ELF_GETDATA(3E) ELF_GETDATA(3E)
elf_getdata, elf_newdata, elf_rawdata - get section data
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);
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.
elf_getdata 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.
elf_getdata translates the data from file representations into memory
representations [see elf_xlate(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 elf_version(3E)] specifies what version of
the memory structures the program wishes elf_getdata to present.
elf_newdata 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_flag(3E)]. If scn is
null or an error occurs, elf_newdata returns a null pointer.
elf_rawdata 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(3E)]
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(3E) for a related
facility that applies to the entire file. When elf_getdata provides the
Page 1
ELF_GETDATA(3E) ELF_GETDATA(3E)
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 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.
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.
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.
Page 2
ELF_GETDATA(3E) ELF_GETDATA(3E)
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 zero, 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
elf_getdata interprets sections' data according to the section type, as
noted in the section header available through elf_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 elf_xlate.
Section Type Elf_Type 32-Bit Type
_____________|____________|______________
SHT_DYNAMIC | ELF_T_DYN | Elf32_Dyn
SHT_DYNSYM | ELF_T_SYM | Elf32_Sym
SHT_HASH | ELF_T_WORD | Elf32_Word
SHT_NOBITS | ELF_T_BYTE | unsigned char
SHT_NOTE | ELF_T_BYTE | unsigned char
SHT_NULL | none | none
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
other | ELF_T_BYTE | unsigned char
_____________|____________|______________
elf_rawdata 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
Page 3
ELF_GETDATA(3E) ELF_GETDATA(3E)
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.
The following fragment obtains the string table that holds section names
(ignoring error checking). See elf_strptr(3E) 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.
elf(3E), elf_cntl(3E), elf_fill(3E), elf_flag(3E), elf_getehdr(3E),
elf_getscn(3E), elf_getshdr(3E), elf_rawfile(3E), elf_version(3E),
elf_xlate(3E).
The 64-bit class functions work identically, simply replacing all
instances of 32 in the descriptions with 64.
PPPPaaaaggggeeee 4444 [ Back ]
|
