Problems with fwrite

我正在为一个赋值做一个外部归并排序，我有两个结构体:

    // 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;

我还得到了这个:

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)

现在，在阶段0，我像这样分配内存:

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));

然后，在我从二进制文件中读取记录(运行顺利)之后，我用以下命令将它们写入多个文件(当然是在排序和其他一些步骤之后):

outputfile = fopen(name.c_str(), "wb");
fwrite(records, recordsIndex, sizeof(record_t), outputfile);

和这样读:

fread(&buffer[b].entries[rec],sizeof(record_t),1,currentFiles[b])

它有效!然后，我想将其中一些文件组合起来，使用priority_queue转换为minheap生成一个更大的排序文件(经过测试，它可以工作)，但是当我尝试使用以下命令写入文件时:

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);

它在文件中写入无意义的内容，就好像它读取内存的随机部分一样。

我知道代码可能远远不够，但是所有的代码都相当大。我要确保在使用新名称再次打开输出文件之前关闭它。此外，我使用memset()(而不是free())来清除缓冲区，然后再填充它。