mremap函数无法分配新的内存

问题描述

我已经编写了以下代码，但是该代码仍然是EEERROR消息，告诉我mremap无法扩展内存.

I have write the following code , but the code is still fiven me EEERROR message , which tells that the mremap failed to extend the memory.

int main()
{
int size_of_mem = 1024
int fd = shm_open("/myregion", O_CREAT | O_RDWR, S_IRWXO | S_IRUSR | S_IWUSR);
    if (fd == -1)
        printf("ERROR in shm_open \n") ;

    if (ftruncate(fd, size_of_mem) == -1)
        printf("ERROR in ftruncate \n") ;

    int shm_address = mmap(0 , size_of_mem , PROT_READ | PROT_WRITE | PROT_EXEC ,MAP_SHARED , fd , 0) ;

    if (shm_address == MAP_FAILED)
    {
        printf("Error mmapping the file \n");
        exit(EXIT_FAILURE);
    }
   int temp = mremap(shm_address , size_of_mem ,4000 , MREMAP_MAYMOVE) ;
   if( temp < 0)
    {
        printf("EEEEEEEERROR\n") ;
    }
return 0 ; 
}

推荐答案

这里有些错误.

第一，mmap()和mremap()返回void*指针，您不能仅将其强制转换为int.

First, mmap() and mremap() return a void* pointer, which you must not just cast to an int.

第二，mremap() man页面状态:

成功时，mremap()返回一个指向新虚拟内存区域的指针.错误时，将返回值MAP_FAILED(即(void *)-1)，并且已正确设置errno.

On success mremap() returns a pointer to the new virtual memory area. On error, the value MAP_FAILED (that is, (void *) -1) is returned, and errno is set appropriately.

所以您的支票temp < 0是错误的.它应该是temp == (void*)-1. mremap()完全有可能在成功时返回一个有效指针，该指针在转换为int时小于零.

So your check temp < 0 is wrong. It should be temp == (void*)-1. It is entirely possible that mremap() returns a valid pointer on success that is smaller than zero when cast to an int.

第三，mmap()和mremap()均设置errno(手册页)变量，当发生错误时.您可以阅读该内容，以获取有关到底出了什么问题的更多信息.要仅输出文本错误消息，请使用perror()函数(手册页).请注意，您必须先#include <errno.h>.

Third, both mmap() and mremap() set the errno (man page) variable when an error occurs. You can read that to get more information about what exactly went wrong. To just output a text error message use the perror() function (man page). Note that you have to #include <errno.h> for that.

第四，如果您检测到错误情况，则始终打印一条消息，但大多数情况下允许执行继续.那没有道理.如果shm_open()失败，则要立即返回(或调用exit(EXIT_FAILURE)).如果您什至无法打开SHM文件，则以下功能均无法使用.

Fourth, if you detect an error condition, you always print a message, but you mostly allow execution to continue. That doesn't make sense. If shm_open() failed, you want to return immediately (or call exit(EXIT_FAILURE)). None of the following functions will work if you couldn't even open the SHM file.

因此，我的清理版本如下:

Thus, my cleaned up version looks like this:

#include <error.h>

int main()
{
    int size_of_mem = 1024;
    int fd = shm_open("/myregion", O_CREAT | O_RDWR,
                      S_IRWXO | S_IRUSR | S_IWUSR);
    if (fd == -1)
    {
        perror("Error in shm_open");
        return EXIT_FAILURE;
    }

    if (ftruncate(fd, size_of_mem) == -1)
    {
        perror("Error in ftruncate");
        return EXIT_FAILURE;
    }

    void *shm_address = mmap(0, size_of_mem,
                             PROT_READ | PROT_WRITE | PROT_EXEC,
                             MAP_SHARED, fd, 0);
    if (shm_address == MAP_FAILED)
    {
        perror("Error mmapping the file");
        return EXIT_FAILURE;
    }

    void *temp = mremap(shm_address, size_of_mem, 4000, MREMAP_MAYMOVE);
    if(temp == (void*)-1)
    {
        perror("Error on mremap()");
        return EXIT_FAILURE;
    }
    return 0;
}

注意:

• 更正数据类型(void*)，更正mremap()的错误检查，使用perror()打印更多有用的错误消息，错误路径终止函数的执行.
• 正确/一致的缩进.
• 函数调用中,之前没有空格.

• Correct data types (void*), correct error checking for mremap(), usage of perror() to print more informative error messages, error paths ending execution of the function.
• Correct/consistent indentation.
• No spaces before , in function calls.

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