program with clang test.c -o test, compilation failed with the following error:

/tmp/test-32c2ea.o: In function `main':
test.c:(.text+0x18): undefined reference to `exp'
clang-11: error: linker command failed with exit code 1 (use -v to see invocation)

This is totally expected since we use exp() function here, which is implemented in the shared object libm.so. For clang, the linker will not link against libm.so by default. In order to compile the program, we need to explicitly link libm.so:

One explanation for the separation of libm.so and libc.so is that in the early days of computing, floating point calculation is expensive. People use various workarounds for floating point calculations. It makes no sense to integrate math functions into libc.so. It makes libc.so larger with little benefit. As computer hardware upgrades with time, the separation of libm and libc does not make much sense any more.

However, if we translate the above program to C++:

#include <iostream>
#include <cmath>

int main() {

    std::cout << "e^{1} = " << std:: exp(1) << std::endl;
    return 0;
}

we can actually compile the program without linking against libm:

The output of command ldd test is:

> ldd test
        linux-vdso.so.1 =>  (0x00007ffd009a4000)
        libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00002b4d4d13c000)
        libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00002b4d4d51f000)
        libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00002b4d4d828000)
        libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00002b4d4da40000)
        /lib64/ld-linux-x86-64.so.2 (0x00002b4d4cf14000)

libm.so is actually implicitly linked. According to post here, for C++ programs, since libstdc++.so requires libm.so, so libm.so will be implicitly linked even if you haven’t instructed its linkage¹.

It is interesting that shared library like libc.so² is linked even if we haven’t specified that. This behaviour can be disabled by providing the -nostdlib option:

clang -nostdlib test.c -o test

You will now see an error message about undefined printf():

test.c:(.text+0x29): undefined reference to `printf'

Ref:

• Why does gcc/clang know to link to libc by default?
• Libraries that are linked by default

Different between GCC and Clang on linkage#

You might think that GCC will also fail if we compile the test.c without linking libm, but it is not true. The following command runs without error for gcc 4.8.0 and 7.5.0 on my servers:

This is because GCC uses its built-in implementation of some standard C library functions to produce faster and smaller executables. According to post here, we can use -fno-builtin to disable this behavior. We can also use -fno-builtin-somefunc to disable a single function named somefunc. Take the above test.c for an example, the following command will result in compilation error:

gcc -fno-builtin-sqrt test.c -o test
# adding -lm will fix the compilation errors.
# gcc -fno-builtin-sqrt test.c -lm -o test

Ref:

• Other Built-in Functions Provided by GCC
• https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html#index-fno-builtin