Tribblix: manual page: elf

ELF_UPDATE(3ELF) ELF Library Functions 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

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().

July 11, 2001 ELF_UPDATE(3ELF)

NAME

SYNOPSIS

DESCRIPTION

ATTRIBUTES

SEE ALSO

NOTES