## 2 BFD Front End

### 2.1 `typedef bfd`

A BFD has type `bfd`; objects of this type are the cornerstone of any application using BFD. Using BFD consists of making references though the BFD and to data in the BFD.

Here is the structure that defines the type `bfd`. It contains the major data about the file and pointers to the rest of the data.

         struct bfd
         {
           /* A unique identifier of the BFD  */
           unsigned int id;

           /* The filename the application opened the BFD with.  */
           const char *filename;

           /* A pointer to the target jump table.  */
           const struct bfd_target *xvec;

           /* The IOSTREAM, and corresponding IO vector that provide access
              to the file backing the BFD.  */
           void *iostream;
           const struct bfd_iovec *iovec;

           /* Is the file descriptor being cached?  That is, can it be closed as
              needed, and re-opened when accessed later?  */
           bfd_boolean cacheable;

           /* Marks whether there was a default target specified when the
              BFD was opened. This is used to select which matching algorithm
              to use to choose the back end.  */
           bfd_boolean target_defaulted;

           /* The caching routines use these to maintain a
              least-recently-used list of BFDs.  */
           struct bfd *lru_prev, *lru_next;

           /* When a file is closed by the caching routines, BFD retains
              state information on the file here...  */
           ufile_ptr where;

           /* ... and here: (``once'' means at least once).  */
           bfd_boolean opened_once;

           /* Set if we have a locally maintained mtime value, rather than
              getting it from the file each time.  */
           bfd_boolean mtime_set;

           /* File modified time, if mtime_set is TRUE.  */
           long mtime;

           /* Reserved for an unimplemented file locking extension.  */
           int ifd;

           /* The format which belongs to the BFD. (object, core, etc.)  */
           bfd_format format;

           /* The direction with which the BFD was opened.  */
           enum bfd_direction
             {
               no_direction = 0,
               read_direction = 1,
               write_direction = 2,
               both_direction = 3
             }
           direction;

           /* Format_specific flags.  */
           flagword flags;

           /* Currently my_archive is tested before adding origin to
              anything. I believe that this can become always an add of
              origin, with origin set to 0 for non archive files.  */
           ufile_ptr origin;

           /* Remember when output has begun, to stop strange things
              from happening.  */
           bfd_boolean output_has_begun;

           /* A hash table for section names.  */
           struct bfd_hash_table section_htab;

           /* Pointer to linked list of sections.  */
           struct bfd_section *sections;

           /* The last section on the section list.  */
           struct bfd_section *section_last;

           /* The number of sections.  */
           unsigned int section_count;

           /* Stuff only useful for object files:
              The start address.  */
           bfd_vma start_address;

           /* Used for input and output.  */
           unsigned int symcount;

           /* Symbol table for output BFD (with symcount entries).  */
           struct bfd_symbol  **outsymbols;

           /* Used for slurped dynamic symbol tables.  */
           unsigned int dynsymcount;

           /* Pointer to structure which contains architecture information.  */
           const struct bfd_arch_info *arch_info;

           /* Flag set if symbols from this BFD should not be exported.  */
           bfd_boolean no_export;

           /* Stuff only useful for archives.  */
           void *arelt_data;
           struct bfd *my_archive;      /* The containing archive BFD.  */
           struct bfd *archive_next;    /* The next BFD in the archive.  */
           struct bfd *archive_head;    /* The first BFD in the archive.  */
           bfd_boolean has_armap;

           /* A chain of BFD structures involved in a link.  */
           struct bfd *link_next;

           /* A field used by _bfd_generic_link_add_archive_symbols.  This will
              be used only for archive elements.  */
           int archive_pass;

           /* Used by the back end to hold private data.  */
           union
             {
               struct aout_data_struct *aout_data;
               struct artdata *aout_ar_data;
               struct _oasys_data *oasys_obj_data;
               struct _oasys_ar_data *oasys_ar_data;
               struct coff_tdata *coff_obj_data;
               struct pe_tdata *pe_obj_data;
               struct xcoff_tdata *xcoff_obj_data;
               struct ecoff_tdata *ecoff_obj_data;
               struct ieee_data_struct *ieee_data;
               struct ieee_ar_data_struct *ieee_ar_data;
               struct srec_data_struct *srec_data;
               struct ihex_data_struct *ihex_data;
               struct tekhex_data_struct *tekhex_data;
               struct elf_obj_tdata *elf_obj_data;
               struct nlm_obj_tdata *nlm_obj_data;
               struct bout_data_struct *bout_data;
               struct mmo_data_struct *mmo_data;
               struct sun_core_struct *sun_core_data;
               struct sco5_core_struct *sco5_core_data;
               struct trad_core_struct *trad_core_data;
               struct som_data_struct *som_data;
               struct hpux_core_struct *hpux_core_data;
               struct hppabsd_core_struct *hppabsd_core_data;
               struct sgi_core_struct *sgi_core_data;
               struct lynx_core_struct *lynx_core_data;
               struct osf_core_struct *osf_core_data;
               struct cisco_core_struct *cisco_core_data;
               struct versados_data_struct *versados_data;
               struct netbsd_core_struct *netbsd_core_data;
               struct mach_o_data_struct *mach_o_data;
               struct mach_o_fat_data_struct *mach_o_fat_data;
               struct bfd_pef_data_struct *pef_data;
               struct bfd_pef_xlib_data_struct *pef_xlib_data;
               struct bfd_sym_data_struct *sym_data;
               void *any;
             }
           tdata;

           /* Used by the application to hold private data.  */
           void *usrdata;

           /* Where all the allocated stuff under this BFD goes.  This is a
              struct objalloc *, but we use void * to avoid requiring the inclusion
              of objalloc.h.  */
           void *memory;
         };

### 2.2 Error reporting

Most BFD functions return nonzero on success (check their individual documentation for precise semantics). On an error, they call `bfd_set_error` to set an error condition that callers can check by calling `bfd_get_error`. If that returns `bfd_error_system_call`, then check `errno`.

The easiest way to report a BFD error to the user is to use `bfd_perror`.

#### 2.2.1 Type `bfd_error_type`

The values returned by `bfd_get_error` are defined by the enumerated type `bfd_error_type`.

         typedef enum bfd_error
         {
           bfd_error_no_error = 0,
           bfd_error_system_call,
           bfd_error_invalid_target,
           bfd_error_wrong_format,
           bfd_error_wrong_object_format,
           bfd_error_invalid_operation,
           bfd_error_no_memory,
           bfd_error_no_symbols,
           bfd_error_no_armap,
           bfd_error_no_more_archived_files,
           bfd_error_malformed_archive,
           bfd_error_file_not_recognized,
           bfd_error_file_ambiguously_recognized,
           bfd_error_no_contents,
           bfd_error_nonrepresentable_section,
           bfd_error_no_debug_section,
           bfd_error_bad_value,
           bfd_error_file_truncated,
           bfd_error_file_too_big,
           bfd_error_on_input,
           bfd_error_invalid_error_code
         }
         bfd_error_type;

##### 2.2.1.1 `bfd_get_error`

Synopsis

         bfd_error_type bfd_get_error (void);

Description
Return the current BFD error condition.

##### 2.2.1.2 `bfd_set_error`

Synopsis

         void bfd_set_error (bfd_error_type error_tag, ...);

Description
Set the BFD error condition to be error_tag. If error_tag is bfd_error_on_input, then this function takes two more parameters, the input bfd where the error occurred, and the bfd_error_type error.

##### 2.2.1.3 `bfd_errmsg`

Synopsis

         const char *bfd_errmsg (bfd_error_type error_tag);

Description
Return a string describing the error error_tag, or the system error if error_tag is `bfd_error_system_call`.

##### 2.2.1.4 `bfd_perror`

Synopsis

         void bfd_perror (const char *message);

Description
Print to the standard error stream a string describing the last BFD error that occurred, or the last system error if the last BFD error was a system call failure. If message is non-NULL and non-empty, the error string printed is preceded by message, a colon, and a space. It is followed by a newline.

#### 2.2.2 BFD error handler

Some BFD functions want to print messages describing the problem. They call a BFD error handler function. This function may be overridden by the program.

The BFD error handler acts like printf.

         typedef void (*bfd_error_handler_type) (const char *, ...);

##### 2.2.2.1 `bfd_set_error_handler`

Synopsis

         bfd_error_handler_type bfd_set_error_handler (bfd_error_handler_type);

Description
Set the BFD error handler function. Returns the previous function.

##### 2.2.2.2 `bfd_set_error_program_name`

Synopsis

         void bfd_set_error_program_name (const char *);

Description
Set the program name to use when printing a BFD error. This is printed before the error message followed by a colon and space. The string must not be changed after it is passed to this function.

##### 2.2.2.3 `bfd_get_error_handler`

Synopsis

         bfd_error_handler_type bfd_get_error_handler (void);

Description
Return the BFD error handler function.

### 2.3 Miscellaneous

#### 2.3.1 Miscellaneous functions

##### 2.3.1.1 `bfd_get_reloc_upper_bound`

Synopsis

         long bfd_get_reloc_upper_bound (bfd *abfd, asection *sect);

Description
Return the number of bytes required to store the relocation information associated with section sect attached to bfd abfd. If an error occurs, return -1.

##### 2.3.1.2 `bfd_canonicalize_reloc`

Synopsis

         long bfd_canonicalize_reloc
            (bfd *abfd, asection *sec, arelent **loc, asymbol **syms);

Description
Call the back end associated with the open BFD abfd and translate the external form of the relocation information attached to sec into the internal canonical form. Place the table into memory at loc, which has been preallocated, usually by a call to `bfd_get_reloc_upper_bound`. Returns the number of relocs, or -1 on error.

The syms table is also needed for horrible internal magic reasons.

##### 2.3.1.3 `bfd_set_reloc`

Synopsis

         void bfd_set_reloc
            (bfd *abfd, asection *sec, arelent **rel, unsigned int count);

Description
Set the relocation pointer and count within section sec to the values rel and count. The argument abfd is ignored.

##### 2.3.1.4 `bfd_set_file_flags`

Synopsis

         bfd_boolean bfd_set_file_flags (bfd *abfd, flagword flags);

Description
Set the flag word in the BFD abfd to the value flags.

Possible errors are:

- `bfd_error_wrong_format` - The target bfd was not of object format.
- `bfd_error_invalid_operation` - The target bfd was open for reading.
- `bfd_error_invalid_operation` - The flag word contained a bit which was not applicable to the type of file. E.g., an attempt was made to set the `D_PAGED` bit on a BFD format which does not support demand paging.

##### 2.3.1.5 `bfd_get_arch_size`

Synopsis

         int bfd_get_arch_size (bfd *abfd);

Description
Returns the architecture address size, in bits, as determined by the object file's format. For ELF, this information is included in the header.

Returns
Returns the arch size in bits if known, `-1` otherwise.

##### 2.3.1.6 `bfd_get_sign_extend_vma`

Synopsis

         int bfd_get_sign_extend_vma (bfd *abfd);

Description
Indicates if the target architecture "naturally" sign extends an address. Some architectures implicitly sign extend address values when they are converted to types larger than the size of an address. For instance, bfd_get_start_address() will return an address sign extended to fill a bfd_vma when this is the case.

Returns
Returns `1` if the target architecture is known to sign extend addresses, `0` if the target architecture is known to not sign extend addresses, and `-1` otherwise.

##### 2.3.1.7 `bfd_set_start_address`

Synopsis

         bfd_boolean bfd_set_start_address (bfd *abfd, bfd_vma vma);

Description
Make vma the entry point of output BFD abfd.

Returns
Returns `TRUE` on success, `FALSE` otherwise.

##### 2.3.1.8 `bfd_get_gp_size`

Synopsis

         unsigned int bfd_get_gp_size (bfd *abfd);

Description
Return the maximum size of objects to be optimized using the GP register under MIPS ECOFF. This is typically set by the `-G` argument to the compiler, assembler or linker.

##### 2.3.1.9 `bfd_set_gp_size`

Synopsis

         void bfd_set_gp_size (bfd *abfd, unsigned int i);

Description
Set the maximum size of objects to be optimized using the GP register under ECOFF or MIPS ELF. This is typically set by the `-G` argument to the compiler, assembler or linker.

##### 2.3.1.10 `bfd_scan_vma`

Synopsis

         bfd_vma bfd_scan_vma (const char *string, const char **end, int base);

Description
Convert, like `strtoul`, a numerical expression string into a `bfd_vma` integer, and return that integer. (Though without as many bells and whistles as `strtoul`.) The expression is assumed to be unsigned (i.e., positive). If given a base, it is used as the base for conversion. A base of 0 causes the function to interpret the string in hex if a leading "0x" or "0X" is found, otherwise in octal if a leading zero is found, otherwise in decimal.

If the value would overflow, the maximum `bfd_vma` value is returned.

##### 2.3.1.11 `bfd_copy_private_header_data`

Synopsis

         bfd_boolean bfd_copy_private_header_data (bfd *ibfd, bfd *obfd);

Description
Copy private BFD header information from the BFD ibfd to the the BFD obfd. This copies information that may require sections to exist, but does not require symbol tables. Return `true` on success, `false` on error. Possible error returns are:

- `bfd_error_no_memory` - Not enough memory exists to create private data for obfd.

         #define bfd_copy_private_header_data(ibfd, obfd) \
              BFD_SEND (obfd, _bfd_copy_private_header_data, \
                        (ibfd, obfd))

##### 2.3.1.12 `bfd_copy_private_bfd_data`

Synopsis

         bfd_boolean bfd_copy_private_bfd_data (bfd *ibfd, bfd *obfd);

Description
Copy private BFD information from the BFD ibfd to the the BFD obfd. Return `TRUE` on success, `FALSE` on error. Possible error returns are:

- `bfd_error_no_memory` - Not enough memory exists to create private data for obfd.

         #define bfd_copy_private_bfd_data(ibfd, obfd) \
              BFD_SEND (obfd, _bfd_copy_private_bfd_data, \
                        (ibfd, obfd))

##### 2.3.1.13 `bfd_merge_private_bfd_data`

Synopsis

         bfd_boolean bfd_merge_private_bfd_data (bfd *ibfd, bfd *obfd);

Description
Merge private BFD information from the BFD ibfd to the the output file BFD obfd when linking. Return `TRUE` on success, `FALSE` on error. Possible error returns are:

- `bfd_error_no_memory` - Not enough memory exists to create private data for obfd.

         #define bfd_merge_private_bfd_data(ibfd, obfd) \
              BFD_SEND (obfd, _bfd_merge_private_bfd_data, \
                        (ibfd, obfd))

##### 2.3.1.14 `bfd_set_private_flags`

Synopsis

         bfd_boolean bfd_set_private_flags (bfd *abfd, flagword flags);

Description
Set private BFD flag information in the BFD abfd. Return `TRUE` on success, `FALSE` on error. Possible error returns are:

- `bfd_error_no_memory` - Not enough memory exists to create private data for obfd.

         #define bfd_set_private_flags(abfd, flags) \
              BFD_SEND (abfd, _bfd_set_private_flags, (abfd, flags))

##### 2.3.1.15 `Other functions`

Description
The following functions exist but have not yet been documented.

         #define bfd_sizeof_headers(abfd, info) \
                BFD_SEND (abfd, _bfd_sizeof_headers, (abfd, info))

         #define bfd_find_nearest_line(abfd, sec, syms, off, file, func, line) \
                BFD_SEND (abfd, _bfd_find_nearest_line, \
                          (abfd, sec, syms, off, file, func, line))

         #define bfd_find_line(abfd, syms, sym, file, line) \
                BFD_SEND (abfd, _bfd_find_line, \
                          (abfd, syms, sym, file, line))

         #define bfd_find_inliner_info(abfd, file, func, line) \
                BFD_SEND (abfd, _bfd_find_inliner_info, \
                          (abfd, file, func, line))

         #define bfd_debug_info_start(abfd) \
                BFD_SEND (abfd, _bfd_debug_info_start, (abfd))

         #define bfd_debug_info_end(abfd) \
                BFD_SEND (abfd, _bfd_debug_info_end, (abfd))

         #define bfd_debug_info_accumulate(abfd, section) \
                BFD_SEND (abfd, _bfd_debug_info_accumulate, (abfd, section))

         #define bfd_stat_arch_elt(abfd, stat) \
                BFD_SEND (abfd, _bfd_stat_arch_elt,(abfd, stat))

         #define bfd_update_armap_timestamp(abfd) \
                BFD_SEND (abfd, _bfd_update_armap_timestamp, (abfd))

         #define bfd_set_arch_mach(abfd, arch, mach)\
                BFD_SEND ( abfd, _bfd_set_arch_mach, (abfd, arch, mach))

         #define bfd_relax_section(abfd, section, link_info, again) \
                BFD_SEND (abfd, _bfd_relax_section, (abfd, section, link_info, again))

         #define bfd_gc_sections(abfd, link_info) \
                BFD_SEND (abfd, _bfd_gc_sections, (abfd, link_info))

         #define bfd_merge_sections(abfd, link_info) \
                BFD_SEND (abfd, _bfd_merge_sections, (abfd, link_info))

         #define bfd_is_group_section(abfd, sec) \
                BFD_SEND (abfd, _bfd_is_group_section, (abfd, sec))

         #define bfd_discard_group(abfd, sec) \
                BFD_SEND (abfd, _bfd_discard_group, (abfd, sec))

         #define bfd_link_hash_table_create(abfd) \
                BFD_SEND (abfd, _bfd_link_hash_table_create, (abfd))

         #define bfd_link_hash_table_free(abfd, hash) \
                BFD_SEND (abfd, _bfd_link_hash_table_free, (hash))

         #define bfd_link_add_symbols(abfd, info) \
                BFD_SEND (abfd, _bfd_link_add_symbols, (abfd, info))

         #define bfd_link_just_syms(abfd, sec, info) \
                BFD_SEND (abfd, _bfd_link_just_syms, (sec, info))

         #define bfd_final_link(abfd, info) \
                BFD_SEND (abfd, _bfd_final_link, (abfd, info))

         #define bfd_free_cached_info(abfd) \
                BFD_SEND (abfd, _bfd_free_cached_info, (abfd))

         #define bfd_get_dynamic_symtab_upper_bound(abfd) \
                BFD_SEND (abfd, _bfd_get_dynamic_symtab_upper_bound, (abfd))

         #define bfd_print_private_bfd_data(abfd, file)\
                BFD_SEND (abfd, _bfd_print_private_bfd_data, (abfd, file))

         #define bfd_canonicalize_dynamic_symtab(abfd, asymbols) \
                BFD_SEND (abfd, _bfd_canonicalize_dynamic_symtab, (abfd, asymbols))

         #define bfd_get_synthetic_symtab(abfd, count, syms, dyncount, dynsyms, ret) \
                BFD_SEND (abfd, _bfd_get_synthetic_symtab, (abfd, count, syms, \
                                                            dyncount, dynsyms, ret))

         #define bfd_get_dynamic_reloc_upper_bound(abfd) \
                BFD_SEND (abfd, _bfd_get_dynamic_reloc_upper_bound, (abfd))

         #define bfd_canonicalize_dynamic_reloc(abfd, arels, asyms) \
                BFD_SEND (abfd, _bfd_canonicalize_dynamic_reloc, (abfd, arels, asyms))

         extern bfd_byte *bfd_get_relocated_section_contents
           (bfd *, struct bfd_link_info *, struct bfd_link_order *, bfd_byte *,
            bfd_boolean, asymbol **);

##### 2.3.1.16 `bfd_alt_mach_code`

Synopsis

         bfd_boolean bfd_alt_mach_code (bfd *abfd, int alternative);

Description
When more than one machine code number is available for the same machine type, this function can be used to switch between the preferred one (alternative == 0) and any others. Currently, only ELF supports this feature, with up to two alternate machine codes.

         struct bfd_preserve
         {
           void *marker;
           void *tdata;
           flagword flags;
           const struct bfd_arch_info *arch_info;
           struct bfd_section *sections;
           struct bfd_section *section_last;
           unsigned int section_count;
           struct bfd_hash_table section_htab;
         };

##### 2.3.1.17 `bfd_preserve_save`

Synopsis

         bfd_boolean bfd_preserve_save (bfd *, struct bfd_preserve *);

Description
When testing an object for compatibility with a particular target back-end, the back-end object_p function needs to set up certain fields in the bfd on successfully recognizing the object. This typically happens in a piecemeal fashion, with failures possible at many points. On failure, the bfd is supposed to be restored to its initial state, which is virtually impossible. However, restoring a subset of the bfd state works in practice. This function stores the subset and reinitializes the bfd.

##### 2.3.1.18 `bfd_preserve_restore`

Synopsis

         void bfd_preserve_restore (bfd *, struct bfd_preserve *);

Description
This function restores bfd state saved by bfd_preserve_save. If MARKER is non-NULL in struct bfd_preserve then that block and all subsequently bfd_alloc'd memory is freed.

##### 2.3.1.19 `bfd_preserve_finish`

Synopsis

         void bfd_preserve_finish (bfd *, struct bfd_preserve *);

Description
This function should be called when the bfd state saved by bfd_preserve_save is no longer needed. ie. when the back-end object_p function returns with success.

##### 2.3.1.20 `bfd_emul_get_maxpagesize`

Synopsis

         bfd_vma bfd_emul_get_maxpagesize (const char *);

Description
Returns the maximum page size, in bytes, as determined by emulation.

Returns
Returns the maximum page size in bytes for ELF, abort otherwise.

##### 2.3.1.21 `bfd_emul_set_maxpagesize`

Synopsis

         void bfd_emul_set_maxpagesize (const char *, bfd_vma);

Description
For ELF, set the maximum page size for the emulation. It is a no-op for other formats.

##### 2.3.1.22 `bfd_emul_get_commonpagesize`

Synopsis

         bfd_vma bfd_emul_get_commonpagesize (const char *);

Description
Returns the common page size, in bytes, as determined by emulation.

Returns
Returns the common page size in bytes for ELF, abort otherwise.

##### 2.3.1.23 `bfd_emul_set_commonpagesize`

Synopsis

         void bfd_emul_set_commonpagesize (const char *, bfd_vma);

Description
For ELF, set the common page size for the emulation. It is a no-op for other formats.

##### 2.3.1.24 `bfd_demangle`

Synopsis

         char *bfd_demangle (bfd *, const char *, int);

Description
Wrapper around cplus_demangle. Strips leading underscores and other such chars that would otherwise confuse the demangler. If passed a g++ v3 ABI mangled name, returns a buffer allocated with malloc holding the demangled name. Returns NULL otherwise and on memory alloc failure.

##### 2.3.1.25 `struct bfd_iovec`

Description
The `struct bfd_iovec` contains the internal file I/O class. Each `BFD` has an instance of this class and all file I/O is routed through it (it is assumed that the instance implements all methods listed below).

         struct bfd_iovec
         {
           /* To avoid problems with macros, a "b" rather than "f"
              prefix is prepended to each method name.  */
           /* Attempt to read/write NBYTES on ABFD's IOSTREAM storing/fetching
              bytes starting at PTR.  Return the number of bytes actually
              transfered (a read past end-of-file returns less than NBYTES),
              or -1 (setting bfd_error) if an error occurs.  */
           file_ptr (*bread) (struct bfd *abfd, void *ptr, file_ptr nbytes);
           file_ptr (*bwrite) (struct bfd *abfd, const void *ptr,
                               file_ptr nbytes);
           /* Return the current IOSTREAM file offset, or -1 (setting bfd_error
              if an error occurs.  */
           file_ptr (*btell) (struct bfd *abfd);
           /* For the following, on successful completion a value of 0 is returned.
              Otherwise, a value of -1 is returned (and  bfd_error is set).  */
           int (*bseek) (struct bfd *abfd, file_ptr offset, int whence);
           int (*bclose) (struct bfd *abfd);
           int (*bflush) (struct bfd *abfd);
           int (*bstat) (struct bfd *abfd, struct stat *sb);
         };

##### 2.3.1.26 `bfd_get_mtime`

Synopsis

         long bfd_get_mtime (bfd *abfd);

Description
Return the file modification time (as read from the file system, or from the archive header for archive members).

##### 2.3.1.27 `bfd_get_size`

Synopsis

         file_ptr bfd_get_size (bfd *abfd);

Description
Return the file size (as read from file system) for the file associated with BFD abfd.

The initial motivation for, and use of, this routine is not so we can get the exact size of the object the BFD applies to, since that might not be generally possible (archive members for example). It would be ideal if someone could eventually modify it so that such results were guaranteed.

Instead, we want to ask questions like "is this NNN byte sized object I'm about to try read from file offset YYY reasonable?" As as example of where we might do this, some object formats use string tables for which the first `sizeof (long)` bytes of the table contain the size of the table itself, including the size bytes. If an application tries to read what it thinks is one of these string tables, without some way to validate the size, and for some reason the size is wrong (byte swapping error, wrong location for the string table, etc.), the only clue is likely to be a read error when it tries to read the table, or a "virtual memory exhausted" error when it tries to allocate 15 bazillon bytes of space for the 15 bazillon byte table it is about to read. This function at least allows us to answer the question, "is the size reasonable?".