[root@VM-0-16-centos openssl]# cat g.c
#include<stdio.h>
#include<openssl/bio.h>
int main()
{
        int n = 0;
        char *p = NULL;
        BIO *mem = BIO_new(BIO_s_mem());
        BIO_puts(mem, "123");
        BIO_puts(mem, "456");
        n=BIO_get_mem_data(mem, &p);
        BIO_set_close(mem, BIO_NOCLOSE);
        printf("%d %s\n", n, p);
        BIO_free(mem);

        return 0;

}
[root@VM-0-16-centos openssl]#
[root@VM-0-16-centos openssl]#
[root@VM-0-16-centos openssl]# gcc g.c -lcrypto
[root@VM-0-16-centos openssl]#
[root@VM-0-16-centos openssl]#
[root@VM-0-16-centos openssl]# ./a.out
6 123456
[root@VM-0-16-centos openssl]#
[root@VM-0-16-centos openssl]#