With few exceptions, mixing Fortran and C input/output calls is asking for trouble. Often the runtime libraries of both languages implement separate buffering schemes in order to improve performance.
Terminal Input
One should never mix READ(5,...) and
fgets(...,stdin)
requests.
On most systems it works as long as the input device is really a terminal.
However, at the latest when stdin is connected to a file or a
pipe the Fortran READ will suck in more than just its share
to satisfy the immediate request.
Any following fgets will then access the wrong information
(or report end-of-file).
Terminal Output
Mixing WRITE(6,...) and fprintf(stdout,...) is safe as
long as the output device is the terminal.
When stdout is redirected the C stream functions usually start
buffering the output.
On a number of Unix systems (RS/6000, Alpha/OSF, Ultrix) the Fortran runtime library does not use the C stream functions, and the Fortran and C output buffers are flushed out of sequence. The C runtime libraries on these systems can be told to limit themselves to line buffering by
if( !isatty( fileno(stdout) ) )
setlinebuf( stdout );
It seems that the Fortran runtime libraries are already disciplined
enough to use for WRITE(6,...) and PRINT ... only line
buffering.
Note that setlinebuf is not a standard Unix function.
Hopefully, if it is not contained in stdio.h, it also is not necessary.
Formatted I/O
There is no general way to associate a Fortran logical unit number with
a C FILE* stream.
KUIP offers C functions ku_read and ku_write
as a convenient interface to Fortran I/O
statements for reading/writing complete lines.
For details refer to the KUIP manual.
(Note that even in Fortran a READ(LUN,'(A)') CHLINE is not
portable because the VS-Fortran runtime library is not able to handle
V-format files.)
Unformatted I/O
Some systems provide a function getfd to inquire about the C file
descriptor associated to a Fortran logical unit number.
This is however non-standard, and often Fortran unformatted sequential
I/O adds system-dependent control information to allow for variable
length records.
The CIO package in KERNLIB provides a Fortran callable set of
interface routines to the C read and write functions in
order to perform system-independent unformatted I/O, both for
sequential and direct access with fixed record sizes.