Update an ELF descriptor
off_t elf_update(Elf * elf, Elf_Cmd cmd);
elf_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.
The ELF_C_WRITE and ELF_C_IMPURE_WRITE (shown below) 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 the elf32_getehdr and elf_getdata subroutines], 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; you 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 cannot update an executable COFF file with the ELF_C_WRITE command (though ELF_C_NULL is allowed).
|cmd||Specifies the action to take and can be set to any of the following
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.
|ELF Header||e_ident[EI_DATA]||Library controls other e_ident values|
|e_phoff||Only when ELF_F_LAYOUT asserted|
|e_shoff||Only when ELF_F_LAYOUT asserted|
|Program Header||p_type||The application controls all|
|p_offset||program header entries|
|sh_offset||Only when ELF_F_LAYOUT asserted|
|sh_size||Only when ELF_F_LAYOUT asserted|
|sh_addralign||Only when ELF_F_LAYOUT asserted|
|d_off||Only when ELF_F_LAYOUT asserted|
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 does not 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 sections most strictly aligned data buffer controls the sections alignment. The library also inserts padding between buffers, as necessary, to ensure the proper alignment of each buffer.
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.
The elf_begin (elf_begin Subroutine) subroutine, elf_flagdata (elf_flagdata, elf_flagehdr, elf_flagelf, elf_flagphdr, elf_flagscn, elf_flagshdr Subroutine) subroutine, elf32_fsize (elf32_fsize, elf64_fsize Subroutine) subroutine, elf_getdata (elf_getdata, elf_newdata, elf_rawdata Subroutine) subroutine, elf32_getehdr (elf32_getehdr, elf32_newehdr, elf64_getehdr, elf64_newehdr Subroutine) subroutine, elf32_getshdr (elf32_getshdr, elf64_getshdr Subroutine) subroutine, elf32_xlatetof (elf32_xlatetof, elf32_xlatetom, elf64_xlatetof, elf64_xlatetom Subroutine) subroutine.
Introduction to ELF Subroutines.