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currently I'm writing little program that reads elf file header and prints some information
I have an unsigned char pointer called buf which points to the location where elf file is located in memory(I used mmap to map it to memory), then I typecast it to a proper elf header pointer
Elf32_Ehdr *ehdr = (Elf32_Ehdr *)buf;
After this I want to get an address of the program header table, I do it like this
Elf32_Phdr *ptbl = (Elf32_Phdr *) (buf + ehdr->e_phoff)
As I noticed the value of ptbl pointer doesn't change and when I try to print the value of the e_phoff member like this
fprintf( stdout , "Offset of program headers : %d\n", ehdr->e_phoff);
I get zero Same stuff happens when I try to print number of program headers and number of section headers - always get zero If I use linux readelf, it prints proper values Does anyone experienced the same problem?
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To find them the ELF header is used, which is located at the very start of the file. The first bytes contain the elf magic "\x7fELF" , followed by the class ID (32 or 64 bit ELF file), the data format ID (little endian/big endian), the machine type, etc. Finally, the entry point of this file is at address 0x0.
you can use readelf and objdump to read parts of an elf file. You can also use 'hexdump filename' to get a hexdump of the contents of a binary file (this is likely only useful if you like reading machine code or you are writing an assembler).
In computing, the Executable and Linkable Format (ELF, formerly named Extensible Linking Format), is a common standard file format for executable files, object code, shared libraries, and core dumps.
Section header (Shdr) A file's section header table lets one locate all the file's sections. The section header table is an array of Elf32_Shdr or Elf64_Shdr structures. The ELF header's e_shoff member gives the byte offset from the beginning of the file to the section header table.
When parsing an ELF object, you would need to keep in mind that:
Rather than handle these cases by hand, it may be easier to use an implementation of the ELF(3) access API to parse the ELF object (see: BSD libelf, or GNU libelf).
The tutorial ``libelf by Example'' contains a readable introduction to the ELF(3) API.
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