openssl_pkcs7_sample.c

#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>

#include "openssl/err.h"
#include "openssl/objects.h"
#include "openssl/evp.h"
#include "openssl/x509.h"
#include "openssl/pkcs7.h"
#include "openssl/pem.h"

int get_file_size(const char* fname) {
    struct stat st;
    if (stat(fname, &st) != 0) {
        fprintf(stderr, "cannot open file");
        return -1;
    }
    return st.st_size;
}

int main() {
    const char* in_file = "test.der";
    FILE* fin = fopen(in_file, "rb");
    if (!fin) {
        fprintf(stderr, "cannot open file");
        return 1;
    }

    BIO* out = BIO_new_fp(stdout, BIO_NOCLOSE | BIO_FP_TEXT);

    const int fsize = get_file_size(in_file);
    char* buffer = (char*) malloc(fsize);
    fread(buffer, sizeof(char), fsize, fin);

    BIO* mem = BIO_new_mem_buf(buffer, fsize);
    PKCS7* p7 = d2i_PKCS7_bio(mem, NULL);
    PKCS7_print_ctx(out, p7, 0, NULL);

    return 0;
}

I don't free any resources in this sample program. If you use this snippet, please call free function to release the allocated resources.