Reading Unformatted Binary file: Unexpected output - Fortran90 - io

Question

Preface: I needed to figure out the structure of a binary grid_data_file. From the Fortran routines I figured that the first record consists of 57 bytes and has information in the following order.
No. of the file :: integer*4
File name :: char*16
file status :: char*3 (i.e. new, old, tmp)
.... so forth (rest is clear from write statement in the program)
Now for the testing I wrote a simple program as follows: (I haven't included all the parameters)
Program testIO
implicit none
integer :: x, nclat, nclon
character :: y, z
real :: lat_gap, lon_gap, north_lat, west_lat
integer :: gridtype
open(11, file='filename', access='direct', form='unformatted', recl='200')
read(11, rec=1) x,y,z,lat_gap,lon_gap, north_lat,west_lat, nclat, nclon, gridtyp
write(*,*) x,y,z,lat_gap,lon_gap, north_lat,west_lat, nclat, nclon, gridtyp
close(11)
END
To my surprise, when I change the declaration part to
integer*4 :: x, nclat, nclon
character*16 :: y
character*3 :: z
real*4 :: lat_gap, lon_gap, north_lat, west_lat
integer*2 :: gridtype
It gives me SOME correct information, albeit not all! I can't understand this. It would help me to improve my Fortran knowledge if someone explains this phenomenon.
Moreover, I can't use ACCESS=stream due to machine being old and not supported, so I conclude that above is the only possibility to figure out the file structure.

Answer 1

From your replies and what others have commented, I think your problem might be a misunderstanding of what a Fortran "record" is:
You say that you have a binary file where each entry (you said record, but more on that later) is 57 bytes.
The problem is that a "record" in Fortran I/O is not what you would expect it is coming from a C (or anywhere else, really) background. See the following document from Intel, which gives a good explanation of the different access modes:
https://software.intel.com/sites/products/documentation/hpc/composerxe/en-us/2011Update/fortran/lin/bldaps_for/common/bldaps_rectypes.htm
In short, it has extra data (a header) describing the data in each entry.
Moreover, I can't use ACCESS=stream due to machine being old and not supported, so I conclude that above is the only possibility to figure out the file structure. Any guidance would be a big help!
If you can't use stream, AFAIK there is really no simple and painless way to read binary files with no record information.
A possible solution which requires a C compiler is to do IO in a C function that you call from Fortran, "minimal" example:
main.f90:
program main
integer, parameter :: dp = selected_real_kind(15)
character(len=*), parameter :: filename = 'test.bin'
real(dp) :: val
call read_bin(filename, val)
print*, 'Read: ', val
end program
read.c:
#include <string.h>
#include <stdio.h>
void read_bin_(const char *fname, double *ret, unsigned int len)
{
char buf[256];
printf("len = %d\n", len);
strncpy(buf, fname, len);
buf[len] = '\0'; // fortran strings are not 0-terminated
FILE* fh = fopen(buf, "rb");
fread(ret, sizeof(double), 1, fh);
fclose(fh);
}
Note that there is an extra parameter needed in the end and some string manipulation because of the way Fortran handles strings, which differs from C.
write.c:
#include <stdio.h>
int main() {
double d = 1.234;
FILE* fh = fopen("test.bin", "wb");
fwrite(&d, sizeof(double), 1, fh);
fclose(fh);
}
Compilation instructions:
gcc -o write write.c
gcc -c -g read.c
gfortran -g -o readbin main.f90 read.o
Create binary file with ./write, then see how the Fortran code can read it back with ./readbin.
This can be extended for different data types to basically emulate access=stream. In the end, if you can recompile the original Fortran code to output the data file differently, this will be the easiest solution, as this one is pretty much a crude hack.
Lastly, a tip for getting into unknown data formats: The tool od is your friend, check its manpage. It can directly convert binary represantations into a variety of different native datatypes. Try with the above example (the z adds the character representation in the right-hand column, not very useful here, in general yes):
od -t fDz test.bin