227 lines
5.6 KiB
Markdown
227 lines
5.6 KiB
Markdown
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Specification of DUMBFILE_SYSTEM
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================================
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DUMB is designed filesystem-agnostic, even though the C standard library
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already defines an abstraction over files on a disk. This is useful because
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Allegro 4 and 5 define their own abstractions.
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To register your own filesystem abstraction with DUMB, you must create an
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instance of struct `DUMBFILE_SYSTEM`, fill in your own function pointers
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according to the specification below, and call `register_dumbfile_system` on
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your instance.
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The header `dumb.h` defines `DUMBFILE_SYSTEM` as a struct of function pointers:
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```
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typedef struct DUMBFILE_SYSTEM
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{
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void *(*open)(const char *filename);
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int (*skip)(void *f, dumb_off_t n);
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int (*getc)(void *f);
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dumb_ssize_t (*getnc)(char *ptr, size_t n, void *f);
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void (*close)(void *f);
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int (*seek)(void *f, dumb_off_t n);
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dumb_off_t (*get_size)(void *f);
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}
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DUMBFILE_SYSTEM;
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```
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Here, `dumb_off_t` is a signed integer at least 64 bits wide, it is intended
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to measure file offsets. The return type `dumb_ssize_t` is a signed integer
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exactly as wide as `size_t`, it is intended to store either a `size_t` or a
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negative error code. Both `dumb_*_t` are defined in `dumb.h`.
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The function pointers `skip` and `getnc` are optional, i.e., you may set
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some of these to `NULL` in your struct instance. DUMB will then try to
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mimick the missing functions' behavior by calling your `getc` several times.
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If DUMB is built with debugging flags, it will assert that all other
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functions are not `NULL`. In release mode, DUMB will silently fail.
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Your non-`NULL` function pointers must conform to the following specification.
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open
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----
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```
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void *(*open)(const char *filename);
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```
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Open a file for reading.
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Arguments:
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* `const char *filename`: A normal filename as understood by the operating
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system. Will be opened for reading.
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Returns as `void *`:
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* the address of a file handle on successfully opening the file.
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DUMB will pass this file handle as argument to other functions of
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the `DUMBFILE_SYSTEM`.
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* `NULL` on error during opening the file.
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Each file has a *position* internally managed by DUMB. A newly opened file
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has a position of 0. Other functions from the `DUMBFILE_SYSTEM` can move
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this position around.
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DUMB allocates memory for the successfully opened file, and will store opaque
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information in that memory, e.g., the DUMB-internal file position. This memory
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be freed when DUMB calls `close` on the file's handle. The memory is separate
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from your own filesystem implementation: You are responsible for supplying the
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data, and DUMB is responsible for storing anything about interpreting that
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data.
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skip
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----
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```
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int (*skip)(void *f, dumb_off_t n);
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```
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Advance the position in the file.
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Arguments:
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* `void *f`: A file handle that `open` returned. Guaranteed non-`NULL`.
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* `dumb_off_t n`: Number of bytes to advance in the file. DUMB will only
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call this with `n >= 0`. For `n < 0`, the behavior of `skip` is undefined.
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Returns as `int`:
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* `0` on successfully skipping ahead by `n` bytes.
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* `-1` on error.
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It is legal to set `skip = NULL` in a `DUMBFILE_SYSTEM`. DUMB will then call
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`getc` a total of `n` times to skip ahead in a file. For speed, it is
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advisable to supply a proper `skip` implementation.
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getc
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----
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```
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int (*getc)(void *f);
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```
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Read a byte from the file.
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Arguments:
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* `void *f`: A file handle that `open` returned. Guaranteed non-`NULL`.
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Returns as `int`:
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* the value of the byte read, on successfully reading one byte.
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* `-1` on error.
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After a succesful read, DUMB will treat the file as advanced by one byte.
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getnc
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-----
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```
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dumb_ssize_t (*getnc)(char *ptr, size_t n, void *f);
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```
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Read up to the given number of bytes from the file into a given buffer.
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* `char *ptr`: The start of a buffer provided by DUMB.
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* `size_t n`: The length of the number of bytes to be read.
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* `void *f`: A file handle that `open` returned. Guaranteed non-`NULL`.
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Returns as `dumb_ssize_t`:
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* the number of bytes successfully read, if it was possible to read at least
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one byte.
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* `-1` on error.
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This function shall bytes from the file `f` and store them in sequence in the
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buffer beginning at `ptr`. It shall read fewer than `n` bytes if end of file
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is encountered before `n` bytes could have been read, otherwise it should read
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`n` bytes.
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It is legal to set `skip = NULL` in a `DUMBFILE_SYSTEM`. DUMB will then call
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`getc` a total of `n` times and store the results in its buffer.
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close
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-----
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```
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void (*close)(void *f);
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```
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Closes a file that has been opened before with `open`.
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Arguments:
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* `void *f`: A file handle that `open` returned. Guaranteed non-`NULL`.
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DUMB will deallocate the memory that it used to interpret the file. You are
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free to treat your resource however you would like: You may deallocate it, or
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keep it around for other things. For example, Allegro 5's implementation
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of `close` takes a void pointer and does nothing with it at all.
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seek
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----
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```
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int (*seek)(void *f, dumb_off_t n);
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```
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Jump to an arbitrary position in the file.
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Arguments:
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* `void *f`: A file handle that `open` returned. Guaranteed non-`NULL`.
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* `dumb_off_t n`: The position in the file, relative to the beginning.
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There is no guarantee whether `n >= 0`.
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Returns as `int`:
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* `0` on successfully seeking in the file.
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* `-1` on error.
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DUMB will modify its internal position of the file accordingly.
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A value of `n < 0` shall set the file into an erroneous state from which no
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bytes can be read.
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get_size
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--------
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```
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dumb_off_t (*get_size)(void *f);
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```
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Get the length in bytes, i.e., the position after the final byte, of a file.
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Arguments:
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* `void *f`: A file handle that `open` returned. Guaranteed non-`NULL`.
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Returns as `dumb_off_t`:
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* the length of the file in bytes.
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