How to get a “bus error”?

问题:

I am trying very hard to get a bus error.

One way is misaligned access and I have tried the examples given here and here, but no error for me - the programs execute just fine.

Is there some situation which is sure to produce a bus error?

回答1:

Bus errors can only be invoked on hardware platforms that:

1. Require aligned access, and
2. Don't compensate for an unaligned access by performing two aligned accesses and combining the results.

You probably do not have access to such a system.

回答2:

This should reliably result in a SIGBUS on a POSIX-compliant system.

#include <unistd.h>
#include <stdio.h>
#include <sys/mman.h>
int main() {
    FILE *f = tmpfile();
    int *m = mmap(0, 4, PROT_WRITE, MAP_PRIVATE, fileno(f), 0);
    *m = 0;
    return 0;
}

From the Single Unix Specification, mmap:

References within the address range starting at pa and continuing for len bytes to whole pages following the end of an object shall result in delivery of a SIGBUS signal.

回答3:

Try something along the lines of:

#include <signal.h>
int main(void)
{
    raise(SIGBUS);
    return 0;
}

(I know, probably not the answer you want, but it's almost sure to get you a "bus error"!)

回答4:

As others have mentioned this is very platform specific. On the ARM system I'm working with (which doesn't have virtual memory) there are large portions of the address space which have no memory or peripheral assigned. If I read or write one of those addresses, I get a bus error.

You can also get a bus error if there's actually a hardware problem on the bus.

If you're running on a platform with virtual memory, you might not be able to intentionally generate a bus error with your program unless it's a device driver or other kernel mode software. An invalid memory access would likely be trapped as an access violation or similar by the memory manager (and it never even has a chance to hit the bus).

回答5:

on linux with an Intel CPU try this:

int main(int argc, char **argv)
{
# if defined i386
    /* enable alignment check (AC) */
    asm("pushf; "
    "orl $(1<<18), (%esp); "
    "popf;");
# endif

    char d[] = "12345678";  /* yep! - causes SIGBUS even on Linux-i386 */
    return 0;
}

the trick here is to set the "alignment check" bit in one of the CPUs "special" registers.

see also: here

回答6:

I am sure that you must be using x86 machines. X86 cpu does not generate bus error unless its AC flag in EFALAGS register is set.

Try this code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void)
{
    char *p;

    __asm__("pushf\n"
            "orl $0x40000, (%rsp)\n"
            "popf");

    /* 
     * malloc() always provides aligned memory.
     * Do not use stack variable like a[9], depending on the compiler you use,
     * a may not be aligned properly.
     */
    p = malloc(sizeof(int) + 1);
    memset(p, 0, sizeof(int) + 1);

    /* making p unaligned */
    p++;

    printf("%d\n", *(int *)p);

    return 0;
}

More about this can be found at http://orchistro.tistory.com/206

回答7:

Also keep in mind that some operating systems report "bus error" for errors other than misaligned access. You didn't mention in your question what it was you were actually trying to acheive. Maybe try thus:

int *x = 0;
*x=1;

---

the Wikipedia page you linked to mentions that access to non-existant memory can also result is a bus error. You might have better luck with loading a known-invalid address into a pointer and dereferwncing that.

回答8:

How about this? untested.

  #include<stdio.h>

    typedef struct
    {
    int a;
    int b;
    } busErr;

    int main()
    {
    busErr err;
    char * cPtr;
    int *iPtr;
    cPtr = (char *)&err;
    cPtr++;
    iPtr = (int *)cPtr;
    *iPtr = 10;
    } 

回答9:

int main(int argc, char **argv)
{
    char *bus_error = new char[1];
    for (int i=0; i<1000000000;i++) {
        bus_error += 0xFFFFFFFFFFFFFFF;
    *(bus_error + 0xFFFFFFFFFFFFFF) = 'X';
    }
}

Bus error: 10 (core dumped)

回答10:

Simple, write to memory that isn't yours:

int main()
{
    char *bus_error = 0;

    *bus_error = 'X';
}

Instant bus error on my PowerPC Mac [OS X 10.4, dual 1ghz PPC7455's], not necessarily on your hardware and/or operating system.

There's even a wikipedia article about bus errors, including a program to make one.

回答11:

For 0x86 arch:

#include <stdio.h>

int main()
{
#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

    int b = 0;
    int a = 0xffffff;
    char *c = (char*)&a;
    c++;
    int *p = (int*)c;
    *p = 10;  //Bus error as memory accessed by p is not 4 or 8 byte aligned
    printf ("%d\n", sizeof(a));
    printf ("%x\n", *p);
    printf ("%x\n", p);
    printf ("%x\n", &a);
}

Note:If asm instructions are removed, code wont generate the SIGBUS error as suggested by others. SIGBUS can occur for other reason too.

回答12:

Bus errors occur if you try to access memory that is not addressable by your computer. For example, your computer's memory has an address range 0x00 to 0xFF but you try to access a memory element at 0x0100 or greater.

In reality, your computer will have a much greater range than 0x00 to 0xFF.

To answer your original post:

Tell me some situation which is sure to produce a bus error.

In your code, index into memory way outside the scope of the max memory limit. I dunno ... use some kind of giant hex value 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF indexed into a char* ...