Segmentation fault after mapping a file to memory - c

Question

I am trying to "mmap" a binary file in order to encrypt it using AES then write the encrypted data to another file(outFile) using the following code. I tried to modify the flags for both functions mmap() and open() but I always get segmentation fault when I run the executable.
int main (void)
{
FILE *outFile; //The output file (encrypted)
/* A 256 bit key */
unsigned char *key = (unsigned char *)"01234567890123456789012345678901";
/* A 128 bit IV */
unsigned char *iv = (unsigned char *)"01234567890123456";
int fd;
struct stat sb;
void * memblock;
fd = open("result0.jpg",O_RDONLY);
outFile=fopen("result0enc.jpg","wb");
fstat(fd, &sb);
printf("Size: %lu\n", sb.st_size);
unsigned char decryptedtext[sb.st_size];
int decryptedtext_len, ciphertext_len;
/* Initialise the library */
ERR_load_crypto_strings();
OpenSSL_add_all_algorithms();
OPENSSL_config(NULL);
memblock = mmap(NULL, sb.st_size,PROT_READ, MAP_SHARED, fd, 0);
if (memblock == MAP_FAILED) {
close(fd);
perror("Error mmapping the file");
exit(EXIT_FAILURE);
}
ciphertext_len = encrypt((unsigned char *)memblock, sb.st_size,key,iv,ciphertext);
fwrite( ciphertext,1, sb.st_size,outFile);
if (munmap(memblock, sb.st_size) == -1) {
perror("Error un-mmapping the file");
/* Decide here whether to close(fd) and exit() or not. Depends... */
}
close(fd);
fclose(outFile);
EVP_cleanup();
ERR_free_strings();
return 0;
}

Answer 1

As yano mentioned in the comments, your error is here:
memcpy(outFile, ciphertext, sb.st_size);
You're trying to memcpy to a FILE * which is completely wrong. That doesn't do at all what you expect. You're overwriting the private internals of the FILE structure to which outFile points.
You should instead operate on a buffer and use fwrite to write to the file.
I suggest you get familiar with basic file I/O operations using f... functions before digging into mmap and encryption.