TODO: use fopen(3), using read_fat_chain is done

A read() analogue wouldn't be particularly useful here since we're
just poking at bytes, but having a way to get the "real" offset in
a file sure would be!

TODO

@@ -1,4 +1,5 @@
 Implement randomness (xorshift? https://www.jstatsoft.org/article/view/v008i14)
-Maybe extract file "read"/"write" operations with read_fat_chain()
 Config parser (config.c)
 Do the actual corrupty things
+
+Use fopen(3) and friends instead of open(2)

src/dosfs.c

@@ -8,6 +8,8 @@
 #include "dosfs.h"
 #include "debug.h"
 
+static unsigned int *read_fat_chain(dosfs_t *, unsigned int, int *);
+
 /*
  * Opens an image a la open(2). Returns 0 on success, or
  * some error number on failure.
@@ -151,6 +153,9 @@ dos_listdir(dosfs_t *fsd, unsigned int offset)
 
         (void)memcpy(&cur->ent, &dirs[i], sizeof(struct dos_dirent));
 
+        /* useful for get_byte_offset() */
+        cur->fat_chain = read_fat_chain(fsd, cur->ent.low_loc,
+            &cur->fc_len);
 
         res = 0;
     }
@@ -234,39 +239,29 @@ read_fat_chain(dosfs_t *fsd, unsigned int first, int *length)
 }
 
 /*
- * TODO: reads a file to a given buffer
+ * Converts the byte offset of a file into the "real"
+ * offset, i.e. one that can be used with lseek()/pread().
  */
 int
-dos_fread(dosfs_t *fsd, dosfile_t *file, void *buf, int max_size)
+get_byte_offset(dosfs_t *fsd, dosfile_t *file, int f_offset)
 {
-    unsigned int *chain = NULL;
-    int clen = 0, i, left, chunk, cbytes, off, res;
+    int coff, csz;
 
-    chain = read_fat_chain(fsd, file->ent.low_loc, &clen);
-    if (chain == NULL) {
-        return -1; /* XXX */
+    if (f_offset > file->ent.size) {
+        DPRINTF("offset requested is out of bounds!\n");
+        return -EINVAL;
     }
 
-    cbytes = fsd->ib.sect_size * fsd->ib.sect_per_cluster;
-    left = max_size;
+    /* size in bytes of a cluster */
+    csz = fsd->ib.sect_per_cluster * fsd->ib.sect_size;
 
-    for (i = 0; i < clen; i++) {
-        /* only copy as much as we can hold */
-        chunk = (left < cbytes) ? left : cbytes;
-        
-        res = pread(fsd->ifd, buf + (i * cbytes),
-            chunk, fsd->data_loc + (chain[i] * cbytes));
-        if (res == -1) {
-            DPRINTF("pread failed\n");
-            free(chain);
-            return -1;
-        }
+    /* get the offset in the fat chain */
+    coff = f_offset / csz;
 
-        left -= chunk;
-
-        if (left <= 0)
-            break;
+    if (coff > file->fc_len) {
+        DPRINTF("offset is within filesize, but not enough clusters??\n");
+        return -EINVAL;
     }
 
-    return 0;
+    return (file->fat_chain[coff] * csz) + (f_offset % csz);
 }

src/dosfs.h

@@ -69,16 +69,17 @@ typedef struct dosfile_t {
     struct dos_dirent ent;
     char   fname[9];
     char   fext[4];
+    unsigned int *fat_chain;
+    int fc_len;
     struct dosfile_t  *next;
 } dosfile_t;
 
 int open_image(const char *, int, dosfs_t *);
-
 void close_image(dosfs_t *);
 
 dosfile_t *dos_listdir(dosfs_t *, unsigned int);
-int dos_fread(dosfs_t *, dosfile_t *, void *, int);
 void dos_freedir(dosfile_t *);
 
+int get_byte_offset(dosfs_t *, dosfile_t *, int);
 
 #endif /* !_FAT12_H */