Source

#include <stdio.h>

int main()
{
    int i = 0x00000001;

    if( ((char *)&i)[0])
        printf ("Little Endian\n");
    else
        printf ("Big Endian\n");

    return 0;
}


Reference site

http://www.joinc.co.kr/modules/moniwiki/wiki.php/Site/Network_Programing/Documents/endian#AEN57

http://www.jopenbusiness.com/mediawiki/index.php/Unix_to_Linux_Migration

man page 설치

yum install man-pages  # 페도라에서 맨 페이지 설치
apt-get install manpages-dev # 우분투에서 맨 페이지 설치
apt-get install manpages-posix-dev # 우분투에서 posix 함수 관련 맨 페이지 설치

Section 번호

맨 페이지에는 명령어 별로 섹션이 분류가 되있으며, 섹션 번호를 이용하면 원하는 명령어의 메뉴얼을 볼 수 있습니다. 섹션 번호는 다음과 같습니다.

Section 1 : user commands

Section 2 : system calls

Section 3 : C library functions

Section 4 : devices and special files

Section 5 : file formats and conventions

Section 8 : system administration tools and deamons

맨 페이지를 사용할 때 ” man [섹션 번호] command ” 와 같이 사용하면 원하는 특정 명령어를 찾을 수 있습니다.

vim에서는 커서를 함수이름에 위치 시킨 후 ” [섹션 번호] Shift + k ” 를 입력하면 맨 페이지로 이동합니다.

출처 : http://openracoon.org/?p=391

A segmentation fault (often shortened to segfault) or bus error is generally an attempt to access memory that the CPU cannot physically address. It occurs when the hardware notifies a Unix-like operating system about a memory access violation. Then the OS kernel sends a signal to the process which caused the exception. By default the process receiving the signal dumps core and terminates. The default signal handler can also be overridden to customize how the signal is handled.^[1]

[edit] Bus error

Bus errors usually are signaled with the SIGBUS signal, but SIGBUS can also be caused by any general device fault that the computer detects. A bus error rarely means that the computer hardware is physically broken - it is normally caused by a bug in a program's source code.

There are two main causes of bus errors:

non-existent address 

The CPU is instructed by software to read or write a specific physical memory address. Accordingly, the CPU sets this physical address on its address bus and requests all other hardware connected to the CPU to respond with the results, if they answer for this specific address. If no other hardware responds, the CPU raises an exception, stating that the requested physical address is unrecognized by the whole computer system. Note that this only covers physical memory addresses. Trying to access an undefined virtual memory address is generally considered to be a segmentation fault rather than a bus error, though if the MMU is separate, the processor can't tell the difference.

unaligned access 

Most CPUs are byte-addressable, where each unique memory address refers to an 8-bit byte. Most CPUs can access individual bytes from each memory address, but they generally cannot access larger units (16 bits, 32 bits, 64 bits and so on) without these units being "aligned" to a specific boundary. For example, if multi-byte accesses must be 16 bit-aligned, addresses 0, 2, 4, and so on would be considered aligned and therefore accessible, while addresses 1, 3, 5, and so on would be considered unaligned. Similarly, if multi-byte accesses must be 32-bit aligned, addresses 0, 4, 8, 12, and so on would be considered aligned and therefore accessible, and all addresses in between would be considered unaligned. Attempting to access a unit larger than a byte at an unaligned address can cause a bus error.

CPUs generally access data at the full width of their data bus at all times. To address bytes, they access memory at the full width of their data bus, then mask and shift to address the individual byte. This is inefficient, but tolerated as it is an essential feature for most software, especially string processing. Unlike bytes, larger units can span two aligned addresses and would thus require more than one fetch on the data bus. It is possible for CPUs to support this, but this functionality is rarely required directly at the machine code level, thus CPU designers normally avoid implementing it and instead issue bus errors for unaligned memory access.

[edit] Segmentation/Page fault

Example of human generated signal

A segmentation fault occurs when a program attempts to access a memory location that it is not allowed to access, or attempts to access a memory location in a way that is not allowed (for example, attempting to write to a read-only location, or to overwrite part of the operating system).

Segmentation is one approach to memory management and protection in the operating system. It has been superseded by paging for most purposes, but much of the terminology of segmentation is still used, "segmentation fault" being an example. Some operating systems still have segmentation at some logical level although paging is used as the main memory management policy.

On Unix-like operating systems, a signal called SIGSEGV is sent to a process that accesses an invalid memory address. On Microsoft Windows, a process that accesses invalid memory receives the STATUS_ACCESS_VIOLATION exception.

[edit] Common causes

A few causes of a segmentation fault can be summarized as follows:

• attempting to execute a program that does not compile correctly. Note that most compilers will not output a binary given a compile-time error.
• a buffer overflow.
• using uninitialized pointers.
• dereferencing NULL pointers.
• attempting to access memory the program does not own.
• attempting to alter memory the program does not own (storage violation).

Generally, segmentation faults occur because: a pointer is either NULL, points to random memory (probably never initialized to anything), or points to memory that has been freed/deallocated/"deleted".

e.g.

    char *p1 = NULL;           // Initialized to null, which is OK,

                               // (but cannot be dereferenced on many systems).

    char *p2;                  // Not initialized at all.

    char *p3  = new char[20];  // Great! it's allocated,

    delete [] p3;              // but now it isn't anymore.

Now, dereferencing any of these variables could cause a segmentation fault.

[edit] Examples

[edit] Bus error example

This is an example of un-aligned memory access, written in the C programming language.

#include <stdlib.h>

 

int main(int argc, char **argv) {

    int *iptr;

    char *cptr;

 

#if defined(__GNUC__)

# if defined(__i386__)

    /* Enable Alignment Checking on x86 */

    __asm__("pushf\norl $0x40000,(%esp)\npopf");

# elif defined(__x86_64__) 

     /* Enable Alignment Checking on x86_64 */

    __asm__("pushf\norl $0x40000,(%rsp)\npopf");

# endif

#endif

 

    /* malloc() always provides aligned memory */

    cptr = malloc(sizeof(int) + 1);

 

    /* Increment the pointer by one, making it misaligned */

    iptr = (int *) ++cptr;

 

    /* Dereference it as an int pointer, causing an unaligned access */

    *iptr = 42;

 

    return 0;

}

Compiling and running the example on Linux on x86 demonstrates the error:

$ gcc -ansi sigbus.c -o sigbus

$ ./sigbus 

Bus error

$ gdb ./sigbus

(gdb) r

Program received signal SIGBUS, Bus error.

0x080483ba in main ()

(gdb) x/i $pc

0x80483ba <main+54>:    mov    DWORD PTR [eax],0x2a

(gdb) p/x $eax

$1 = 0x804a009

(gdb) p/t $eax & (sizeof(int) - 1)

$2 = 1

The GDB debugger shows that the immediate value 0x2a is being stored at the location stored in the EAX register, using X86 assembly language. This is an example of register indirect addressing.

Printing the low order bits of the address shows that it is not aligned to a word boundary ("dword" using x86 terminology).

[edit] Segmentation fault example

Here is an example of ANSI C code that should create a segmentation fault on platforms with memory protection:

 int main(void)

 {

     char *s = "hello world";

     *s = 'H';

 }

When the program containing this code is compiled, the string "hello world" is placed in the section of the program binary marked as read-only; when loaded, the operating system places it with other strings and constant data in a read-only segment of memory. When executed, a variable, s, is set to point to the string's location, and an attempt is made to write an H character through the variable into the memory, causing a segmentation fault. Compiling such a program with a compiler that does not check for the assignment of read-only locations at compile time, and running it on a Unix-like operating system produces the following runtime error:

$ gcc segfault.c -g -o segfault

$ ./segfault

Segmentation fault

Backtrace from gdb:

Program received signal SIGSEGV, Segmentation fault.

0x1c0005c2 in main () at segfault.c:6

6               *s = 'H';

The conditions under which segmentation violations occur and how they manifest themselves are specific to an operating system.

Because a very common program error is a null pointer dereference (a read or write through a null pointer, used in C to mean "pointer to no object" and as an error indicator), most operating systems map the null pointer's address such that accessing it causes a segmentation fault.

 int *ptr = NULL;

 *ptr = 1;

This sample code creates a null pointer, and tries to assign a value to its non-existent target. Doing so causes a segmentation fault at runtime on many operating systems.

Another example is recursion without a base case:

 int main(void)

 {

    main();

 }

which causes the stack to overflow which results in a segmentation fault.^[2]

[edit] See also

• Buffer overflow
• Core dump
• General protection fault
• Segfault, a humor website named after this error.
• Page fault
• Storage violation

[edit] External links

• A FAQ: User contributed answers regarding the definition of a segmentation fault
• A "null pointer" explained
• Answer to: NULL is guaranteed to be 0, but the null pointer is not?
• Resolving crashes and segmentation faults, an article from the Real-Time embedded blog.

[edit] References

관리자 권한으로 cmd.exe를 실행 후 아래 명령어 실행하면 됩니다.

net user administrator /active:yes

administrator로 로그인시 모든 명령들이 관리자 권한으로 실행됩니다.

CFG_I2C_NOPROBES

이 옵션은 'i2c probe' 명령이 issue되었을 때, skipp될 I2C device의 list를 명시한다.
만약  CONFIG_I2C_MULTI_BUS 가 설정되었다면 bus-device pairs의 list를 명시하고, 그렇지 않으면 device address의 1차 배열을 명시해서 사용한다.

e.g. 

하나의 i2c bus에서  0x50, 0x68의  address를 skip할 것이다.

bus0에서 0x50, 0x68의 address를 skip하고, bus1에서는 0x54의 address를 skip할 것이다.

⊙ cpu_init_crit
중요한 registers 및 memory timing 등을 setup한다.

cpu_init_crit:

#ifndef CONFIG_EVT1
#if 0  
    bl  v7_flush_dcache_all
#else
    bl  disable_l2cache

    mov r0, #0x0    @
    mov r1, #0x0    @ i
    mov r3, #0x0
    mov r4, #0x0
lp1:
    mov r2, #0x0    @ j
lp2:
    mov r3, r1, LSL #29     @ r3 = r1(i) <<29
    mov r4, r2, LSL #6      @ r4 = r2(j) <<6
    orr r4, r4, #0x2        @ r3 = (i<<29)|(j<<6)|(1<<1)
    orr r3, r3, r4
    mov r0, r3          @ r0 = r3
    bl  CoInvalidateDCacheIndex
    add r2, #0x1        @ r2(j)++
    cmp r2, #1024       @ r2 < 1024
    bne lp2         @ jump to lp2
    add r1, #0x1        @ r1(i)++
    cmp r1, #8          @ r1(i) < 8
    bne lp1         @ jump to lp1

    bl  set_l2cache_auxctrl

    bl  enable_l2cache
#endif
#endif

    bl  disable_l2cache

    bl  set_l2cache_auxctrl_cycle

    bl  enable_l2cache

       /*
        * Invalidate L1 I/D
        */
        mov r0, #0                  @ set up for MCR
        mcr p15, 0, r0, c8, c7, 0   @ invalidate TLBs
        mcr p15, 0, r0, c7, c5, 0   @ invalidate icache

       /*
        * disable MMU stuff and caches
        */
        mrc p15, 0, r0, c1, c0, 0
        bic r0, r0, #0x00002000     @ clear bits 13 (--V-)
        bic r0, r0, #0x00000007     @ clear bits 2:0 (-CAM)
        orr r0, r0, #0x00000002     @ set bit 1 (--A-) Align
        orr r0, r0, #0x00000800     @ set bit 12 (Z---) BTB
        mcr     p15, 0, r0, c1, c0, 0

        /* Read booting information */
        ldr r0, =PRO_ID_BASE
        ldr r1, [r0,#OMR_OFFSET]

        bic r2, r1, #0xffffffc1

#ifdef CONFIG_VOGUES
    /* PS_HOLD(GPH0_0) set to output high */
    ldr r0, =ELFIN_GPIO_BASE
    ldr r1, =0x00000001
    str r1, [r0, #GPH0CON_OFFSET]

    ldr r1, =0x5500
    str r1, [r0, #GPH0PUD_OFFSET]

    ldr r1, =0x01
    str r1, [r0, #GPH0DAT_OFFSET]
#endif