Simple way to Get filesize in x86 Assembly Language

Question

Assuming that I have opened a file in assembly and have the file handle for that file in register eax. How would I go about getting the size of the file so I can allocate en

Answer 1

Here is how is implemented in FreshLib. It is a wrapper in order to provide portability. You can simplify it of course (see below).

  struct STAT
    .st_dev     dw  ?     ; ID of device containing file
    .pad1       dw  ?
    .st_ino     dd  ?     ; inode number
    .st_mode    dw  ?     ; protection
    .st_nlink   dw  ?     ; number of hard links
    .st_uid     dw  ?     ; user ID of owner
    .st_gid     dw  ?     ; group ID of owner
    .st_rdev    dw  ?     ; device ID (if special file)
    .pad2       dw  ?
    .st_size    dd  ?     ; total size, in bytes
    .st_blksize dd  ?     ; block size
    .st_blocks  dd  ?

    .st_atime   dd  ?     ; time of last access
    .unused1    dd  ?

    .st_mtime   dd  ?     ; time of last modification
    .unused2    dd  ?

    .st_ctime   dd  ?     ; time of last status change
    .unused3    dd  ?
    .unused4    dd  ?
    .unused5    dd  ?
  ends

  sys_newfstat =  $6c

  proc FileSize, .handle
  .stat STAT
  begin
          push    edx ecx ebx

          mov     eax, sys_newfstat
          mov     ebx, [.handle]
          lea     ecx, [.stat]
          int     $80

          cmp     eax, -1
          jle     .error

          mov     eax, [.stat.st_size]
          clc
          pop     ebx ecx edx
          return

  .error:
          neg     eax       ; error code
          stc
          pop     ebx ecx edx
          return
  endp

The minimal version can look this way (much less readable and not thread safe):

  ; argument: file handle in ebx
  ; returns:  the size in EDX; error code in EAX
  FileSize:
          mov     eax, $6c
          mov     ecx, file_stat
          int     $80
          mov     edx, [file_stat+$14]
          retn

  file_stat rd $10

Answer 2

My solution -- Just use .lcomm to create locations for all the named variables

movl    inputfile, %ebx         #Move file handler into ebx for system call
movl    $0x6c, %eax          #Stat Sys Call into eax
leal    statlocation, %ecx      #Move reserved location for stat structure into
int     $0x80                    #Execute System Call
movl    20(%ecx), %eax          #20 to location of size variable, size is now in eax