FSOP(File Stream Oriented Programming) 1편 - FILE Struct란 무엇인가

//gcc -o file_struct file_struct.c #include <stdio.h> int main(){ printf("File Structure Debugging\n"); read(0, stdin, 0x100); puts("Hello World!"); return 1; }

#ifndef __FILE_defined #define __FILE_defined 1 struct _IO_FILE; /* The opaque type of streams. This is the definition used elsewhere. */ typedef struct _IO_FILE FILE; #endif

struct _IO_FILE { int _flags; /* High-order word is _IO_MAGIC; rest is flags. */ /* The following pointers correspond to the C++ streambuf protocol. */ char *_IO_read_ptr; /* Current read pointer */ char *_IO_read_end; /* End of get area. */ char *_IO_read_base; /* Start of putback+get area. */ char *_IO_write_base; /* Start of put area. */ char *_IO_write_ptr; /* Current put pointer. */ char *_IO_write_end; /* End of put area. */ char *_IO_buf_base; /* Start of reserve area. */ char *_IO_buf_end; /* End of reserve area. */ /* The following fields are used to support backing up and undo. */ char *_IO_save_base; /* Pointer to start of non-current get area. */ char *_IO_backup_base; /* Pointer to first valid character of backup area */ char *_IO_save_end; /* Pointer to end of non-current get area. */ struct _IO_marker *_markers; struct _IO_FILE *_chain; int _fileno; int _flags2; __off_t _old_offset; /* This used to be _offset but it's too small. */ /* 1+column number of pbase(); 0 is unknown. */ unsigned short _cur_column; signed char _vtable_offset; char _shortbuf[1]; _IO_lock_t *_lock; #ifdef _IO_USE_OLD_IO_FILE }; struct _IO_FILE_complete { struct _IO_FILE _file; #endif __off64_t _offset; /* Wide character stream stuff. */ struct _IO_codecvt *_codecvt; struct _IO_wide_data *_wide_data; struct _IO_FILE *_freeres_list; void *_freeres_buf; size_t __pad5; int _mode; /* Make sure we don't get into trouble again. */ char _unused2[15 * sizeof (int) - 4 * sizeof (void *) - sizeof (size_t)]; };

/* Magic number and bits for the _flags field. The magic number is mostly vestigial, but preserved for compatibility. It occupies the high 16 bits of _flags; the low 16 bits are actual flag bits. */ #define _IO_MAGIC 0xFBAD0000 /* Magic number */ #define _IO_MAGIC_MASK 0xFFFF0000 #define _IO_USER_BUF 0x0001 /* Don't deallocate buffer on close. */ #define _IO_UNBUFFERED 0x0002 #define _IO_NO_READS 0x0004 /* Reading not allowed. */ #define _IO_NO_WRITES 0x0008 /* Writing not allowed. */ #define _IO_EOF_SEEN 0x0010 #define _IO_ERR_SEEN 0x0020 #define _IO_DELETE_DONT_CLOSE 0x0040 /* Don't call close(_fileno) on close. */ #define _IO_LINKED 0x0080 /* In the list of all open files. */ #define _IO_IN_BACKUP 0x0100 #define _IO_LINE_BUF 0x0200 #define _IO_TIED_PUT_GET 0x0400 /* Put and get pointer move in unison. */ #define _IO_CURRENTLY_PUTTING 0x0800 #define _IO_IS_APPENDING 0x1000 #define _IO_IS_FILEBUF 0x2000 /* 0x4000 No longer used, reserved for compat. */ #define _IO_USER_LOCK 0x8000

gef➤ disas main Dump of assembler code for function main: 0x000060b66f697169 <+0>: endbr64 0x000060b66f69716d <+4>: push rbp 0x000060b66f69716e <+5>: mov rbp,rsp 0x000060b66f697171 <+8>: lea rax,[rip+0xe8c] # 0x60b66f698004 0x000060b66f697178 <+15>: mov rdi,rax 0x000060b66f69717b <+18>: call 0x60b66f697060 <puts@plt> 0x000060b66f697180 <+23>: mov rax,QWORD PTR [rip+0x2e89] # 0x60b66f69a010 <stdin@GLIBC_2.2.5> 0x000060b66f697187 <+30>: mov edx,0x100 0x000060b66f69718c <+35>: mov rsi,rax 0x000060b66f69718f <+38>: mov edi,0x0 0x000060b66f697194 <+43>: mov eax,0x0 0x000060b66f697199 <+48>: call 0x60b66f697070 <read@plt> 0x000060b66f69719e <+53>: lea rax,[rip+0xe78] # 0x60b66f69801d 0x000060b66f6971a5 <+60>: mov rdi,rax 0x000060b66f6971a8 <+63>: call 0x60b66f697060 <puts@plt> 0x000060b66f6971ad <+68>: mov eax,0x1 0x000060b66f6971b2 <+73>: pop rbp 0x000060b66f6971b3 <+74>: ret End of assembler dump.

b *main p *stdin

#define _IO_MAGIC 0xFBAD0000 /* Magic number */ #define _IO_IS_FILEBUF 0x2000 #define _IO_LINKED 0x0080 /* In the list of all open files. */ #define _IO_NO_WRITES 0x0008 /* Writing not allowed. */ _flags = 0xfbad2088 _flags = _IO_MAGIC | _IO_IS_FILEBUF | _IO_LINKED | _IO_NO_WRITES _flags = 0xFBAD0000 | 0x2000 | 0x0080 | 0x0008

ptype *stdin

/* A streammarker remembers a position in a buffer. */ struct _IO_marker { struct _IO_marker *_next; FILE *_sbuf; /* If _pos >= 0 it points to _buf->Gbase()+_pos. FIXME comment */ /* if _pos < 0, it points to _buf->eBptr()+_pos. FIXME comment */ int _pos; };

struct _IO_codecvt { _IO_iconv_t __cd_in; _IO_iconv_t __cd_out; };

struct _IO_wide_data { wchar_t *_IO_read_ptr; /* Current read pointer */ wchar_t *_IO_read_end; /* End of get area. */ wchar_t *_IO_read_base; /* Start of putback+get area. */ wchar_t *_IO_write_base; /* Start of put area. */ wchar_t *_IO_write_ptr; /* Current put pointer. */ wchar_t *_IO_write_end; /* End of put area. */ wchar_t *_IO_buf_base; /* Start of reserve area. */ wchar_t *_IO_buf_end; /* End of reserve area. */ /* The following fields are used to support backing up and undo. */ wchar_t *_IO_save_base; /* Pointer to start of non-current get area. */ wchar_t *_IO_backup_base; /* Pointer to first valid character of backup area */ wchar_t *_IO_save_end; /* Pointer to end of non-current get area. */ __mbstate_t _IO_state; __mbstate_t _IO_last_state; struct _IO_codecvt _codecvt; wchar_t _shortbuf[1]; const struct _IO_jump_t *_wide_vtable; };