BFD tries to maintain as much symbol information as it can when
it moves information from file to file. BFD passes information
to applications though the asymbol
structure. When the
application requests the symbol table, BFD reads the table in
the native form and translates parts of it into the internal
format. To maintain more than the information passed to
applications, some targets keep some information "behind the
scenes" in a structure only the particular back end knows
about. For example, the coff back end keeps the original
symbol table structure as well as the canonical structure when
a BFD is read in. On output, the coff back end can reconstruct
the output symbol table so that no information is lost, even
information unique to coff which BFD doesn't know or
understand. If a coff symbol table were read, but were written
through an a.out back end, all the coff specific information
would be lost. The symbol table of a BFD
is not necessarily read in until a canonicalize request is
made. Then the BFD back end fills in a table provided by the
application with pointers to the canonical information. To
output symbols, the application provides BFD with a table of
pointers to pointers to asymbol
s. This allows applications
like the linker to output a symbol as it was read, since the "behind
the scenes" information will be still available.
There are two stages to reading a symbol table from a BFD: allocating storage, and the actual reading process. This is an excerpt from an application which reads the symbol table:
long storage_needed; asymbol **symbol_table; long number_of_symbols; long i; storage_needed = bfd_get_symtab_upper_bound (abfd); if (storage_needed < 0) FAIL if (storage_needed == 0) { return ; } symbol_table = (asymbol **) xmalloc (storage_needed); ... number_of_symbols = bfd_canonicalize_symtab (abfd, symbol_table); if (number_of_symbols < 0) FAIL for (i = 0; i < number_of_symbols; i++) { process_symbol (symbol_table[i]); }
All storage for the symbols themselves is in an objalloc connected to the BFD; it is freed when the BFD is closed.
Writing of a symbol table is automatic when a BFD open for
writing is closed. The application attaches a vector of
pointers to pointers to symbols to the BFD being written, and
fills in the symbol count. The close and cleanup code reads
through the table provided and performs all the necessary
operations. The BFD output code must always be provided with an
"owned" symbol: one which has come from another BFD, or one
which has been created using bfd_make_empty_symbol
. Here is an
example showing the creation of a symbol table with only one element:
#include "bfd.h" main() { bfd *abfd; asymbol *ptrs[2]; asymbol *new; abfd = bfd_openw("foo","a.out-sunos-big"); bfd_set_format(abfd, bfd_object); new = bfd_make_empty_symbol(abfd); new->name = "dummy_symbol"; new->section = bfd_make_section_old_way(abfd, ".text"); new->flags = BSF_GLOBAL; new->value = 0x12345; ptrs[0] = new; ptrs[1] = (asymbol *)0; bfd_set_symtab(abfd, ptrs, 1); bfd_close(abfd); } ./makesym nm foo 00012345 A dummy_symbol
Many formats cannot represent arbitary symbol information; for
instance, the a.out
object format does not allow an
arbitary number of sections. A symbol pointing to a section
which is not one of .text
, .data
or .bss
cannot
be described.
Mini symbols provide read-only access to the symbol table. They use less memory space, but require more time to access. They can be useful for tools like nm or objdump, which may have to handle symbol tables of extremely large executables.
The bfd_read_minisymbols
function will read the symbols
into memory in an internal form. It will return a void *
pointer to a block of memory, a symbol count, and the size of
each symbol. The pointer is allocated using malloc
, and
should be freed by the caller when it is no longer needed.
The function bfd_minisymbol_to_symbol
will take a pointer
to a minisymbol, and a pointer to a structure returned by
bfd_make_empty_symbol
, and return a asymbol
structure.
The return value may or may not be the same as the value from
bfd_make_empty_symbol
which was passed in.
An asymbol
has the form:
typedef struct symbol_cache_entry { /* A pointer to the BFD which owns the symbol. This information is necessary so that a back end can work out what additional information (invisible to the application writer) is carried with the symbol. This field is *almost* redundant, since you can use section->owner instead, except that some symbols point to the global sections bfd_{abs,com,und}_section. This could be fixed by making these globals be per-bfd (or per-target-flavor). FIXME. */ struct _bfd *the_bfd; /* Use bfd_asymbol_bfd(sym) to access this field. */ /* The text of the symbol. The name is left alone, and not copied; the application may not alter it. */ CONST char *name; /* The value of the symbol. This really should be a union of a numeric value with a pointer, since some flags indicate that a pointer to another symbol is stored here. */ symvalue value; /* Attributes of a symbol: */ #define BSF_NO_FLAGS 0x00 /* The symbol has local scope;static
inC
. The value is the offset into the section of the data. */ #define BSF_LOCAL 0x01 /* The symbol has global scope; initialized data inC
. The value is the offset into the section of the data. */ #define BSF_GLOBAL 0x02 /* The symbol has global scope and is exported. The value is the offset into the section of the data. */ #define BSF_EXPORT BSF_GLOBAL /* no real difference */ /* A normal C symbol would be one of:BSF_LOCAL
,BSF_FORT_COMM
,BSF_UNDEFINED
orBSF_GLOBAL
*/ /* The symbol is a debugging record. The value has an arbitary meaning. */ #define BSF_DEBUGGING 0x08 /* The symbol denotes a function entry point. Used in ELF, perhaps others someday. */ #define BSF_FUNCTION 0x10 /* Used by the linker. */ #define BSF_KEEP 0x20 #define BSF_KEEP_G 0x40 /* A weak global symbol, overridable without warnings by a regular global symbol of the same name. */ #define BSF_WEAK 0x80 /* This symbol was created to point to a section, e.g. ELF's STT_SECTION symbols. */ #define BSF_SECTION_SYM 0x100 /* The symbol used to be a common symbol, but now it is allocated. */ #define BSF_OLD_COMMON 0x200 /* The default value for common data. */ #define BFD_FORT_COMM_DEFAULT_VALUE 0 /* In some files the type of a symbol sometimes alters its location in an output file - ie in coff aISFCN
symbol which is alsoC_EXT
symbol appears where it was declared and not at the end of a section. This bit is set by the target BFD part to convey this information. */ #define BSF_NOT_AT_END 0x400 /* Signal that the symbol is the label of constructor section. */ #define BSF_CONSTRUCTOR 0x800 /* Signal that the symbol is a warning symbol. The name is a warning. The name of the next symbol is the one to warn about; if a reference is made to a symbol with the same name as the next symbol, a warning is issued by the linker. */ #define BSF_WARNING 0x1000 /* Signal that the symbol is indirect. This symbol is an indirect pointer to the symbol with the same name as the next symbol. */ #define BSF_INDIRECT 0x2000 /* BSF_FILE marks symbols that contain a file name. This is used for ELF STT_FILE symbols. */ #define BSF_FILE 0x4000 /* Symbol is from dynamic linking information. */ #define BSF_DYNAMIC 0x8000 /* The symbol denotes a data object. Used in ELF, and perhaps others someday. */ #define BSF_OBJECT 0x10000 flagword flags; /* A pointer to the section to which this symbol is relative. This will always be non NULL, there are special sections for undefined and absolute symbols. */ struct sec *section; /* Back end special data. */ union { PTR p; bfd_vma i; } udata; } asymbol;
bfd_get_symtab_upper_bound
Description
Return the number of bytes required to store a vector of pointers
to asymbols
for all the symbols in the BFD abfd,
including a terminal NULL pointer. If there are no symbols in
the BFD, then return 0. If an error occurs, return -1.
#define bfd_get_symtab_upper_bound(abfd) \ BFD_SEND (abfd, _bfd_get_symtab_upper_bound, (abfd))
bfd_is_local_label
Synopsis
boolean bfd_is_local_label(bfd *abfd, asymbol *sym);
Description
Return true if the given symbol sym in the BFD abfd is
a compiler generated local label, else return false.
bfd_is_local_label_name
Synopsis
boolean bfd_is_local_label_name(bfd *abfd, const char *name);
Description
Return true if a symbol with the name name in the BFD
abfd is a compiler generated local label, else return
false. This just checks whether the name has the form of a
local label.
#define bfd_is_local_label_name(abfd, name) \ BFD_SEND (abfd, _bfd_is_local_label_name, (abfd, name))
bfd_canonicalize_symtab
Description
Read the symbols from the BFD abfd, and fills in
the vector location with pointers to the symbols and
a trailing NULL.
Return the actual number of symbol pointers, not
including the NULL.
#define bfd_canonicalize_symtab(abfd, location) \ BFD_SEND (abfd, _bfd_canonicalize_symtab,\ (abfd, location))
bfd_set_symtab
Synopsis
boolean bfd_set_symtab (bfd *abfd, asymbol **location, unsigned int count);
Description
Arrange that when the output BFD abfd is closed,
the table location of count pointers to symbols
will be written.
bfd_print_symbol_vandf
Synopsis
void bfd_print_symbol_vandf(PTR file, asymbol *symbol);
Description
Print the value and flags of the symbol supplied to the
stream file.
bfd_make_empty_symbol
Description
Create a new asymbol
structure for the BFD abfd
and return a pointer to it.
This routine is necessary because each back end has private
information surrounding the asymbol
. Building your own
asymbol
and pointing to it will not create the private
information, and will cause problems later on.
#define bfd_make_empty_symbol(abfd) \ BFD_SEND (abfd, _bfd_make_empty_symbol, (abfd))
bfd_make_debug_symbol
Description
Create a new asymbol
structure for the BFD abfd,
to be used as a debugging symbol. Further details of its use have
yet to be worked out.
#define bfd_make_debug_symbol(abfd,ptr,size) \ BFD_SEND (abfd, _bfd_make_debug_symbol, (abfd, ptr, size))
bfd_decode_symclass
Description
Return a character corresponding to the symbol
class of symbol, or '?' for an unknown class.
Synopsis
int bfd_decode_symclass(asymbol *symbol);
bfd_symbol_info
Description
Fill in the basic info about symbol that nm needs.
Additional info may be added by the back-ends after
calling this function.
Synopsis
void bfd_symbol_info(asymbol *symbol, symbol_info *ret);
bfd_copy_private_symbol_data
Synopsis
boolean bfd_copy_private_symbol_data(bfd *ibfd, asymbol *isym, bfd *obfd, asymbol *osym);
Description
Copy private symbol information from isym in the BFD
ibfd to the symbol osym in 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 osec.
#define bfd_copy_private_symbol_data(ibfd, isymbol, obfd, osymbol) \ BFD_SEND (obfd, _bfd_copy_private_symbol_data, \ (ibfd, isymbol, obfd, osymbol))
This document was generated on 7 April 1999 using the texi2html translator version 1.52.