Tribblix: manual page: elf

ELF_GETDATA(3ELF) ELF Library Functions ELF_GETDATA(3ELF)

NAME

elf_getdata, elf_newdata, elf_rawdata - get section data

SYNOPSIS

cc [ flag ... ] file ... -lelf [ library ... ]
#include <libelf.h>

Elf_Data *elf_getdata(Elf_Scn *scn, Elf_Data *data);

Elf_Data *elf_newdata(Elf_Scn *scn);

Elf_Data *elf_rawdata(Elf_Scn *scn, Elf_Data *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.

The elf_getdata() function 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, elf_getdata() returns
a null pointer.

The elf_getdata() function translates the data from file
representations into memory representations (see
elf32_xlatetof(3ELF)) and presents objects with memory data types to
the program, based on the file's class (see elf(3ELF)). The working
library version (see elf_version(3ELF)) specifies what version of the
memory structures the program wishes elf_getdata() to present.

The elf_newdata() function 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 (d_type below) is set to ELF_T_BYTE, and the
version (d_version 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 ELF_F_DIRTY bit for the
section's data (see elf_flagdata(3ELF)). If scn is null or an error
occurs, elf_newdata() returns a null pointer.

The elf_rawdata() function differs from elf_getdata() 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 elf_cntl(3ELF)) the file descriptor
associated with elf before the initial raw operation, because
elf_rawdata() might read the data from the file to ensure it doesn't
interfere with elf_getdata(). See elf_rawfile(3ELF) for a related
facility that applies to the entire file. When elf_getdata()
provides the right translation, its use is recommended over
elf_rawdata(). If scn is null or an error occurs, elf_rawdata()
returns a null pointer.

The Elf_Data structure includes the following members:

void *d_buf;
Elf_Type d_type;
size_t d_size;
off_t d_off;
size_t d_align;
unsigned d_version;

These members are available for direct manipulation by the program.
Descriptions appear below.

d_buf
A pointer to the data buffer resides here. A data
element with no data has a null pointer.

d_type
This member's value specifies the type of the data to
which d_buf points. A section's type determines how to
interpret the section contents, as summarized below.

d_size
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 SHT_NOBITS
below for more information.)

d_off
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.

d_align
This member holds the buffer's required alignment, from
the beginning of the section. That is, d_off will be a
multiple of this member's value. For example, if this
member's value is 4, 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.

d_version
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 elf_fill() 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 0, and elf_fill() 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

The elf_getdata() function interprets sections' data according to the
section type, as noted in the section header available through
elf32_getshdr(). 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 elf32_xlatetof(3ELF)

Section Type Elf_Type 32-bit Type
SHT_DYNAMIC ELF_T_DYN Elf32_Dyn
SHT_DYNSYM ELF_T_SYM Elf32_Sym
SHT_FINI_ARRAY ELF_T_ADDR Elf32_Addr
SHT_GROUP ELF_T_WORD Elf32_Word
SHT_HASH ELF_T_WORD Elf32_Word
SHT_INIT_ARRAY ELF_T_ADDR Elf32_Addr
SHT_NOBITS ELF_T_BYTE unsigned char
SHT_NOTE ELF_T_NOTE unsigned char
SHT_NULL none none
SHT_PREINIT_ARRAY ELF_T_ADDR Elf32_Addr
SHT_PROGBITS ELF_T_BYTE unsigned char
SHT_REL ELF_T_REL Elf32_Rel
SHT_RELA ELF_T_RELA Elf32_Rela
SHT_STRTAB ELF_T_BYTE unsigned char
SHT_SYMTAB ELF_T_SYM Elf32_Sym
SHT_SUNW_comdat ELF_T_BYTE unsigned char
SHT_SUNW_move ELF_T_MOVE Elf32_Move (sparc)
SHT_SUNW_move ELF_T_MOVEP Elf32_Move (ia32)
SHT_SUNW_syminfo ELF_T_SYMINFO Elf32_Syminfo
SHT_SUNW_verdef ELF_T_VDEF Elf32_Verdef
SHT_SUNW_verneed ELF_T_VNEED Elf32_Verneed
SHT_SUNW_versym ELF_T_HALF Elf32_Versym
other ELF_T_BYTE unsigned char

The elf_rawdata() function creates a buffer with type ELF_T_BYTE.

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 ELF_T_BYTE, just as
it would for an unrecognized section type.

A section with a special type, SHT_NOBITS, occupies no space in an
object file, even when the section header indicates a non-zero size.
elf_getdata() and elf_rawdata() 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 d_size value is set to the
size from the section header. When a program is creating a new
section of type SHT_NOBITS, it should use elf_newdata() to add data
buffers to the section. These empty data buffers should have the
d_size members set to the desired size and the d_buf members set to
NULL.

EXAMPLES

Example 1: A sample program of calling elf_getdata().

The following fragment obtains the string table that holds section
names (ignoring error checking). See elf_strptr(3ELF) for a variation
of string table handling.

ehdr = elf32_getehdr(elf);
scn = elf_getscn(elf, (size_t)ehdr->e_shstrndx);
shdr = elf32_getshdr(scn);
if (shdr->sh_type != SHT_STRTAB)
{
/* not a string table */
}
data = 0;
if ((data = elf_getdata(scn, data)) == 0 || data->d_size == 0)
{
/* error or no data */
}

The e_shstrndx 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->d_buf points at the first byte of
the string table, and data->d_size holds the string table's size in
bytes.

ATTRIBUTES