GCOBOL(3)

NAME
gcobol - GCC COBOL Front-end I/O function API

LIBRARY
libgcobol

SYNOPSIS
#include <symbols.h>
#include <io.h>
#include <gcobolio.h>
gcobol_io_t gcobol_fileops();

    class gcobol_io_t {
    public:
      static const char constexpr marquee[64];
      typedef void (open_t)( cblc_file_t *file,
                             char *filename,
                             int mode_char,
                             int is_quoted );
      typedef void (close_t)( cblc_file_t *file,
                              int how );
      typedef void (start_t)( cblc_file_t *file,
                              int relop, // needs enum
                              int first_last_key,
                              size_t length );
      typedef void (read_t)( cblc_file_t *file,
                             int where );
      typedef void (write_t)( cblc_file_t *file,
                              unsigned char  *data,
                              size_t length,
                              int after,
                              int lines,
                              int is_random );
      typedef void (rewrite_t)( cblc_file_t *file,
                                size_t length, bool is_random );
      typedef void (delete_t)( cblc_file_t *file,
                              bool is_random );
      open_t      *Open;
      close_t     *Close;
      start_t     *Start;
      read_t      *Read;
      write_t     *Write;
      rewrite_t   *Rewrite;
      delete_t    *Delete;
      ...
    };

DESCRIPTION
gcobol supplies replaceable I/O functionality via gcobol_fileops(). It returns a pointer to a structure of C function pointers that implement sequential, relative, and indexed file operations over files whose On Disk Format (ODF) is defined by gcobol. A user wishing to use another library that implements the same functionality over a different ODF must supply a different implementation of gcobol_fileops(), plus 7 functions, as described in this document. The pointers to those user-implemented functions are placed in a C++ object of type gcobol_io_t and an instantiation of that type is returned by gcobol_fileops(). The compiled program initializes I/O operations by calling that function the first time any file is opened.

Each function takes as its first argument a pointer to a cblc_file_t object, which is analogous to a FILE object used in the C stdio functions. The cblc_file_t structure acts as a communication area between the compiled program and the I/O library. Any information needed about the file is kept there. Notably, the outcome of any operation is stored in that structure in the file_status member, not as a return code. Information about the operation (as opposed to the file) appear as parameters to the function.

cblc_file_t has one member, not used by gcobol, that is reserved for the user:
    void * implementation.

User-supplied I/O functions may assign and dereference implementation. gcobol will preserve the value, but never references it.

The 7 function pointers in gcobol_io_t are

Open
    void open_t(cblc_file_t *file, char *filename, int mode_char, int is_quoted)
    parameters:
    filename
        is the filename, as known to the OS
    mode_char
        is one of
        ‘r’ OPEN INPUT: read-only mode
        ‘w’ OPEN OUTPUT: create a new file or overwrite an existing one
        ‘a’ EXTEND: append to sequential file
        ‘+’ modify existing file
    is_quoted
        If true, filename is taken literally. If false, filename is interpreted as the name of an environment variable, the contents of which, if extant, are taken as the name of the file to be opened. If no such variable exists, then filename is used verbatim.

Close
    void close_t(cblc_file_t *file, int how)
    parameters:
    how A value of 0x08 closes a “REEL unit”. Because no such thing is supported, the function sets the file status to “07”, meaning not a tape.

Start
    void start_t(cblc_file_t *file, int relop, int first_last_key, size_t length)
    parameters:
    relop
        is one of
        0   means ‘<’
        1   means ‘<=’
        2   means ‘=’
        3   means ‘!=’
        4   means ‘>=’
        5   means ‘>’
    first_last_key
        is the key number (starting at 1) of the key within the cblc_file_t structure.
    length
        is the size of the key (TODO: per the START statement?)

Read
    void read_t(cblc_file_t *file, int where) parameters:
    where
        -2  PREVIOUS
        -1  NEXT
        N   represents a key number, starting with 1, in the cblc_file_t structure. The value of that key is used to find the record, and read it.

Write
    void write_t(cblc_file_t *file, unsigned char *data, size_t length, int after, int lines, int is_random)
    parameters:
    data
        address of in-memory buffer to write
    length
        length of in-memory buffer to write
    after
        has the value 1 if the
            AFTER ADVANCING n LINES
        phrase was present in the WRITE statement, else 0
    lines
        may be one of
        -666
            ADVANCING PAGE
        -1  no ADVANCING phrase appeared
        0   ADVANCING 0 LINES is valid
        >0  the value of n in ADVANCING n LINES
    is_random
        is true if the access mode is RANDOM

Rewrite
    void rewrite_t(cblc_file_t *file, size_t length, bool is_random) parameters:
    length
        number of bytes to write
    is_random
        true if access mode is RANDOM

Delete
    void delete_t(cblc_file_t *file, bool is_random) parameters:
    is_random
        true if access mode is RANDOM

The library implements one function that the gcobol -produced binary calls directly:

gcobol_io_t * gcobol_fileops()
This function populates a gcobol_io_t object with the above function pointers. The compiled binary begins by calling gcobol_fileops(), and then uses the supplied pointers to effect I/O.

RETURN VALUES
I/O functions return void. gcobol_fileops() returns gcobol_io_t*.

STANDARDS
The I/O library supplied by gcobol, libgcobolio.so, supports the I/O semantics defined by ISO COBOL. It is not intended to be compatible with any other ODF. That is, libgcobolio.so cannot be used to exchange data with any other COBOL implementation.

The purpose of the gcobol_io_t structure is to allow the use of other I/O implementations with other ODF representations.

CAVEATS
The library is not well tested, not least because it is not implemented.

BUGS
The future is yet to come.