Problems with fwrite - c++

Question

I'm doing an external merge sort for an assignment and I'm given two structs:
// This is the definition of a record of the input file. Contains three fields, recid, num and str
typedef struct {
unsigned int recid;
unsigned int num;
char str[STR_LENGTH];
bool valid; // if set, then this record is valid
int blockID; //The block the record belongs too -> Used only for minheap
} record_t;
// This is the definition of a block, which contains a number of fixed-sized records
typedef struct {
unsigned int blockid;
unsigned int nreserved; // how many reserved entries
record_t entries[MAX_RECORDS_PER_BLOCK]; // array of records
bool valid; // if set, then this block is valid
unsigned char misc;
unsigned int next_blockid;
unsigned int dummy;
} block_t;
Also I'm given this:
void MergeSort (char *infile, unsigned char field, block_t *buffer,
unsigned int nmem_blocks, char *outfile,
unsigned int *nsorted_segs, unsigned int *npasses,
unsigned int *nios)
Now, at phase 0 I'm allocating memory like this:
buffer = (block_t *) malloc (sizeof(block_t)*nmem_blocks);
//Allocate disc space for records in buffer
record_t *records = (record_t*)malloc(nmem_blocks*MAX_RECORDS_PER_BLOCK*sizeof(record_t));
And then after I read the records from a binary file (runs smoothly), I write them to multiple files (after sorting of course and some other steps) with this command:
outputfile = fopen(name.c_str(), "wb");
fwrite(records, recordsIndex, sizeof(record_t), outputfile);
and read like this:
fread(&buffer[b].entries[rec],sizeof(record_t),1,currentFiles[b])
And it works! Then I want to combine some of these files to produce a larger sorted file using a priority_queue turned to minheap (it's tested, it works), but when I try to write to files using this command:
outputfile = fopen(outputName.c_str(), "ab"); //Opens file for appending
fwrite(&buffer[nmem_blocks-1].entries, buffer[nmem_blocks-1].
nreserved, sizeof(record_t), outputfile);
It writes nonsense in the file, as if it reads random parts of memory.
I know that the code is probably not nearly enough, but all of it is quite large.
I'm making sure I'm closing the output file before I open it again using a new name. Also I use memset() (and not free()) to clear the buffer before I fill it again.

Answer 1

Finally the main problem was the way I was trying to open the file:
outputfile = fopen(outputName.c_str(), "ab"); //Opens file for appending
Instead I should have used again:
outputfile = fopen(outputName.c_str(), "wb"); //Opens file for writing to end of file
Because the file in the meantime never closed, so it was trying to open an already open file for appending and it didn't work quite well. But you couldn't have known since you didn't have the full code. Thank you for your help though! :)