我的问题是关于bfd以及以下代码中的部分数量如何工作。 bfd结构的gdb调试器中的代码和转储及其中的部分结构如下所示。我还包括了下面bfd和bfd-＆gt;部分的数据结构定义。我的问题是：为什么当我运行这段代码时（可执行文件称为getsections）：getsections getsections.o（所以传递给它自己的目标文件） - 为什么段结构为空（全为零），如果你看看bfd- ＆gt; section_count，它的编号是4218960？如果你使用Linux命令：objdump -h getsections.o，它显示14个部分（.text，.data，.bss，.rodata，.debug_info，.debug_abbrev，.debug_loc，.debug_aranges，.debug_line，.debug_str ,. comment，.comment.SUSE.OPTS，.note-GNU-stack，.eh_frame）。我已经阅读了我在网上的BFD文档中可以找到的内容，而且我不确定我在这里做错了什么，因为我的目标文件结构远不及你在objdump（14节）中看到的那样。任何帮助表示赞赏。谢谢。
unsigned int number_of_sections(bfd *abfd)
{
unsigned int numSections = 0;
numSections = bfd_count_sections(abfd);
return numSections;
}
int main (int argc, char *argv[])
{
bfd *ibfd = NULL;
char filename[80];
char *fptr = &filename[0];
unsigned int numSections = 0;
if (argc < 2)
{
printf("Argc < 2\n");
exit(EXIT_FAILURE);
}
else
{
bfd_init();
printf("filename = %s\n", argv[1]);
ibfd = bfd_openr(argv[1], NULL);
numSections = number_of_sections(ibfd);
printf("num sections = %d\n", numSections);
bfd_close(ibfd);
}
return 1;
}
在number_of_sections（）处设置断点，bfd的转储产生以下内容：
(gdb) print abfd
$1 = (bfd *) 0x85c010
(gdb) print *abfd
$2 = {filename = 0x0, xvec = 0x7fffffffe06f, iostream = 0x7fff20 <bfd_elf64_x86_64_vec> "\211\345X", cacheable = (unknown: 8773984), target_defaulted = false, lru_prev = 0x7f6d40 <cache_iovec>, lru_next = 0x85c010, where = 8765456, opened_once = false, mtime_set = false, mtime = 0, ifd = 0, format = bfd_unknown, direction = read_direction, flags = 0, origin = 0, output_has_begun = false, sections = 0x85d180, section_count = 4218960, start_address = 8769872, symcount = 251, outsymbols = 0x130, arch_info = 0x0, arelt_data = 0x0, my_archive = 0x0, next = 0x0, archive_head = 0x0, has_armap = false, link_next = 0x0, archive_pass = 8673856, tdata = {aout_data = 0x0, aout_ar_data = 0x0, oasys_obj_data = 0x0, oasys_ar_data = 0x0, coff_obj_data = 0x0, pe_obj_data = 0x0, xcoff_obj_data = 0x0, ecoff_obj_data = 0x0, ieee_data = 0x0, ieee_ar_data = 0x0, srec_data = 0x0, tekhex_data = 0x0, elf_obj_data = 0x0, nlm_obj_data = 0x0, bout_data = 0x0, sun_core_data = 0x0, trad_core_data = 0x0, som_data = 0x0, hpux_core_data = 0x0, hppabsd_core_data = 0x0, sgi_core_data = 0x0, lynx_core_data = 0x0, osf_core_data = 0x0, cisco_core_data = 0x0, versados_data = 0x0, any = 0x0}, usrdata = 0x0, memory = {chunk_size = 0, chunk = 0x0, object_base = 0x0, next_free = 0x0, chunk_limit = 0x0, temp = {tempint = 0, tempptr = 0x0}, alignment_mask = 0, chunkfun = 0x85c140, freefun = 0x7, extra_arg = 0x21, use_extra_arg = 0, maybe_empty_object = 0, alloc_failed = 0}}
abfd-＆gt;部分的转储似乎没有任何内容：
(gdb) print *(abfd->sections)
$4 = {name = 0x0, index = 0, next = 0x0, flags = 0, vma = 0, user_set_vma = false, lma = 0, _cooked_size = 0, _raw_size = 0, output_offset = 0, output_section = 0x0, alignment_power = 0, relocation = 0x0, orelocation = 0x0, reloc_count = 0, filepos = 0, rel_filepos = 0, line_filepos = 0, userdata = 0x0, contents = 0x0, lineno = 0x0, lineno_count = 0, moving_line_filepos = 0, target_index = 0, used_by_bfd = 0x0, constructor_chain = 0x0, owner = 0x0, reloc_done = false, symbol = 0x0, symbol_ptr_ptr = 0x0, link_order_head = 0x0, link_order_tail = 0x0}
bfd结构如下：
struct _bfd
{
/* The filename the application opened the BFD with. */
CONST char *filename;
/* A pointer to the target jump table. */
const struct bfd_target *xvec;
/* To avoid dragging too many header files into every file that
includes `<<bfd.h>>', IOSTREAM has been declared as a "char
*", and MTIME as a "long". Their correct types, to which they
are cast when used, are "FILE *" and "time_t". The iostream
is the result of an fopen on the filename. */
char *iostream;
/* Is the file descriptor being cached? That is, can it be closed as
needed, and re-opened when accessed later? */
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. */
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: */
file_ptr where;
/* and here: (``once'' means at least once) */
boolean opened_once;
/* Set if we have a locally maintained mtime value, rather than
getting it from the file each time: */
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 the BFD was opened with*/
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. */
file_ptr origin;
/* Remember when output has begun, to stop strange things
from happening. */
boolean output_has_begun;
/* Pointer to linked list of sections*/
struct sec *sections;
/* 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 symbol_cache_entry **outsymbols;
/* Pointer to structure which contains architecture information*/
const struct bfd_arch_info *arch_info;
/* Stuff only useful for archives:*/
PTR arelt_data;
struct _bfd *my_archive; /* The containing archive BFD. */
struct _bfd *next; /* The next BFD in the archive. */
struct _bfd *archive_head; /* The first BFD in the archive. */
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 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 sun_core_struct *sun_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;
PTR any;
} tdata;
/* Used by the application to hold private data*/
PTR usrdata;
/* Where all the allocated stuff under this BFD goes */
struct obstack memory;
};
和bfd-＆gt;部分结构（sec结构）如下所示：
typedef struct sec
{
/* The name of the section; the name isn't a copy, the pointer is
the same as that passed to bfd_make_section. */
CONST char *name;
/* Which section is it; 0..nth. */
int index;
/* The next section in the list belonging to the BFD, or NULL. */
struct sec *next;
/* The field flags contains attributes of the section. Some
flags are read in from the object file, and some are
synthesized from other information. */
flagword flags;
#define SEC_NO_FLAGS 0x000
/* Tells the OS to allocate space for this section when loading.
This is clear for a section containing debug information
only. */
#define SEC_ALLOC 0x001
/* Tells the OS to load the section from the file when loading.
This is clear for a .bss section. */
#define SEC_LOAD 0x002
/* The section contains data still to be relocated, so there is
some relocation information too. */
#define SEC_RELOC 0x004
#if 0 /* Obsolete ? */
#define SEC_BALIGN 0x008
#endif
/* A signal to the OS that the section contains read only
data. */
#define SEC_READONLY 0x010
/* The section contains code only. */
#define SEC_CODE 0x020
/* The section contains data only. */
#define SEC_DATA 0x040
/* The section will reside in ROM. */
#define SEC_ROM 0x080
/* The section contains constructor information. This section
type is used by the linker to create lists of constructors and
destructors used by <<g++>>. When a back end sees a symbol
which should be used in a constructor list, it creates a new
section for the type of name (e.g., <<__CTOR_LIST__>>), attaches
the symbol to it, and builds a relocation. To build the lists
of constructors, all the linker has to do is catenate all the
sections called <<__CTOR_LIST__>> and relocate the data
contained within - exactly the operations it would peform on
standard data. */
#define SEC_CONSTRUCTOR 0x100
/* The section is a constuctor, and should be placed at the
end of the text, data, or bss section(?). */
#define SEC_CONSTRUCTOR_TEXT 0x1100
#define SEC_CONSTRUCTOR_DATA 0x2100
#define SEC_CONSTRUCTOR_BSS 0x3100
/* The section has contents - a data section could be
<<SEC_ALLOC>> | <<SEC_HAS_CONTENTS>>; a debug section could be
<<SEC_HAS_CONTENTS>> */
#define SEC_HAS_CONTENTS 0x200
/* An instruction to the linker to not output the section
even if it has information which would normally be written. */
#define SEC_NEVER_LOAD 0x400
/* The section is a COFF shared library section. This flag is
only for the linker. If this type of section appears in
the input file, the linker must copy it to the output file
without changing the vma or size. FIXME: Although this
was originally intended to be general, it really is COFF
specific (and the flag was renamed to indicate this). It
might be cleaner to have some more general mechanism to
allow the back end to control what the linker does with
sections. */
#define SEC_COFF_SHARED_LIBRARY 0x800
/* The section is a common section (symbols may be defined
multiple times, the value of a symbol is the amount of
space it requires, and the largest symbol value is the one
used). Most targets have exactly one of these (which we
translate to bfd_com_section_ptr), but ECOFF has two. */
#define SEC_IS_COMMON 0x8000
/* The section contains only debugging information. For
example, this is set for ELF .debug and .stab sections.
strip tests this flag to see if a section can be
discarded. */
#define SEC_DEBUGGING 0x10000
/* The contents of this section are held in memory pointed to
by the contents field. This is checked by
bfd_get_section_contents, and the data is retrieved from
memory if appropriate. */
#define SEC_IN_MEMORY 0x20000
/* End of section flags. */
/* The virtual memory address of the section - where it will be
at run time. The symbols are relocated against this. The
user_set_vma flag is maintained by bfd; if it's not set, the
backend can assign addresses (for example, in <<a.out>>, where
the default address for <<.data>> is dependent on the specific
target and various flags). */
bfd_vma vma;
boolean user_set_vma;
/* The load address of the section - where it would be in a
rom image; really only used for writing section header
information. */
bfd_vma lma;
/* The size of the section in bytes, as it will be output.
contains a value even if the section has no contents (e.g., the
size of <<.bss>>). This will be filled in after relocation */
bfd_size_type _cooked_size;
/* The original size on disk of the section, in bytes. Normally this
value is the same as the size, but if some relaxing has
been done, then this value will be bigger. */
bfd_size_type _raw_size;
/* If this section is going to be output, then this value is the
offset into the output section of the first byte in the input
section. E.g., if this was going to start at the 100th byte in
the output section, this value would be 100. */
bfd_vma output_offset;
/* The output section through which to map on output. */
struct sec *output_section;
/* The alignment requirement of the section, as an exponent of 2 -
e.g., 3 aligns to 2^3 (or 8). */
unsigned int alignment_power;
/* If an input section, a pointer to a vector of relocation
records for the data in this section. */
struct reloc_cache_entry *relocation;
/* If an output section, a pointer to a vector of pointers to
relocation records for the data in this section. */
struct reloc_cache_entry **orelocation;
/* The number of relocation records in one of the above */
unsigned reloc_count;
/* Information below is back end specific - and not always used
or updated. */
/* File position of section data */
file_ptr filepos;
/* File position of relocation info */
file_ptr rel_filepos;
/* File position of line data */
file_ptr line_filepos;
/* Pointer to data for applications */
PTR userdata;
/* If the SEC_IN_MEMORY flag is set, this points to the actual
contents. */
unsigned char *contents;
/* Attached line number information */
alent *lineno;
/* Number of line number records */
unsigned int lineno_count;
/* When a section is being output, this value changes as more
linenumbers are written out */
file_ptr moving_line_filepos;
/* What the section number is in the target world */
int target_index;
PTR used_by_bfd;
/* If this is a constructor section then here is a list of the
relocations created to relocate items within it. */
struct relent_chain *constructor_chain;
/* The BFD which owns the section. */
bfd *owner;
boolean reloc_done;
/* A symbol which points at this section only */
struct symbol_cache_entry *symbol;
struct symbol_cache_entry **symbol_ptr_ptr;
struct bfd_link_order *link_order_head;
struct bfd_link_order *link_order_tail;
} asection ;
答案 0 :(得分：2)
我没有尝试你的程序。但是，我注意到它没有调用bfd_check_format。这是必需的。关于这些事情的BFD文档并不总是很清楚（欢迎改进！）所以当我需要处理BFD时，我通常最终会阅读示例程序，比如objdump。
我的一个BFD测试开始了：
fd = open (argv[1], O_RDONLY | O_CLOEXEC, 0);
if (fd == -1)
die ("open");
abfd = bfd_fopen (argv[1], NULL, "r", fd);
if (!bfd_check_format (abfd, bfd_object))
{
bfd_close (abfd);
die ("bfd_check_format");
}