SSLeay reading a PEM file - c++

Question

I am trying to use the PEM_read_bio function to get data from a file.
The version of SSLeay we are using is from 1997, so documentation is a bit thin on the ground. Thankfully in this case it seems there is a matching function documented here: https://www.openssl.org/docs/man1.1.0/crypto/PEM_read_bio.html
I originally tried this:
char ** names;
char ** headers;
unsigned char ** data;
long len;
BIO *in = BIO_new_file("C:\\filename.txt", "r");
if (!in)
{
// error
}
else
{
int result = PEM_read_bio(in, names, headers, data, &len);
}
BIO_free(in);
OPENSSL_free(names);
OPENSSL_free(headers);
OPENSSL_free(data);
However this results in a run-time check failure: The variable 'names' is being used without being initialized.
The documentation mentions OPENSSL_malloc( num ) is used to initialize memory, but it fails to mention whether it does this behind the scenes, or the user does it.
OPENSSL_malloc is similar in usage to C's malloc, but how are we supposed to know how much memory to allocate in advance, before reading the file?
I have tried the following at the beginning:
char ** names = reinterpret_cast<char **>(OPENSSL_malloc(2));
char ** headers = reinterpret_cast<char **>(OPENSSL_malloc(2));
unsigned char ** data = reinterpret_cast<unsigned char **>(OPENSSL_malloc(2));
long len;
This results in apparently random data.

Answer 1

The documentation you linked to says:
The name, header and data pointers are allocated via OPENSSL_malloc() and should be freed by the caller via OPENSSL_free() when no longer needed.
That means PEM_read_bio() calls OPENSSL_malloc() for you, and then you call OPENSSL_free() on the allocated memory it returns when you are doing with it.
You are passing uninitialized pointers to PEM_read_bio(), that is why it is failing. The name, header and data parameters are all output parameters. You need to pass in the addresses of your own pointer variables to receive the memory that PEM_read_bio() allocates for you, eg:
char *name;
char *headers;
unsigned char *data;
long len;
BIO *in = BIO_new_file("C:\\filename.txt", "r");
if (!in)
{
// error
}
else
{
int result = PEM_read_bio(in, &name, &headers, &data, &len);
if (!result)
{
// error
}
else
{
...
OPENSSL_free(name);
OPENSSL_free(headers);
OPENSSL_free(data);
}
BIO_free(in);
}