is this implementation real salsa20? https://raw.githubusercontent.com/andres-erbsen/salsa20/master/salsa20.c

I tried it, and it's working fine. Now question: is it real salsa20?
The Salsa20 specification paper contains examples for testing for each function. quarterround 7 example, rowround 2 example, etc..
Indeed this paper is written as a tool for programmers, or the analyzers so that they can use these test vectors.

Salsa20 exists in the Crypto++ which is a C++ library originally written by Wei Dai. You can use this code to test, too.

Also, the snuffle page of the Bernstein contains code for various platforms.