What does SEGMENT_START("text-segment", 0x400000) represent?

Question

I'm learning about the layout of executable binaries. My end goal is to analyze a specific executable for things that could be refactored (in its source) to reduce the compiled output size.

I've been using https://www.embeddedrelated.com/showarticle/900.php and https://www.geeksforgeeks.org/memory-layout-of-c-program/ as references for this initial learning.

From what I've learned, a linker script specifies the addresses where sections of compiled binaries are placed. E.g.

> ld --verbose | grep text
  PROVIDE (__executable_start = SEGMENT_START("text-segment", 0x400000)); . = SEGMENT_START("text-segment", 0x400000) + SIZEOF_HEADERS;
      *(.rela.text .rela.text.* .rela.gnu.linkonce.t.*)

I think this means that the text segments of compiled binaries starts at memory address 0x400000 - true?

What does that value, 0x400000, represent? I'm probably not understanding something properly, but surely that 0x400000 does not represent a physical memory location, does it? E.g. if I were to run two instances of my compiled a.out executable in parallel, they couldn't both simultaneously occupy the space at 0x400000, right?

Answer 1

0x4000000 is not a physical address in the sense how your memory chips see it. This is a virtual address as it's seen from CPU's point of view.

Loader of your program will map a few pages of physical memory to VA 0x400000 and copy the contents of text-segment to it. And yes, another instance of your program could occupy the same physical and virtual block of memory for the text-segment, because text (code) is readable and executable but not writeable. Other segments (data, bss, stack, heap) may have identical VA but each will be mapped to their private protected physical block of memory.