Below is the assembly code generated from my c program:
SECTION .text align=1 execute ; section number 1, code main: ; Function begin push rbp ; 0000 _ 55 mov rbp, rsp ; 0001 _ 48: 89. E5 mov dword [rbp-4H], edi ; 0004 _ 89. 7D, FC mov qword [rbp-10H], rsi ; 0007 _ 48: 89. 75, F0 mov eax, 1 ; 000B _ B8, 00000001 pop rbp ; 0010 _ 5D ret ; 0011 _ C3 ; main End of function SECTION .data align=1 noexecute ; section number 2, data SECTION .bss align=1 noexecute ; section number 3, bss Variables are aligned to some extent to help faster access to them. The alignment value is usually an implementation detail, but compilers usually follow Intel's recommendations to align/place variables next to each other.
Performance penalty of unaligned access to the stack happens when a memory reference splits a cache line. This means that one out of eight spatially consecutive unaligned quadword accesses is always penalized, similarly for one out of 4 consecutive, non-aligned double-quadword accesses, etc.
Aligning the stack may be beneficial any time there are data objects that exceed the default stack alignment of the system. For example, on 32/64bit Linux, and 64bit Windows, the default stack alignment is 16 bytes, while 32bit Windows is 4 bytes.
Assembly/Compiler Coding Rule 55. (H impact, M generality) Make sure that the stack is aligned at the largest multi-byte granular data type boundary matching the register width.
In your case on x64 the registers are 8 bytes wide and argc being an int is 4 bytes wide. Following the guideline above the variables are aligned to 8-byte boundary.
► 0x5555555545fe mov dword ptr [rbp - 4], edi 0x555555554601 mov qword ptr [rbp - 0x10], rsi 0x555555554605 mov eax, 0x2a 0x55555555460a pop rbp 0x55555555460b ret
edi stores argc a 32 bit value - packed in a 64 bit boundary with the upper 32 bits of no use for you.
pwndbg> tele rbp-0x10 00:0000│ 0x7fffffffdf10 —▸ 0x7fffffffe000 ◂— 0x1 01:0008│ 0x7fffffffdf18 ◂— 0x0 02:0010│ rbp rsp 0x7fffffffdf20 —▸ 0x555555554610 (__libc_csu_init) ◂— push r15 So the argv starts 8 bytes above argv