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    Subjectx86 Endiannes and libc printf
    Hi all,
    i am searching for a few hours now the endianness in the x86
    environment,and i have the following
    snippets of code,which in some places i cant understand.Please help me!!!

    #include <stdio.h>
    #include <fcntl.h>
    #include <sys/types.h>

    int main()
    char *filename= "endian.txt";
    unsigned long buf;
    char *k=(char *)&buf;
    int fd;

    fd = open("makis",O_RDONLY);


    printf("&buf: %p %#x %p\n",&buf,*k,k);
    return 0;


    &buf: 0xbffff8b0 0x44 0xbffff8b0

    In the first printf everything is fine.In the second printf we see that
    the 0x44,0x43,0x42,0x41 byte-data is printed in the revserse order,while
    we can see
    that in memory it is in the right order after the read system call.Why
    this happens?Is it being internal in printf???

    I tried to explain that with similar approaches like unions, but the same
    #include <stdio.h>
    #include <unistd.h>

    int main()

    union {
    long s;
    char c[sizeof(long)];
    } un;

    un.s = 0x41424344;
    if (sizeof(short) == 2) {
    if (un.c[0] == 0x41 && un.c[1] == 0x42)
    else if (un.c[0] == 0x44 && un.c[1] == 0x43)
    } else
    printf("sizeof(short) = %d\n", sizeof(short));



    The same as above.Should i assume that an internal operation in printf is
    doing this???

    I also used the above assembly example,to see what
    happens.Memory-to-memory movements (with push & pop) dont inherit the
    little-endian way.Is this
    happens only from memory-to-register and the opposite????

    section .bss
    buf resd 1

    section .data
    pathname db "makis",0
    section .text

    global _start


    mov eax,5
    mov ebx,pathname
    mov ecx,02
    int 0x80

    mov ebx,eax
    mov eax,3
    mov ecx,buf
    mov edx,4
    int 0x80

    mov eax,4
    mov ebx,1
    mov ecx,buf
    mov edx,4
    int 0x80

    mov eax,1
    mov ebx,0
    int 0x80

    Everything works just fine.Can anynone knows how can i revserse the order
    of the data,from 0x44434241 to 0x41424344 into the stack?? Without using
    AND and OR.Can
    this be done????

    The last two examples is the output from gcc,one "fixed" from me to find
    out what is in the stack and the other is the default output from the
    first example.My example
    has been changed only in the printf call from the library,after the read
    call,which i suppose is the "black box" to the "problem" i cant
    .file "read.c"
    .version "01.01"
    .section .rodata
    .string "makis"
    .string "%#x\n"
    .align 4
    .globl main
    .type main,@function
    pushl %ebp
    movl %esp, %ebp
    subl $24, %esp
    movl $.LC0, -4(%ebp)
    subl $8, %esp
    pushl $0
    pushl $.LC0
    call open

    addl $16, %esp
    movl %eax, %eax
    movl %eax, -12(%ebp)
    subl $4, %esp
    pushl $4
    leal -8(%ebp), %eax
    pushl %eax
    pushl -12(%ebp)
    call read

    ---->Before it was the printf call which retrieves its arguments from the
    stack.Which as we can see its different for every conversion specifier..

    movl $4,%eax
    movl $1,%ebx
    leal -8(%ebp),%ecx
    movl $4,%edx
    int $0x80

    movl $1, %eax
    movl $0,%ebx
    int $0x80
    .size main,.Lfe1-main
    .ident "GCC: (GNU) 2.96 20000731 (Red Hat Linux 7.3 2.96-110)"

    Can someone please help me with that???

    Thanks in advance,Chris.

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      Last update: 2005-07-08 21:04    [W:0.027 / U:4.640 seconds]
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