C: One pointer for reading and one pointer for updating the same file

I need to build a program that reads each record, and according to that record information would update some other records on the same file. For that, I was thinking in this approach: ``` int main(int argc, char *argv[]) { FILE *my_file; int files_read; struct my_struct an_struct; my_file = fopen("myfile.dat", "rb"); files_read = fread(&an_struct, sizeof(struct my_struct), 1, my_file); printf("main->files_read: %d \n", files_read); //This prints one while (files_read == 1) { do_update(); files_read = fread(&an_struct, sizeof(struct my_struct), 1, my_file); printf("main->files_read: %d \n", files_read); //This prints one } fclose(archivo_paises); return 0; } ``` In the main function I'm reading the contents of the file, and every time I call `read` I get one as a response until I reach the end of the file. The problem is in the `do_update` function: ``` void do_update() { FILE *my_file; int files_read; struct my_struct an_struct; struct my_struct another_struct; my_files = fopen("myfile.dat", "wb+"); //Using rb+ solves it files_read = fread(&an_struct, sizeof(struct my_struct), 1, my_file); printf("do_update->files_read: %d \n", files_read); //This printed zero!. Prints one using rb+ while (files_read == 1) { //This never gets executed. Unless you use rb+ if(something){ fwrite(&another_struct, sizeof(struct my_struct), 1, my_file); // Using rb+, this returns zero and didn't update } files_read = fread(&an_struct, sizeof(struct my_struct), 1, my_file); printf("do_update->files_read: %d \n", files_read); } fclose(my_file); } ``` What's happening is that the `files_read` variable gets the value of zero after the `read` call, so the logic to update the file is never executed. Why is `read` returning zero when opening a file for `wb+`? Update: Using `rb+` as file mode on `do_update()` works, but now the call to `fwrite()` always returns zero, and it didn't update the file. Is is related to the mode?