9.13. Unformatted I/O

This is simple: only two functions provide this facility, one for reading and one for writing:

#include <stdio.h>

size_t fread(void *ptr, size_t size, size_t nelem, FILE *stream);
size_t fwrite(const void *ptr, size_t size, size_t nelem, FILE *stream);

In each case, the appropriate read or write is performed on the data pointed to by ptr. Up to nelem elements, of size size, are transferred. Failure to transfer the full number is an error only when writing; End of File can prevent the full number on input. The number of elements actually transferred is returned. To distinguish between End of File on input, or an error, use feof or ferror.

If size or nelem is zero, fread does nothing except to return zero.

An example may help.

#include <stdio.h>
#include <stdlib.h>

struct xx{
        int xx_int;
        float xx_float;
}ar[20];

main(){

        FILE *fp = fopen("testfile", "w");

        if(fwrite((const void *)ar,
                sizeof(ar[0]), 5, fp) != 5){

                fprintf(stderr,"Error writing\n");
                exit(EXIT_FAILURE);
        }

        rewind(fp);

        if(fread((void *)&ar[10],
                sizeof(ar[0]), 5, fp) != 5){

                if(ferror(fp)){
                        fprintf(stderr,"Error reading\n");
                        exit(EXIT_FAILURE);
                }
                if(feof(fp)){
                        fprintf(stderr,"End of File\n");
                        exit(EXIT_FAILURE);
                }
        }
        exit(EXIT_SUCCESS);
}