What does this mean in below file? 2**2 and 2**0
$ objdump -h main.o main.o: file format elf32-i386 Sections: Idx Name Size VMA LMA File off Algn 0 .text 0000000b 00000000 00000000 00000034 2**2 CONTENTS, ALLOC, LOAD, READONLY, CODE 1 .data 00000000 00000000 00000000 00000040 2**2 CONTENTS, ALLOC, LOAD, DATA 2 .bss 00000000 00000000 00000000 00000040 2**2 ALLOC 3 .note.GNU-stack 00000000 00000000 00000000 00000040 2**0 CONTENTS, READONLY, CODE
I would assume that 2**2 means 22, or 4 byte alignment, while 2**0 means no (one byte) alignment.
This value comes from the sh_addralign field of the ELF section header. The ELF specification states (emphasis mine):
sh_addralign Some sections have address alignment constraints. For example, if a section holds a doubleword, the system must ensure doubleword alignment for the entire section. That is, the value of sh_addr must be congruent to 0, modulo the value of sh_addralign. Currently, only 0 and positive integral powers of two are allowed. Values 0 and 1 mean the section has no alignment constraints.
As Ray Toal mentioned, since the alignment must be a power of two, it only makes sense that objdump would express this value as a power of two with the 2**x notation.
Note that in some languages, like Python and FORTRAN, ** is a power or exponentiation operator.
Looking at objdump.c, we see:
static void dump_section_header (bfd *abfd, asection *section, void *ignored ATTRIBUTE_UNUSED) { // ... printf (" %08lx 2**%u", (unsigned long) section->filepos, bfd_get_section_alignment (abfd, section));
And in objdump.h:
#define bfd_get_section_alignment(bfd, ptr) ((ptr)->alignment_power + 0)
where the alignment_power member of bfd is:
/* The alignment requirement of the section, as an exponent of 2 - e.g., 3 aligns to 2^3 (or 8). */ unsigned int alignment_power;
