为什么会出现此细分错误？

Question

int main() {
    int fd;     //  File Descriptor for the integer file
    int pagesize=getpagesize(); //  To store the size of virtual page

    printf("\nPage size is %d bytes.\n100 integers require %lu bytes\n",pagesize,100*sizeof(int));
    void *data; //  This is the pointer which will store the returned pointer when mmap() is called

    fd=open("integer", O_RDWR); 

    data=mmap((caddr_t)0, pagesize, PROT_WRITE | PROT_READ, MAP_SHARED, fd,0);
    close(fd);

    sprintf((char*)data,"%d",100);

    return 0;
}

This returns a Segmentation Fault 11

Answer 1

You should check for errors from open and mmap . Also, compile with warnings and include the necessary include files. mmap especially needs a correct prototype on many systems because of its off_t arguments which could break things when you pass it a 0 as a last argument.

But that's not enough. Does the file exist and is non-zero size? mmap can happily map pages beyond the end of a file, but access to those pages will result in faults. If you actually want to the data to be written to the file you need to make sure it's long enough for it. Use ftruncate for that.

Also, I wonder which museum you found your documentation if it mentions caddr_t as the first argument to mmap , but that's beside the point, if that compiles it should also work.