Openssl for RSA: undefined reference to RSA_new

The propblem is that you are linking with libssl and you are using RSA crypto which is part of libcrypto, another error : there is no function called : RSA_new_:

toc@UnixServer:/usr/include/openssl$ grep RSA_new *rsa.h:RSA * RSA_new(void);rsa.h:RSA * RSA_new_method(ENGINE *engine);

So correct your code:

rsa = RSA_new();

And compile like that:

gcc -I/usr/include/openssl/ -Wall my_rsa.c -o my_rsa  -lcrypto

EDIT : for the last error(dl functions):

gcc -I/usr/include/openssl/ -Wall my_rsa.c -o my_rsa  -lcrypto -ldl

You need to link with the library as well:

gcc -I/usr/local/ssl/include -o etc etc.c -L/usr/local/lib -lssl

The -L option tells GCC where to look for library file, and -l (small L) tells the linker that it should link with the library.

Replace the library folder and library name with what you got.

You have to link against the libSSL library. Something like

gcc -I /usr/local/ssl/include -o myprog myprog.c -lssl

will do the trick.

(Maybe it's not actually -lssl but -lopenssl, -lssl-rsa or whatever; you can find this out by typing

pkg-config --libs PACKAGENAME

where PACKAGENAME is the name of the package which contains libssl, something like libssl, openssl, libssl-dev, openssl-devel etc.)