Sample code from "Basic blockchain programming", a developer-oriented series about Bitcoin.

http://davidederosa.com/basic-blockchain-programming/


1. how to solve compilation error: /usr/bin/ld: cannot find -lcrypto

hopkin@ubuntu:~/workspace/basic-blockchain-programming$ ./test.sh ex-sample-nameex-sample-name.c:1:0: warning: ISO C forbids an empty translation unit [-Wpedantic]
/usr/bin/ld: cannot find -lcrypto

Solution: installing libssl-dev.
  
hopkin@ubuntu:~/workspace/basic-blockchain-programming$ sudo apt-get install libssl-dev


2. compilation error

hopkin@ubuntu:~/workspace/basic-blockchain-programming$ . all-tests.sh 
=== ex-address ===

/tmp/ccfAVqub.o: In function `bbp_sha256':
ex-address.c:(.text+0x43c): undefined reference to `SHA256_Init'
ex-address.c:(.text+0x459): undefined reference to `SHA256_Update'
ex-address.c:(.text+0x46f): undefined reference to `SHA256_Final'

Problem reason,wrong command line:
gcc -ansi -Wall -Wno-deprecated-declarations -I $OPENSSL_INCLUDE -pedantic -lcrypto -o $1.out $1.c

You make a very common beginners mistake... Putting the libraries you link with in the wrong place on the command line when you build.

Dependencies are reversed on the command line, so something that depends on something else should actually be put before what it depends on on the command line.

In your example, you have a source file main.cpp that depends on some set of libraries, then the source file should be before the libraries it depend on:

$ g++ -o main main.cpp -lssl -lcrypto
To be safe, always put libraries last, after any source or object files listed on the command line.

solution: change the compilation to:

gcc -o $1.out $1.c -ansi -Wall -Wno-deprecated-declarations -I $OPENSSL_INCLUDE -pedantic -lssl -lcrypto



http://stackoverflow.com/questions/13784434/gcc-use-openssls-sha256-functions