在 mmap 之后在 memcpy 中获取分段错误 SIGSEGV [英] Getting segmentation fault SIGSEGV in memcpy after mmap

问题描述

我编写了一个简单的 Android 原生函数，它获取文件名和更多参数，并通过映射 (mmap) 它的内存来读取文件.

I wrote a simple Android native function that get a filename and some more arguments and read the file by mmapping (mmap) it's memory.

因为是mmap，所以我不需要调用read()"，所以我只是从mmap()返回的地址中memcpy().

Because it's mmap, I don't really need to call "read()" so I just memcpy() from the address returned from the mmap().

但是，我在某个地方得到了 SIGSEGV，可能是因为我试图访问我不允许的内存.但是我不明白为什么，我已经要求映射所有文件的内存！

But, somewhere I'm getting a SIGSEGV probably because I'm trying to access a memory which I not permitted. But I don't understand why, I already asked all file's memory to be mapped!

我附上了我的代码和我得到的错误:

I'm attaching my code and the error I got:

编辑

我修复了终止循环，但在读取 25001984 个字节后仍然得到 SIGSEGV.该函数适用于这些参数:jn_bytes = 100,000,000jbuffer_size = 8192jshared=jpopulate=jadvice=0

I fixed the unterminating loop, but still getting SIGSEGV after 25001984 bytes have been read. The function works on those arguments: jn_bytes = 100,000,000 jbuffer_size = 8192 jshared=jpopulate=jadvice=0

void Java_com_def_benchmark_Benchmark_testMmapRead(JNIEnv* env, jobject javaThis,
        jstring jfile_name, unsigned int jn_bytes, unsigned int jbuffer_size, jboolean jshared, jboolean jpopulate, jint jadvice) {
    const char *file_name = env->GetStringUTFChars(jfile_name, 0);

    /* *** start count  *** */
    int fd = open(file_name, O_RDONLY);
    //get the size of the file
    size_t length = lseek(fd, 0L, SEEK_END);
    lseek(fd, 0L, SEEK_SET);
    length = length>jn_bytes?jn_bytes:length;

    // man 2 mmap: MAP_POPULATE is only supported for private mappings since Linux 2.6.23
    int flags =  0;
    if (jshared) flags |= MAP_SHARED; else flags |= MAP_PRIVATE;
    if(jpopulate) flags |= MAP_POPULATE;
    //int flags = MAP_PRIVATE;
    int *  addr = reinterpret_cast<int *>(mmap(NULL, length , PROT_READ, flags , fd, 0));
    if (addr == MAP_FAILED) {
        __android_log_write(ANDROID_LOG_ERROR, "NDK_FOO_TAG", strerror(errno));
        return;
    }
    int * initaddr = addr;
    if(jadvice > 0)
        madvise(addr,length,jadvice==1?(MADV_SEQUENTIAL|MADV_WILLNEED):(MADV_DONTNEED));
    close(fd);

    char buffer[jbuffer_size];
    void *ret_val = buffer;
    int read_length = length;
    while(ret_val == buffer || read_length<jbuffer_size) {
/*****GETTING SIGSEGV SOMWHERE HERE IN THE WHILE************/
        ret_val = memcpy(buffer, addr,jbuffer_size);
        addr+=jbuffer_size;
        read_length -= jbuffer_size;
    }
    munmap(initaddr,length);
    /* stop count */
    env->ReleaseStringUTFChars(jfile_name, file_name);
}

和错误日志:

    15736^done
(gdb) 
15737 info signal SIGSEGV
&"info signal SIGSEGV
"
~"Signal        Stop	Print	Pass to program	Description
"
~"SIGSEGV       Yes	Yes	Yes		Segmentation fault
"
15737^done
(gdb) 
15738-stack-list-arguments 0 0 0
15738^done,stack-args=[frame={level="0",args=[]}]
(gdb) 
15739-stack-list-locals 0
15739^done,locals=[]
(gdb) 

推荐答案

这里有个大问题:

    addr+=jbuffer_size;

您将 addr 增加 sizeof(int) * jbuffer_size 字节，而您只想将其增加 jbuffer_size 字节.

You're bumping addr by sizeof(int) * jbuffer_size bytes whereas you just want to increment it by jbuffer_size bytes.

我的猜测是 sizeof(int) 在你的系统上是 4，因此你在循环的大约 25% 处崩溃，因为你正在递增 addr每次迭代增加 4 倍.

My guess is sizeof(int) is 4 on your system, hence you crash at around 25% of the way through your loop, because you're incrementing addr by a factor of 4x too much on each iteration.

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