ref: 7dd890bbf123e4790540a64866eb68e97cd66210
parent: 925fec1d9998e8d9eb9553bf26cccd877bac577c
parent: 5b10b56c38c47f611df03f1e63299240c5b13874
author: Christopher Snowhill <kode54@gmail.com>
date: Sat Sep 16 09:47:26 EDT 2017
Merge pull request #47 from SimonN/spec DUMBFILE_SYSTEM spec
--- /dev/null
+++ b/DUMBFILE_SYSTEM.md
@@ -1,0 +1,221 @@
+Specification of DUMBFILE_SYSTEM
+================================
+
+The header `dumb.h` defines `DUMBFILE_SYSTEM` as a struct of function pointers:
+
+```
+typedef struct DUMBFILE_SYSTEM
+{+ void *(*open)(const char *filename);
+ int (*skip)(void *f, long n);
+ int (*getc)(void *f);
+ long (*getnc)(char *ptr, long n, void *f);
+ void (*close)(void *f);
+ int (*seek)(void *f, long n);
+ long (*get_size)(void *f);
+}
+DUMBFILE_SYSTEM;
+```
+
+DUMB is designed filesystem-agnostic, even though the C standard library
+already defines an abstraction over files on a disk. This is useful because
+Allegro 4 and 5 define their own abstractions.
+
+To register your own filesystem abstraction with DUMB, you must create an
+instance of struct `DUMBFILE_SYSTEM`, fill in your own function pointers
+according to the specification below, and call `register_dumbfile_system` on
+your instance.
+
+The function pointers `skip` and `getnc` are optional, i.e., you may set
+some of these to `NULL` in your struct instance. DUMB will then try to
+mimick the missing functions' behavior by calling your `getc` several times.
+If DUMB is built with debugging flags, it will assert that all other
+functions are not `NULL`. In release mode, DUMB will silently fail.
+
+Your non-`NULL` function pointers must conform to the following specification.
+
+
+
+open
+----
+
+```
+void *(*open)(const char *filename);
+```
+
+Open a file for reading.
+
+Arguments:
+
+* `const char *filename`: A normal filename as understood by the operating
+ system. Will be opened for reading.
+
+Returns as `void *`:
+
+* the address of a file handle on successfully opening the file.
+ DUMB will pass this file handle as argument to other functions of
+ the `DUMBFILE_SYSTEM`.
+
+* `NULL` on error during opening the file.
+
+Each file has a *position* internally managed by DUMB. A newly opened file
+has a position of 0. Other functions from the `DUMBFILE_SYSTEM` can move
+this position around.
+
+DUMB allocates memory for the successfully opened file, and will store opaque
+information in that memory, e.g., the DUMB-internal file position. This memory
+be freed when DUMB calls `close` on the file's handle. The memory is separate
+from your own filesystem implementation: You are responsible for supplying the
+data, and DUMB is responsible for storing anything about interpreting that
+data.
+
+
+
+skip
+----
+
+```
+int (*skip)(void *f, long n);
+```
+
+Advance the position in the file.
+
+Arguments:
+
+* `void *f`: A file handle that `open` returned. Guaranteed non-`NULL`.
+
+* `long n`: Number of bytes to advance in the file. DUMB guarantees to
+ call this function only with `n >= 0`.
+
+Returns as `int`:
+
+* `0` on successfully skipping ahead by `n` bytes.
+
+* `-1` on error.
+
+It is legal to set `skip = NULL` in a `DUMBFILE_SYSTEM`. DUMB will then call
+`getc` a total of `n` times to skip ahead in a file. For speed, it is
+advisable to supply a proper `skip` implementation.
+
+
+
+getc
+----
+
+```
+int (*getc)(void *f);
+```
+
+Read a byte from the file.
+
+Arguments:
+
+* `void *f`: A file handle that `open` returned. Guaranteed non-`NULL`.
+
+Returns as `int`:
+
+* the value of the byte read, on successfully reading one byte.
+
+* `-1` on error.
+
+After a succesful read, DUMB will treat the file as advanced by one byte.
+
+
+
+getnc
+-----
+
+```
+long (*getnc)(char *ptr, long n, void *f);
+```
+
+Read up to the given number of bytes from the file into a given buffer.
+
+* `char *ptr`: The start of a buffer provided by DUMB.
+
+* `long n`: The length of the number of bytes to be read. DUMB guarantees
+ to call this function only with `n >= 0`.
+
+* `void *f`: A file handle that `open` returned. Guaranteed non-`NULL`.
+
+Returns as `long`:
+
+* the number of bytes successfully read.
+
+* `-1` on error.
+
+This function shall bytes from the file `f` and store them in sequence in the
+buffer beginning at `ptr`. It shall read fewer than `n` bytes if end of file
+is encountered before `n` bytes could have been read, otherwise it should read
+`n` bytes.
+
+It is legal to set `skip = NULL` in a `DUMBFILE_SYSTEM`. DUMB will then call
+`getc` a total of `n` times and store the results in its buffer.
+
+
+
+close
+-----
+
+```
+void (*close)(void *f);
+```
+
+Closes a file that has been opened before with `open`.
+
+Arguments:
+
+* `void *f`: A file handle that `open` returned. Guaranteed non-`NULL`.
+
+DUMB will deallocate the memory that it used to interpret the file. You are
+free to treat your resource however you would like: You may deallocate it, or
+keep it around for other things. For example, Allegro 5's implementation
+of `close` takes a void pointer and does nothing with it at all.
+
+
+
+seek
+----
+
+```
+int (*seek)(void *f, long n);
+```
+
+Jump to an arbitrary position in the file.
+
+Arguments:
+
+* `void *f`: A file handle that `open` returned. Guaranteed non-`NULL`.
+
+* `long n`: The position in the file, relative to the beginning.
+ There is no guarantee whether `n >= 0`.
+
+Returns as `int`:
+
+* `0` on successfully seeking in the file.
+
+* `-1` on error.
+
+DUMB will modify its internal position of the file accordingly.
+
+A value of `n < 0` shall set the file into an erroneous state from which no
+bytes can be read.
+
+
+
+get_size
+--------
+
+```
+long (*get_size)(void *f);
+```
+
+Get the length in bytes, i.e., the position after the final byte, of a file.
+
+Arguments:
+
+* `void *f`: A file handle that `open` returned. Guaranteed non-`NULL`.
+
+Returns as `long`:
+
+* the length of the file in bytes.
--- a/src/core/dumbfile.c
+++ b/src/core/dumbfile.c
@@ -106,6 +106,7 @@
+/* Move forward in the file from the current position by n bytes. */
int dumbfile_skip(DUMBFILE *f, long n)
{int rv;
@@ -372,6 +373,9 @@
+/* Move to an arbitrary position n in the file, specified relative to origin,
+ * where origin shall be one of the DFS_SEEK_* constants.
+ */
int dumbfile_seek(DUMBFILE *f, long n, int origin)
{switch ( origin )
@@ -378,6 +382,7 @@
{case DFS_SEEK_CUR: n += f->pos; break;
case DFS_SEEK_END: n += (*f->dfs->get_size)(f->file); break;
+ default: break; /* keep n, seek position from beginning of file */
}
f->pos = n;
return (*f->dfs->seek)(f->file, n);