man pages section 3: Extended Library Functions, Volume 1

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Updated: July 2014
 
 

elf_update(3ELF)

Name

elf_update - update an ELF descriptor

Synopsis

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

off_t elf_update(Elf *elf, Elf_Cmd cmd);

Description

The elf_update() function causes the library to examine the information associated with an ELF descriptor, elf, and to recalculate the structural data needed to generate the file's image.

The cmd argument can have the following values:

ELF_C_NULL

This value tells elf_update() to recalculate various values, updating only the ELF descriptor's memory structures. Any modified structures are flagged with the ELF_F_DIRTY bit. A program thus can update the structural information and then reexamine them without changing the file associated with the ELF descriptor. Because this does not change the file, the ELF descriptor may allow reading, writing, or both reading and writing (see elf_begin(3ELF)).

ELF_C_WRITE

If cmd has this value, elf_update() duplicates its ELF_C_NULL actions and also writes any ``dirty'' information associated with the ELF descriptor to the file. That is, when a program has used elf_getdata(3ELF) or the elf_flagdata(3ELF) facilities to supply new (or update existing) information for an ELF descriptor, those data will be examined, coordinated, translated if necessary (see elf32_xlatetof(3ELF)), and written to the file. When portions of the file are written, any ELF_F_DIRTY bits are reset, indicating those items no longer need to be written to the file (see elf_flagdata(3ELF)). The sections' data are written in the order of their section header entries, and the section header table is written to the end of the file. When the ELF descriptor was created with elf_begin(), it must have allowed writing the file. That is, the elf_begin() command must have been either ELF_C_RDWR or ELF_C_WRITE.

If elf_update() succeeds, it returns the total size of the file image (not the memory image), in bytes. Otherwise an error occurred, and the function returns −1.

When updating the internal structures, elf_update() sets some members itself. Members listed below are the application's responsibility and retain the values given by the program.

The following table shows ELF Header members:

Member
Notes
e_ident[EI_DATA]
Library controls other e_ident values
e_type
e_machine
e_version
e_entry
e_phoff
Only when ELF_F_LAYOUT asserted
e_shoff
Only when ELF_F_LAYOUT asserted
e_flags
e_shstrndx

The following table shows the Program Header members:

Member
Notes
p_type
The application controls all
p_offset
program header entries
p_vaddr
p_paddr
p_filesz
p_memsz
p_flags
p_align

The following table shows the Section Header members:

Member
Notes
sh_name
sh_type
sh_flags
sh_addr
sh_offset
Only when ELF_F_LAYOUT asserted
sh_size
Only when ELF_F_LAYOUT asserted
sh_link
sh_info
sh_addralign
Only when ELF_F_LAYOUT asserted
sh_entsize

The following table shows the Data Descriptor members:

Member
Notes
d_buf
d_type
d_size
d_off
Only when ELF_F_LAYOUT asserted
d_align
d_version

Note that the program is responsible for two particularly important members (among others) in the ELF header. The e_version member controls the version of data structures written to the file. If the version is EV_NONE, the library uses its own internal version. The e_ident[EI_DATA] entry controls the data encoding used in the file. As a special case, the value may be ELFDATANONE to request the native data encoding for the host machine. An error occurs in this case if the native encoding doesn't match a file encoding known by the library.

Further note that the program is responsible for the sh_entsize section header member. Although the library sets it for sections with known types, it cannot reliably know the correct value for all sections. Consequently, the library relies on the program to provide the values for unknown section types. If the entry size is unknown or not applicable, the value should be set to 0.

When deciding how to build the output file, elf_update() obeys the alignments of individual data buffers to create output sections. A section's most strictly aligned data buffer controls the section's alignment. The library also inserts padding between buffers, as necessary, to ensure the proper alignment of each buffer.

Attributes

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
MT-Safe

See Also

elf(3ELF), elf32_fsize(3ELF), elf32_getehdr(3ELF), elf32_getshdr(3ELF), elf32_xlatetof(3ELF), elf_begin(3ELF), elf_flagdata(3ELF), elf_getdata(3ELF), libelf( 3LIB), attributes(5)

Notes

As mentioned above, the ELF_C_WRITE command translates data as necessary, before writing them to the file. This translation is not always transparent to the application program. If a program has obtained pointers to data associated with a file (for example, see elf32_getehdr(3ELF) and elf_getdata(3ELF)), the program should reestablish the pointers after calling elf_update().