elf_getdata(3E) SDK R4.11 elf_getdata(3E)
NAME
elfgetdata, elfnewdata, elfrawdata - get section data
SYNOPSIS
cc [flag ...] file ... -lelf [library ...]
#include <libelf.h>
ElfData *elfgetdata(ElfScn *scn, ElfData *data);
ElfData *elfnewdata(ElfScn *scn);
ElfData *elfrawdata(ElfScn *scn, ElfData *data);
DESCRIPTION
These functions access and manipulate the data associated with a
section descriptor, scn. When reading an existing file, a section
will have a single data buffer associated with it. A program may
build a new section in pieces, however, composing the new data from
multiple data buffers. For this reason, ``the'' data for a section
should be viewed as a list of buffers, each of which is available
through a data descriptor.
elfgetdata lets a program step through a section's data list. If
the incoming data descriptor, data, is null, the function returns the
first buffer associated with the section. Otherwise, data should be
a data descriptor associated with scn, and the function gives the
program access to the next data element for the section. If scn is
null or an error occurs, elfgetdata returns a null pointer.
elfgetdata translates the data from file representations into memory
representations [see elfxlate(3E)] and presents objects with memory
data types to the program, based on the file's class [see elf(3E)].
The working library version [see elfversion(3E)] specifies what
version of the memory structures the program wishes elfgetdata to
present.
elfnewdata creates a new data descriptor for a section, appending it
to any data elements already associated with the section. As
described below, the new data descriptor appears empty, indicating
the element holds no data. For convenience, the descriptor's type
(dtype below) is set to ELFTBYTE, and the version (dversion
below) is set to the working version. The program is responsible for
setting (or changing) the descriptor members as needed. This
function implicitly sets the ELFFDIRTY bit for the section's data
[see elfflag(3E)]. If scn is null or an error occurs, elfnewdata
returns a null pointer.
elfrawdata differs from elfgetdata by returning only uninterpreted
bytes, regardless of the section type. This function typically
should be used only to retrieve a section image from a file being
read, and then only when a program must avoid the automatic data
translation described below. Moreover, a program may not close or
disable [see elfcntl(3E)] the file descriptor associated with elf
before the initial raw operation, because elfrawdata might read the
data from the file to ensure it doesn't interfere with elfgetdata.
See elfrawfile(3E) for a related facility that applies to the entire
file. When elfgetdata provides the right translation, its use is
recommended over elfrawdata. If scn is null or an error occurs,
elfrawdata returns a null pointer.
The ElfData structure includes the following members:
void *dbuf;
ElfType dtype;
sizet dsize;
offt doff;
sizet dalign;
unsigned dversion;
These members are available for direct manipulation by the program.
Descriptions appear below.
dbuf A pointer to the data buffer resides here. A data
element with no data has a null pointer.
dtype This member's value specifies the type of the data to
which dbuf points. A section's type determines how to
interpret the section contents, as summarized below.
dsize This member holds the total size, in bytes, of the
memory occupied by the data. This may differ from the
size as represented in the file. The size will be zero
if no data exist. [See the discussion of SHTNOBITS
below for more information.]
doff This member gives the offset, within the section, at
which the buffer resides. This offset is relative to
the file's section, not the memory object's.
dalign This member holds the buffer's required alignment, from
the beginning of the section. That is, doff will be a
multiple of this member's value. For example, if this
member's value is four, the beginning of the buffer
will be four-byte aligned within the section.
Moreover, the entire section will be aligned to the
maximum of its constituents, thus ensuring appropriate
alignment for a buffer within the section and within
the file.
dversion This member holds the version number of the objects in
the buffer. When the library originally read the data
from the object file, it used the working version to
control the translation to memory objects.
Data Alignment
As mentioned above, data buffers within a section have explicit
alignment constraints. Consequently, adjacent buffers sometimes will
not abut, causing ``holes'' within a section. Programs that create
output files have two ways of dealing with these holes.
First, the program can use elffill to tell the library how to set
the intervening bytes. When the library must generate gaps in the
file, it uses the fill byte to initialize the data there. The
library's initial fill value is zero, and elffill lets the
application change that.
Second, the application can generate its own data buffers to occupy
the gaps, filling the gaps with values appropriate for the section
being created. A program might even use different fill values for
different sections. For example, it could set text sections' bytes
to no-operation instructions, while filling data section holes with
zero. Using this technique, the library finds no holes to fill,
because the application eliminated them.
Section and Memory Types
elfgetdata interprets sections' data according to the section type,
as noted in the section header available through elfgetshdr. The
following table shows the section types and how the library
represents them with memory data types for the 32-bit file class.
Other classes would have similar tables. By implication, the memory
data types control translation by elfxlate.
Section Type ElfType 32-Bit Type
-------------+------------+---------------
SHTDYNAMIC | ELFTDYN | Elf32Dyn
SHTDYNSYM | ELFTSYM | Elf32Sym
SHTHASH | ELFTWORD | Elf32Word
SHTNOBITS | ELFTBYTE | unsigned char
SHTNOTE | ELFTBYTE | unsigned char
SHTNULL | none | none
SHTPROGBITS | ELFTBYTE | unsigned char
SHTREL | ELFTREL | Elf32Rel
SHTRELA | ELFTRELA | Elf32Rela
SHTSTRTAB | ELFTBYTE | unsigned char
SHTSYMTAB | ELFTSYM | Elf32Sym
other | ELFTBYTE | unsigned char
-------------+------------+---------------
elfrawdata creates a buffer with type ELFTBYTE.
As mentioned above, the program's working version controls what
structures the library creates for the application. The library
similarly interprets section types according to the versions. If a
section type ``belongs'' to a version newer than the application's
working version, the library does not translate the section data.
Because the application cannot know the data format in this case, the
library presents an untranslated buffer of type ELFTBYTE, just as
it would for an unrecognized section type.
A section with a special type, SHTNOBITS, occupies no space in an
object file, even when the section header indicates a non-zero size.
elfgetdata and elfrawdata ``work'' on such a section, setting the
data structure to have a null buffer pointer and the type indicated
above. Although no data are present, the dsize value is set to the
size from the section header. When a program is creating a new
section of type SHTNOBITS, it should use elfnewdata to add data
buffers to the section. These ``empty'' data buffers should have the
dsize members set to the desired size and the dbuf members set to
null.
EXAMPLE
The following fragment obtains the string table that holds section
names (ignoring error checking). See elfstrptr(3E) for a variation
of string table handling.
ehdr = elf32getehdr(elf);
scn = elfgetscn(elf, (sizet)ehdr->eshstrndx);
shdr = elf32getshdr(scn);
if (shdr->shtype != SHTSTRTAB)
{
/* not a string table */
}
data = 0;
if ((data = elfgetdata(scn, data)) == 0 || data->dsize == 0)
{
/* error or no data */
}
The eshstrndx member in an ELF header holds the section table index
of the string table. The program gets a section descriptor for that
section, verifies it is a string table, and then retrieves the data.
When this fragment finishes, data->dbuf points at the first byte of
the string table, and data->dsize holds the string table's size in
bytes.
SEE ALSO
elf(3E), elfcntl(3E), elffill(3E), elfflag(3E), elfgetehdr(3E),
elfgetscn(3E), elfgetshdr(3E), elfrawfile(3E), elfversion(3E),
elfxlate(3E).
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