elf_update(S-osr5)
makes the library examine the information associated with an
ELF descriptor,
elf,
and recalculate the structural data needed to generate the file's image.
cmd
may 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(S-osr5)).
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(S-osr5)
or the
elf_flag(S-osr5)
facilities to supply new (or update existing) information for an
ELF
descriptor, those data will be examined,
coordinated, translated if necessary (see
elf_xlate(S-osr5)),
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_flag( )).
The sections' data is 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.
Member
Notes
ELF Header
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
Program Header
p_type
The application controls all
p_offset
program header entries
p_vaddr
p_paddr
p_filesz
p_memsz
p_flags
p_align
Section Header
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
Data Descriptor
d_buf
d_type
d_size
d_off
Only when ELF_F_LAYOUT asserted
d_align
d_version
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.
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 type.
If the entry size is unknown or not applicable, the
value should be set to zero.
In building 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.
Diagnostics
Error conditions are identified through the routine
elf_error(S-osr5).
Warning
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
elf_getehdr(S-osr5)
and
elf_getdata( )),
the program should reestablish the pointers after calling
elf_update( ).
As
elf_begin( )
describes, a program may ``update'' a
COFF
file to
make the image consistent for
ELF.
(
COFF
is an object file format that preceded
ELF
on some computer architectures (Intel, for example)).
When a program calls
elf_begin( )
on a
COFF
file, the library translates
COFF
structures to their
ELF
equivalents, allowing programs to read (but not to write)
a
COFF
file as if it were
ELF.
This conversion happens only to the memory image and not to the file itself.)
The
ELF_C_NULL
command updates only the memory image;
one can use the
ELF_C_WRITE
command to modify the file as well.
Absolute executable files
(a.out files)
require special alignment, which cannot normally be preserved between
COFF
and
ELF.
Consequently, you can not update an executable
COFF
file with the
ELF_C_WRITE
command (though
ELF_C_NULL is allowed).