使用memcpy（）将一个文件的内容复制到另一文件

Question

I need to copy file1 context to file2. Here's my files file1

My name is John

file2

I like water

so my resilt should be file2

I like water
My name is John

I have to use mmap and memcpy for this. So here what I do First I open both files

int da_open(const char *name){
   int dskr;
   dskr = open( name, O_RDWR );
   if( dskr == -1 ){
      perror( name );
      exit(1);
   }
   printf( "dskr1 = %d\n", dskr );
   return dskr;
}

Then I mmap my file's

void *da_mmap1( int d, int size ){
   void *a = NULL;
   a = mmap( NULL, size, PROT_WRITE | PROT_READ, MAP_SHARED, d, 0 );
   if( a == MAP_FAILED ){
      perror( "mmap failed" );
      abort();
   }
   return a;
}

and here's my memcpy

void *da_memcpy(void *str1, const void *str2, size_t n){
    return memcpy(str1, str2, n);
}

and my main

int main(int argc, char *argv[] ){
    struct stat fileStat;
    int st;
    int d1;
    int d2;
    void *r = NULL;
    void *w = NULL;
    if(argc == 3){
        d1 = da_open(argv[1]);
        d2 = da_open(argv[2]);
        r = da_mmap1(d1, SIZE);
        w = da_mmap1(d2, SIZE);
        st = stat(r, &fileStat);
        printf("Tik skaitoma: %d\n", fileStat.st_size);
        da_memcpy(w,r,fileStat.st_size);
        da_munamp(r, SIZE);
        da_munamp(w, SIZE);
        da_close(d1);
        da_close(d2);
    }
    return 0;
}

But after this my file2 looks like that

 My name i

Why? what's wrong?

Answer 1

You need to get the sizes of both files, and write the first one at the end of the second file.

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Also, the first argument to stat is the file name and not a descriptor. If you have a descriptor you can use fstat instead.

When you pass r you pass it a non-terminated contents of the first file, leading to undefined behavior . If you checked what stat returned you would probably have noticed it returned -1 as it failed.