printenv.s

/*** printenv.s ***/

// Description
//   printenv - print the environment to stdout
// Synopsis
//   printenv
// Build
//   $ as --32 -o printenv.o printenv.s
//   $ ld -m elf_i386 -o printenv printenv.o

  .section .text

# ************************************
# * print macro
# * Caller is responsible for setting
# * %ecx and %edx, and saving %eax and
# * %ebx if necessary.
# ************************************
.macro print
  movl $4, %eax
  movl $1, %ebx
  int $0x80
.endm

# ************************************
# * int strlen(char* str);
# * Returns the length of a string.
# ************************************
  .type strlen, @function
strlen:
  pushl %ebp
  movl  %esp, %ebp
  movl  $0, %eax           # Index
  movl  8(%ebp), %ecx      # Address of str
strlen_loop:
  movb  (%ecx,%eax,1), %dl # Current char
  cmpb  $0, %dl
  je    strlen_end
  incl  %eax
  jmp   strlen_loop
strlen_end:
  movl  %ebp, %esp
  popl  %ebp
  ret

  .section .rodata
newline_char:
  .ascii "\n"

  .section .text

# ************************************
# * printenv
# * Prints the environment to stdout.
# ************************************
  .globl _start
_start:
  movl  %esp, %ebp
  # Local variables (offset from %ebp)
  .equ  index, -4          # Stack index being operated on
  .equ  address, -8        # Current env var address
  .equ  length, -12        # Current env var length
  subl  $12, %esp
  # Start at index argc+1 (this skips the argument vector)
  movl  $0, index(%ebp)
  movl  (%ebp), %eax       # argc
  incl  %eax
  addl  %eax, index(%ebp)
printenv_loop:
  # Set address local variable
  movl  index(%ebp), %ecx
  movl  4(%ebp, %ecx, 4), %eax
  movl  %eax, address(%ebp)
  # Check if we're done (reached a NULL pointer)
  cmpl  $0, address(%ebp)
  je    printenv_loop_end
  # Calculate length of string
  pushl address(%ebp)
  call  strlen
  addl  $4, %esp
  # Set length local variable
  movl  %eax, length(%ebp)
  # Print current env var
  movl  address(%ebp), %ecx
  movl  length(%ebp), %edx
  print
  # Print newline char
  movl  $newline_char, %ecx
  movl  $1, %edx
  print
  incl  index(%ebp)
  jmp printenv_loop
printenv_loop_end:
  # Exit
  movl  $1, %eax
  movl  $0, %ebx
  int   $0x80