illumos: 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