Select a file to disk without zeroing

Question

I need to select a huge file without nullifying its contents. I do the following steps fopen => ftruncate => fclose => mmap => (...work...) => munmap with huge file sizes (hundreds of gigabytes). The application freezes at the end for several minutes while the system tries to reset the file bytes - IMHO due to the use of ftruncate .

 ftruncate(ofd, 0); #ifdef HAVE_FALLOCATE int ret = fallocate(ofd, 0, 0, cache_size); if (ret == -1) { printf("Failed to expand file to size %llu (errno %d - %s).\n", cache_size, errno, strerror(errno)); exit(-1); } #elif defined(HAVE_POSIX_FALLOCATE) int ret = posix_fallocate(ofd, 0, cache_size); if (ret == -1) { printf("Failed to expand file to size %llu (errno %d - %s).\n", cache_size, errno, strerror(errno)); exit(-1); } #elif defined(__APPLE__) fstore_t store = {F_ALLOCATECONTIG, F_PEOFPOSMODE, 0, cache_size, 0}; int ret = fcntl(ofd, F_PREALLOCATE, &store); if (ret == -1) { store.fst_flags = F_ALLOCATEALL; ret = fcntl(ofd, F_PREALLOCATE, &store); } if (ret == -1) { // read fcntl docs - must test against -1 printf("Failed to expand file to size %llu (errno %d - %s).\n", cache_size, errno, strerror(errno)); exit(-1); } struct stat sb; ret = fstat(ofd, &sb); if (ret != 0) { printf("Failed to write to file to establish the size.\n"); exit(-1); } //ftruncate(ofd, cache_size); <-- [1] #endif 

This does not seem to work with the commented line [1] . But uncommenting this line leads to zeroing the files that I am trying to avoid. I really don't care about the dirty contents of the file before writing. I just want to avoid the hang of application termination.

DECISION:

According to @torfo , replaced all of my Apple related code with a few lines:

 unsigned long long result_size = cache_size; int ret = fcntl(ofd, F_SETSIZE, &result_size); if(ret == -1) { printf("Failed set size %llu (errno %d - %s).\n", cache_size, errno, strerror(errno)); exit(-1); } 

But it works only for the superuser!

Answer 1

This is apparently MacOS X.

You can try replacing the ftruncate call with

 fcntl(ofd, F_SETSIZE, &size); 

(A note requires root privileges and can create a security hole, since it can provide access to previously saved file contents, so it should be handled with extreme care. “Not sufficiently dirty file contents”, be the passwords of the user account that he deleted a week ago. ..)

MacOS X does not actually support sparse files - it creates and supports them, but its file system driver really wants to fill the holes as soon as possible.