2.4 Stream I/O

A new “stream” I/O scheme of the Fortran 2003 standard is implemented in f95. Stream I/O access treats a data file as a continuous sequence of bytes, addressable by a positive integer starting from 1. Declare a stream I/O file with the ACCESS=’STREAM’ specifier on the OPEN statement. File positioning to a byte address requires a POS=scalar_integer_expression specifier on a READ or WRITE statement. The INQUIRE statement accepts ACCESS=’STREAM’, a specifier STREAM=scalar_character_variable, and POS=scalar_integer_variable.

Stream I/O is very useful when interoperating with files created or read by C programs, as is shown in the following example:

Fortran 95 program reads files created by C fwrite()

program reader
 integer:: a(1024), i, result
 open(file="test", unit=8, access="stream",form="unformatted")
! read all of a
 read(8) a
 do i = 1,1024
   if (a(i) .ne. i-1) print *,’error at ’, i
 enddo
! read the file backward
 do i = 1024,1,-1
   read(8, pos=(i-1)*4+1) result
   if (result .ne. i-1) print *,’error at ’, i
 enddo
 close(8)
end

C program writes to a file

#include <stdio.h>
int binary_data[1024];

/* Create a file with 1024 32-bit integers */
int
main(void)
{
    int i;
    FILE *fp;

    for (i = 0; i < 1024; ++i)
        binary_data[i] = i;
    fp = fopen("test", "w");
    fwrite(binary_data, sizeof(binary_data), 1, fp);
    fclose(fp);
}

The C program writes 1024 32-bit integers to a file using C fwrite(). The Fortran 95 reader reads them once as an array, and then reads them individually going backwards through the file. The pos= specifier in the second read statement illustrates that positions are in bytes, starting from byte 1 (as opposed to C, where they start from byte 0).