In the code below, I have saved the values 1 - 9 in an Excel file and I want to insert an "a" between "4" and "5". I have set the pointer to position 7 but it is still inserting it at the end. Please help me understand this. 

```
FILE *ExcelFile = fopen("testdata.csv","a");
 if (ExcelFile == NULL)
 return -1;
 fprintf(ExcelFile,"1 2 3 4 5 6 7 8 9");
 fseek (ExcelFile, 7, SEEK_SET );
 //printf("pos is %ld bytes\n", pos);
 fprintf(ExcelFile,"a");
 fclose(ExcelFile);

```