Need sample C code for encrypting and decrypting files using openssl

Am writing code in Linux C. I need to encrypt and decrypt files using openssl. Currently Am using system command "des3 -e -开发者_如何学Pythonnosalt -k 0123456789012345 -in inp_file -out out_file" for encrypting and "des3 -d -nosalt -k 0123456789012345 -in inp_file -out out_file" for decrypting. Where shall I get sample code to do the same with C API's.

I would look here: http://www.openssl.org/docs/crypto/EVP_EncryptInit.html#EXAMPLES

and here: http://www.nlnetlabs.nl/downloads/publications/hsm/hsm_node17.html