Undefined reference to SSL_library_init and SSL_load_error_strings

Question

I am implementing a OpenSSL code and have already included required header files but still I am getting errors like *

undefined reference to SSL_library_init

I guess it's a linking error rather than a compilation error.

I am implementing it in Linux box using slickeditor.

Solution 1

Link against libssl and libcrypto. Your LDFLAGS and LDLIBS would be as follows. Order matters for LDLIBS:

LDFLAGS = -L/usr/local/ssl/lib
LDLIBS = -lssl -lcrypto

Don't worry about adding the "lib" in front of library name, or the "so" or "a" suffix. The linker will do it for you.

If you are building from the command line, then you would use the following. Again, order matters.

gcc foo.c -o foo.exe -L/usr/local/ssl/lib -lssl -lcrypto

If you are using the system's OpenSSL, then you can omit -L/usr/local/ssl/lib.

Solution 2

For me this meant to install

 apt install libssl1.0-dev

Solution 3

These methods are deprecated in OpenSSL 1.1. You don't need to use it more. You can just remove it. More info in OpenSSL manual.

Comments

[rlibby]

Apologies, SSL_library_init is in libssl, so the link option would be -lssl. ldd $(which openssl) will show you how your openssl is linked and where those libraries are. If it still doesn't work, perhaps that directory is not on the path for the linker. You can add that path with -Lpath, such as -L/lib/

[rlibby]

You should get a minimal test case working on the command line. #include "whatever" \n int main(void) { SSL_library_init(blah, blah, blah); return 0; } and then g++ my_minimal_test_case.c++ -lssl. If this works then you don't understand your editor/IDE. If it doesn't then you have some configuration issue.

[Craig Scott]

According to comments on the question, SSL_library_init is in libssl, so shouldn't the correct order be LDLIBS = -lcrypto -lssl instead?

[jww]

@CraigScott - Sorry about the late reply. Order matters because its a single pass linker. When LD encounters unsatisfied link symbols, it notes them and looks for them in libraries that follow. If libcrypto was first, then libssl would have unsatisfied symbols like BN_new because libcrypto needs to follow libssl to satisfy the missing symbols. The other option is a two-pass linker. Sometimes you will also see something like -la -lb -la (liba followed by libb followed by liba). Its usually due to a circular reference lurking behind the scenes.

[Ray]

"Order matters" was the key for me. Thanks.

[Rick Sanchez]

You're a lifesaver. Was trying to compile AOSP on Ubuntu 18, and had to downgrade openssl 1.1 -> 1.0.1 , but had installed libssl-dev

[Brent Writes Code]

High five to both of you. I had the same issue and the combination of these two comments made me realize it.