android-mmap之后在memcpy中获取分段错误SIGSEGV

我编写了一个简单的Android本机函数,该函数获取文件名和更多参数,并通过映射(mmap)内存来读取文件.

因为它是mmap,所以我真的不需要调用“ read()”,所以我只是从mmap()返回的地址使用memcpy().

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

我正在附上我的代码和出现的错误：

编辑

我修复了无限循环,但在读取了25001984个字节后仍然得到SIGSEGV.
该函数适用于这些参数：
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\n"
~"Signal        Stop\tPrint\tPass to program\tDescription\n"
~"SIGSEGV       Yes\tYes\tYes\t\tSegmentation fault\n"
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) 

解决方法:

这里有个大问题：

    addr+=jbuffer_size;

您通过sizeof(int)* jbuffer_size个字节增加了addr,而您只想将其增加jbuffer_size个字节.

我的猜测是系统上的sizeof(int)为4,因此您在整个循环的大约25％时间内崩溃,因为每次迭代时addr的增量都增加了4倍.

标签：android,c,segmentation-fault,mmap,memcpy
来源： https://codeday.me/bug/20191012/1901201.html