segments within a executable C program

Question

I was reading about sections and segments. Seems you could list the mapping between sections and segments as below.

$ readelf -l test

Elf file type is EXEC          


        

Answer 1

Program headers in an ELF binary describe how the binary should be run. The interesting parts are the LOAD headers which load part of the binary into different places in memory. There could be almost arbitrary number of LOAD headers in a binary, but usually the linker puts everything read-only and executable into one and everything read/write into another. There are operating systems which will have read-only data LOAD header, read-write data and read-only executable code for slightly increased security.

Segments here just mean parts of the binary loaded in different places in memory. So basically the different LOAD headers.

Sections is how the data was organized during linking. For various reasons you want to have better granularity organizing things than just data/code. Some data is read-only, it's put in ".rodata" in your example. The code is in ".text", initialized data is in ".data" while data in variables that are zeroed on program start are in ".bss".

The "section to segment mapping" tells you which sections are in which segments (different LOAD headers). So ".text" and ".rodata" are in the first LOAD header (the third program header) and ".data" is in the second LOAD header (fourth program header).

The stack is something that the operating system gives you on execution and it's not described by an ELF binary.