mmap不存在物理地址错误

时间：2011-10-14 07:00:15

标签： c mmap sigbus

我正在尝试mmap一个文件，它似乎成功但是当我访问它时，我收到一个SIGBUS错误。 代码：

int main(int argc, char* argv[]) {
int pid = getpid();
char cmd [1024];
int file;
if (argc != 2) {
    printf("Wrong arguments\n");
    return 1;
}
printf("%s", argv[1]);
int numpages = atoi(argv[1]);
sprintf(cmd, "cat /proc/%d/maps", pid);
system(cmd);
long page_size = sysconf(_SC_PAGESIZE);
file = open(argv[1], O_RDWR, (mode_t)0600);
ftruncate(file, page_size  * numpages);
if(file == -1) {
    perror("file open failed!\n");
    return 1;
}
printf("\n\n mapping file - numpages: %d \n\n", numpages);
printf("mapping %ld KB\n", page_size  * numpages/1024);
int* mapped_file = mmap(0, page_size  * numpages,  PROT_READ | PROT_WRITE, MAP_PRIVATE, file, 0 );
if (mapped_file == MAP_FAILED) {
    perror("Map failed");
    return 1;
}
printf("mapped file at %p\n\n", mapped_file);
int* current_pos = mapped_file;

system(cmd);

int j;
for (j=0; j<numpages; j++) {
    printf("%d   ", *current_pos);
    current_pos += page_size/4;
}
if (munmap (mapped_file, page_size  * numpages) == -1) {
        perror ("munmap");
        return 1;
}
if (close(file) == -1) {
    perror("close");
    return 1;
}
return 0;
}

输出： 


(gdb) run 64
Starting program: /home/jords/engcode/workspace/SE370Assignment3/A3Program3 64
00400000-00401000 r-xp 00000000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
00600000-00601000 r--p 00000000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
00601000-00602000 rw-p 00001000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
7ffff7a48000-7ffff7bd2000 r-xp 00000000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7ffff7bd2000-7ffff7dd1000 ---p 0018a000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7ffff7dd1000-7ffff7dd5000 r--p 00189000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7ffff7dd5000-7ffff7dd6000 rw-p 0018d000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7ffff7dd6000-7ffff7ddc000 rw-p 00000000 00:00 0 
7ffff7ddc000-7ffff7dfd000 r-xp 00000000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7ffff7fd0000-7ffff7fd3000 rw-p 00000000 00:00 0 
7ffff7ff8000-7ffff7ffb000 rw-p 00000000 00:00 0 
7ffff7ffb000-7ffff7ffc000 r-xp 00000000 00:00 0                          [vdso]
7ffff7ffc000-7ffff7ffd000 r--p 00020000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7ffff7ffd000-7ffff7fff000 rw-p 00021000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7ffffffde000-7ffffffff000 rw-p 00000000 00:00 0                          [stack]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
64



mapping file - numpages: 64 



mapping 256 KB
mapped file at 0x7ffff7ff4000



00400000-00401000 r-xp 00000000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
00600000-00601000 r--p 00000000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
00601000-00602000 rw-p 00001000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
7ffff7a48000-7ffff7bd2000 r-xp 00000000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7ffff7bd2000-7ffff7dd1000 ---p 0018a000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7ffff7dd1000-7ffff7dd5000 r--p 00189000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7ffff7dd5000-7ffff7dd6000 rw-p 0018d000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7ffff7dd6000-7ffff7ddc000 rw-p 00000000 00:00 0 
7ffff7ddc000-7ffff7dfd000 r-xp 00000000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7ffff7fd0000-7ffff7fd3000 rw-p 00000000 00:00 0 
7ffff7ff4000-7ffff7ff8000 rw-p 00000000 08:03 3288973                    /home/jords/engcode/workspace/SE370Assignment3/64
7ffff7ff8000-7ffff7ffb000 rw-p 00000000 00:00 0 
7ffff7ffb000-7ffff7ffc000 r-xp 00000000 00:00 0                          [vdso]
7ffff7ffc000-7ffff7ffd000 r--p 00020000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7ffff7ffd000-7ffff7fff000 rw-p 00021000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7ffffffde000-7ffffffff000 rw-p 00000000 00:00 0                          [stack]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]



Program received signal SIGBUS, Bus error.
0x0000000000400abb in main (argc=2, argv=0x7fffffffe058) at A3Program3.c:49
49      printf("%d", mapped_file[0]);
(gdb)
这让我感到困惑，因为你可以在地图输出中清楚地看到文件已被映射并且地址是正确的 - 但每当我尝试访问它时它都会出错。

编辑：Valgrind：



ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
==16295== 
==16295== Process terminating with default action of signal 7 (SIGBUS)
==16295==  Non-existent physical address at address 0x4024000
==16295==    at 0x400AB7: main (A3Program3.c:49)
==16295==

更新：所以我已经用ftruncate调用阻止它崩溃了（感谢:)）但是它打印全部为零： 

jords@jords-desktop ~/engcode/workspace/SE370Assignment3 (master) $ ./A3Program3 64
00400000-00401000 r-xp 00000000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
00600000-00601000 r--p 00000000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
00601000-00602000 rw-p 00001000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
7f96ec7a1000-7f96ec92b000 r-xp 00000000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7f96ec92b000-7f96ecb2a000 ---p 0018a000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7f96ecb2a000-7f96ecb2e000 r--p 00189000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7f96ecb2e000-7f96ecb2f000 rw-p 0018d000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7f96ecb2f000-7f96ecb35000 rw-p 00000000 00:00 0 
7f96ecb35000-7f96ecb56000 r-xp 00000000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7f96ecd2a000-7f96ecd2d000 rw-p 00000000 00:00 0 
7f96ecd52000-7f96ecd55000 rw-p 00000000 00:00 0 
7f96ecd55000-7f96ecd56000 r--p 00020000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7f96ecd56000-7f96ecd58000 rw-p 00021000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7fff9be10000-7fff9be31000 rw-p 00000000 00:00 0                          [stack]
7fff9bf0b000-7fff9bf0c000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
64



mapping file - numpages: 64 



mapping 256 KB
mapped file at 0x7f96eccea000



00400000-00401000 r-xp 00000000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
00600000-00601000 r--p 00000000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
00601000-00602000 rw-p 00001000 08:03 3288976                            /home/jords/engcode/workspace/SE370Assignment3/A3Program3
7f96ec7a1000-7f96ec92b000 r-xp 00000000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7f96ec92b000-7f96ecb2a000 ---p 0018a000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7f96ecb2a000-7f96ecb2e000 r--p 00189000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7f96ecb2e000-7f96ecb2f000 rw-p 0018d000 00:10 8103                       /lib/x86_64-linux-gnu/libc-2.13.so
7f96ecb2f000-7f96ecb35000 rw-p 00000000 00:00 0 
7f96ecb35000-7f96ecb56000 r-xp 00000000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7f96eccea000-7f96ecd2a000 rw-p 00000000 08:03 3288973                    /home/jords/engcode/workspace/SE370Assignment3/64
7f96ecd2a000-7f96ecd2d000 rw-p 00000000 00:00 0 
7f96ecd52000-7f96ecd55000 rw-p 00000000 00:00 0 
7f96ecd55000-7f96ecd56000 r--p 00020000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7f96ecd56000-7f96ecd58000 rw-p 00021000 00:10 8090                       /lib/x86_64-linux-gnu/ld-2.13.so
7fff9be10000-7fff9be31000 rw-p 00000000 00:00 0                          [stack]
7fff9bf0b000-7fff9bf0c000 r-xp 00000000 00:00 0                          [vdso]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0                  [vsyscall]
0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0   0
我认为这是因为文件没有被正确读取，所以我正在阅读的是填充零。但我不确定为什么会这样

更新，解决了，它实际上是正确读取文件我只是在如何阅读它时犯了一个错误。谢谢大家

2 个答案:

答案 0 :(得分：5)

问题是你用O_TRUNC打开文件会截断它，而且mmap只能映射文件的现有内容，它会给你一个sigbus。在映射之前，使用truncate()（和朋友）将文件截断为正确的大小。

来自mmap的手册页：

   SIGBUS Attempted access to a portion of the buffer that does not correspond to the file (for example, beyond the end of the file, including the case where another process has truncated the file).

答案 1 :(得分：2)

有一个错误，但它无法解释mapped_file[0]中的崩溃。

sysconf(_SC_PAGESIZE)为您提供页面大小，以字节为单位。

在current_pos += page_size的每次迭代中，您将指针递增sysconf(_SC_PAGESIZE)*sizeof(int)，以便快速离开映射区域。

此外，page_size * 4中的mmap看起来有问题：肯定num_pages应该进入等式？

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