Every ELF object has an ELF header at offset 0 in the file. Executable and sharable objects also contain program headers, which are accessed through the ELF header. The link-editor normally arranges for these items to be included as part of the first loadable segment. The information contained in these headers is therefore visible within the mapped image, and is typically used by the runtime linker. The HDR_NOALLOC directive prevents this.
When HDR_NOALLOC is specified, the ELF header and program header array still appear at the start of the resulting output object file, but are not contained in a loadable segment, and virtual address calculations for the image start at the first section of the first segment rather than at the base of the ELF header.