ELF_UPDATE(3E)ELF_UPDATE(3E)NAMEelf_update - update an ELF descriptor
SYNOPSIS
cc [flag ...] file ... -lelf [library ...]
#include <libelf.h>
off_t elf_update(Elf *elf, Elf_Cmd cmd);
DESCRIPTIONelf_update 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.
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(3E)].
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 or
the elf_flag 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 ELF_F_DIRTY bits
are reset, indicating those items no longer need to be
written to the file [see elf_flag(3E)]. 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.
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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
_________________________________________________________
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
ELF Header e_shstrndx
_________________________________________________________
|
Member Notes
________________________________________
p_type The application controls all
p_offset program header entries
p_vaddr
p_paddr
p_filesz
p_memsz
p_flags
Program Header p_align
________________________________________
|
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
Section Header sh_entsize
_______________________________________________
|
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ELF_UPDATE(3E)ELF_UPDATE(3E)
Member Notes
____________________________________________
d_buf
d_type
d_size
d_off Only when ELF_F_LAYOUT asserted
d_align
Data Descriptor d_version
____________________________________________
|
Note 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 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, 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.
SEE ALSOelf(3E), elf_begin(3E), elf_flag(3E), elf_fsize(3E), elf_getdata(3E),
elf_getehdr(3E), elf_getshdr(3E), elf_xlate(3E).
NOTE
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(3E)
and elf_getdata(3E)], the program should reestablish the pointers after
calling elf_update.
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