SMIME with BIO, char * and binary data - c

Question

The problem
1- I'd like to create a MIME message. Something like this:
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary="---12345"
This is a multipart message in MIME format.
---12345
Content-Type: text/plain
This is the plain text
---12345
Content-Type: application/pdf
>> PDF binary code here <<
---12345
2- Then i need to pass it to the OpenSSL functions in the form of BIO * data.
PKCS7 *PKCS7_sign(..., ..., ..., BIO *data, ...);
The first approach
Load the plain text and attachment data from the filesystem and assign it to char * data, manipulate the data to add the respective MIME headers, finally assign it to BIO * memoryBIO with BIO_puts(memoryBIO, data);.
But this approach doesnt work because the binary data contains "\0"(NULL) which wont go well with char type.
The second approach
Assign plain text and attachment to a BIO each and then "concatenate" them. But couldn't find a way to do this.
Conclusion
I'd like to know if there is a way to accomplish such feat.
I'd also like to avoid using intermediary files and build everything in-memory.

Answer 1

After following jww's suggestion, what i ended up using is BIO_write();.
BIO * inBIO = NULL;
std::vector<unsigned char> data = GetData();
inBIO = BIO_new(BIO_s_mem());
BIO_write(inBIO, data.data(), data.size());
Again thanks to jww for this answer and for all the other answers regarding OpenSSL in SO, you've helped me a great deal.