How to link OpenSSL with emscripten?

Question

I'm trying to compile some C code that uses OpenSSL with emscripten, but I get unresolved symbol warnings like:

warning: unresolved symbol: SHA256_Init
warning: unresolved symbol: SHA256_Final
warning: unresolved symbol: SHA256_Update

I compiled the code using this command:

emcc SHA256.c -lssl -lcrypto -L /usr/local/openssl-1.0.2k/lib/ -I /usr/local/openssl-1.0.2k/include -s WASM=1 -o SHA256.html --emrun

With the following source code

#include <stdio.h>
#include <string.h>
#include <stdlib.h>


#include <openssl/sha.h>
#include <openssl/hmac.h>
#include <openssl/evp.h>


void sha256(char *string, char outputBuffer[65])
{
    unsigned char hash[SHA256_DIGEST_LENGTH];
    SHA256_CTX sha256;
    SHA256_Init(&sha256);
    SHA256_Update(&sha256, string, strlen(string));
    SHA256_Final(hash, &sha256);
    int i = 0;
    for(i = 0; i < SHA256_DIGEST_LENGTH; i++)
    {
        sprintf(outputBuffer + (i * 2), "%02x", hash[i]);
    }
    outputBuffer[64] = 0;
}


int main (void)
{
    static unsigned char buffer[65];
    sha256("string", buffer);
    printf("%s\n", buffer);

    return 0;
}

Answer 1

Compile the two libraries : crypto and openssl with emscripten, rather than using the system versions.

Use the generated .bc files for these libraries as linker arguments to the em++ command and it should work.