elf_getdata(3E)

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.

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 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.

                Elf_Void        *d_buf;
                Elf_Type        d_type;
                Elf64_Xword     d_size;
                Elf64_Off       d_off;
                Elf64_Xword     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.

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 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(3E). 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(3E).

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 structure to have a null buffer pointer and the type indicated above. Although no data is 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

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_getdata(3E)

Technical documentation

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