elf_update(3E)elf_update(3E)NAMEelf_update - update an ELF descriptor
SYNOPSIS
[flag... ] file... [library] ...
DESCRIPTION
causes the library to examine the information associated with an ELF
descriptor, elf, and to recalculate the structural data needed to gen‐
erate the file's image.
cmd may have the following values.
This value tells to recalculate various values, updating only the ELF
descriptor's memory structures. Any modified struc‐
tures are flagged with the 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(3E)).
If cmd has this value, duplicates its actions and also
writes any ``dirty'' information associated with the
ELF descriptor to the file. That is, when a program
has used or the facilities to supply new (or update
existing) information for an ELF descriptor, those
data will be examined, coordinated, translated if
necessary (see elf_xlate(3E)), and written to the
file. When portions of the file are written, any
bits are reset, indicating those items no longer need
to be written to the file (see elf_flag(3E)). The
sections' data is written in the order of their sec‐
tion header entries, and the section header table is
written to the end of the file.
When the ELF descriptor was created with it must have
allowed writing the file. That is, the command must
have been either or
If succeeds, it returns the total size of the file image (not the mem‐
ory image), in bytes. Otherwise an error occurred, and the function
returns -1.
When updating the internal structures, sets some members itself. Mem‐
bers listed below are the application's responsibility and retain the
values given by the program.
Member Notes
───────────────────────────────────────────────
e_ident[EI_DATA] Library controls other
e_ident values
e_type
e_machine
e_version
ELF Header e_entry
e_phoff Only when ELF_F_LAYOUT
asserted
e_shoff Only when ELF_F_LAYOUT
asserted
e_flags
e_shstrndx
───────────────────────────────────────────────
Member Notes
───────────────────────────────────────────────────────────
p_type The application controls all
p_offset program header entries
p_vaddr
p_paddr
Program Header p_filesz
p_memsz
p_flags
p_align
───────────────────────────────────────────────────────────
Member Notes
──────────────────────────────────────────────────
sh_name
sh_type
sh_flags
sh_addr
sh_offset Only when ELF_F_LAYOUT
asserted
Section Header sh_size Only when ELF_F_LAYOUT
asserted
sh_link
sh_info
sh_addralign Only when ELF_F_LAYOUT
asserted
sh_entsize
──────────────────────────────────────────────────
Member Notes
─────────────────────────────────────────────────
d_buf
d_type
d_size
Data Descriptor d_off Only when ELF_F_LAYOUT
asserted
d_align
d_version
─────────────────────────────────────────────────
Note the program is responsible for two particularly important members
(among others) in the ELF header. The member controls the version of
data structures written to the file. If the version is the library
uses its own internal version. The entry controls the data encoding
used in the file. As a special case, the value may be 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 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.
When deciding how to build the output file, 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.
Note
As mentioned above, the commands translate 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(3E) and elf_get‐
data(3E)), the program should reestablish the pointers after calling
As elf_begin(3E) 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 command updates
only the memory image; one can use the command to modify the file as
well. Absolute executable files files) require special alignment,
which cannot normally be preserved between COFF and ELF. Consequently,
one may not update an executable COFF file with the command (though is
allowed).
SEE ALSOelf(3E), elf_begin(3E), elf_flag(3E), elf_fsize(3E), elf_getdata(3E),
elf_getehdr(3E), elf_getshdr(3E), elf_xlate(3E).
elf_update(3E)
