As you probably know, given struct stat info as returned by a call to stat(), info.st_size is the total size of the file in bytes, and info.st_blocks*512 is the number of bytes stored on disk.
In Linux, filesystems store data in aligned chunks of info.st_blksize bytes. (This also means that info.st_blocks*512 can be larger than info.st_size (by at most info.st_blksize-1 bytes). It can also be smaller, if the file is sparse.)
Both unstored data (holes in a sparse file), and explicitly zeroed stored data, read as zeroes.
If you want to know how many zero-filled blocks there are in the file, you will need to read the entire file. Use a buffer with size an integer multiple of info.st_blksize bytes. For each aligned block of st_blksize bytes, check if they are all zeroes. Let the total number of blocks (including the last possibly partial block) be total_blocks, and the number of blocks with all zero contents be zero_blocks.
struct stat info;
/* Number of filesystem blocks for the file */
total_blocks = info.st_size / info.st_blksize
+ (info.st_size % info.st_blksize) ? 1 : 0;
/* Number of bytes stored for the file */
stored_bytes = 512 * info.st_blocks;
/* Number of filesystem blocks used for file data */
stored_blocks = stored_bytes / info.st_blksize
+ (stored_bytes % info.st_blksize) ? 1 : 0;
/* Number of sparse blocks */
sparse_blocks = total_blocks - stored_blocks;
/* TODO: count zero_blocks,
* by reading file in info.st_blksize chunks,
* and saving the number of all-zero chunks
* in zero_blocks. */
/* Number of stored zero blocks */
zeroed_blocks = zero_blocks - sparse_blocks;
Converted into bytes, you have
info.st_size is the file size in bytesstored_blocks*info.st_blksize is the number of bytes used on disksparse_blocks*info.st_blksize is the number of bytes in sparse holes on diskzeroed_blocks*info.st_blksize is the number of unnecessarily stored zero bytes on disk; that could have been stored as a sparse hole instead
Note that you can use cp --sparse=always --preserve=all SOURCEFILE TARGETFILE to create an identical copy of a file, but "optimizing" sparseness, so that sufficiently long runs of zero bytes are stored as holes instead; this may help you in testing your program. See man 1 cp for details. You can also create long sequences of zeros by using dd if=/dev/zero of=TARGETFILE bs=BLOCKSIZE count=BLOCKS; see man 1 dd and man 4 null for details.
Edited to add:
Here is an example function, examine(), that opens the specified file, obtains the statistics, and if necessary (i.e. the amount of unnecessarily stored zeroes is requested), reads the entire file.
I have only lightly tested it, but it should implement the logic above.
It is very rough; I paid most attention to correct error checking and correctness of dynamic memory allocation/freeing. (It should check for and return all error conditions, even some should-never-occur ones, and never leak memory. That is, unless I have a bug or thinko in the code -- corrections are most welcome.)
It would be better to split it into smaller, easier to manage functions.
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <stdint.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
/* Return nonzero if the buffer is all zeros.
*/
static inline int is_zero(const void *const ptr, const size_t len)
{
const char *p = (const char *)ptr;
const char *const q = (const char *const)ptr + len;
while (p < q)
if (*(p++))
return 0;
return 1;
}
/* Return 0 if success, errno error code otherwise.
* (*sizeptr): File size in bytes
* (*blocksizeptr): File block size in bytes
* (*storedptr): Bytes stored on disk
* (*sparseptr): Bytes in sparse holes
* (*zeroedptr): Unnecessarily stored zero bytes
* If zeroedptr is NULL, the file is only opened and
* statistics obtained via fstat(). Otherwise, the entire
* file will be read.
* Special errors:
* EINVAL: NULL or empty file name
* EISDIR: Name refers to a directory
* EISNAM: Name refers to a pipe or device
* EBUSY: File was modified during read
*/
int examine(const char *const filename,
uint64_t *const sizeptr,
uint64_t *const blocksizeptr,
uint64_t *const storedptr,
uint64_t *const sparseptr,
uint64_t *const zeroedptr)
{
struct stat info;
int fd, result;
size_t size, have;
uint64_t total, nonzero, stored;
int cause = 0;
char *data = NULL;
/* Check for NULL or empty filename. */
if (!filename || !*filename)
return errno = EINVAL;
/* Open the specified file. */
do {
fd = open(filename, O_RDONLY | O_NOCTTY);
} while (fd == -1 && errno == EINTR);
if (fd == -1)
return errno;
do {
/* Obtain file statistics. */
if (fstat(fd, &info) == -1) {
cause = errno;
break;
}
/* Count total, rounding up to next multiple of block size. */
total = (uint64_t)info.st_size;
if (total % (uint64_t)info.st_blksize)
total += (uint64_t)info.st_blksize - ((uint64_t)total % (uint64_t)info.st_blksize);
/* Count total stored bytes. */
stored = (uint64_t)512 * (uint64_t)info.st_blocks;
/* Fill in immediately known fields. */
if (sizeptr)
*sizeptr = (uint64_t)info.st_size;
if (blocksizeptr)
*blocksizeptr = (uint64_t)info.st_blksize;
if (storedptr)
*storedptr = stored;
if (sparseptr) {
if (total > stored)
*sparseptr = total - stored;
else
*sparseptr = 0;
}
if (zeroedptr)
*zeroedptr = 0;
/* Verify we have a regular file. */
if (S_ISDIR(info.st_mode)) {
cause = EISDIR;
break;
} else
if (!S_ISREG(info.st_mode)) {
cause = EISNAM;
break;
}
/* Verify we have a valid block size. */
if (info.st_blksize < (blksize_t)1) {
cause = ENOTSUP;
break;
}
/* If zeroedptr is NULL, we do not need to read the file. */
if (!zeroedptr) {
/* Close descriptor and return success. */
do {
result = close(fd);
} while (result == -1 && errno == EINTR);
if (result == -1)
return errno;
return 0;
}
/* Use large enough chunks for I/O. */
if (info.st_blksize < (blksize_t)131072) {
const size_t chunks = (size_t)131072 / (size_t)info.st_blksize;
size = chunks * (size_t)info.st_blksize;
} else
size = (size_t)info.st_blksize;
/* Allocate buffer. */
data = malloc(size);
if (!data) {
cause = ENOMEM;
break;
}
/* Clear counters. */
total = 0;
nonzero = 0;
have = 0;
/* Read loop. */
while (1) {
size_t i;
ssize_t bytes;
int ended = 0;
while (have < (size_t)info.st_blksize) {
bytes = read(fd, data + have, size - have);
if (bytes > (ssize_t)0) {
have += bytes;
total += (uint64_t)bytes;
} else
if (bytes == (ssize_t)0) {
/* Clear the end of the buffer; just to be sure */
memset(data + have, 0, size - have);
ended = 1;
break;
} else
if (bytes != (ssize_t)-1) {
cause = EIO;
break;
} else
if (errno != EINTR) {
cause = errno;
break;
}
}
if (cause)
break;
/* Count number of zero/nonzero chunks in buffer, but add up as bytes. */
i = have / (size_t)info.st_blksize;
while (i-->0)
if (!is_zero(data + i * (size_t)info.st_blksize, (size_t)info.st_blksize))
nonzero += (uint64_t)info.st_blksize;
/* Followed by a partial chunk? */
{ const size_t overlap = have % (size_t)info.st_blksize;
if (overlap) {
if (have > overlap)
memcpy(data, data + have - overlap, overlap);
have = overlap;
} else
have = 0;
}
/* Next round of the loop, unless end of input. */
if (!ended)
continue;
/* Entire file has been processed. */
/* Partial chunk in buffer? */
if (have) {
if (!is_zero(data, have))
nonzero += (uint64_t)info.st_blksize;
}
/* If file size changed, update statistics. */
if (total != (uint64_t)info.st_size) {
if (fstat(fd, &info) == -1) {
cause = errno;
break;
}
/* File changed from under us? */
if (total != (uint64_t)info.st_size) {
cause = EBUSY;
break;
}
}
/* Align total size to (next) multiple of block size. */
if (total % (uint64_t)info.st_blksize)
total += (uint64_t)info.st_blksize - (total % (uint64_t)info.st_blksize);
/* Bytes stored on disk. */
stored = (uint64_t)512 * (uint64_t)info.st_blocks;
/* Sanity check. (File changed while we read it?) */
if (stored > total || nonzero > stored) {
cause = EBUSY;
break;
}
/* Update fields. */
if (sizeptr)
*sizeptr = (uint64_t)info.st_size;
if (storedptr)
*storedptr = (uint64_t)512 * (uint64_t)info.st_blocks;
if (sparseptr)
*sparseptr = total - stored;
if (zeroedptr)
*zeroedptr = (total - nonzero) - (total - stored);
/* Discard buffer. */
free(data);
/* Close file and return. */
do {
result = close(fd);
} while (result == -1 && errno == EINTR);
if (result == -1)
return errno;
return 0;
}
} while (0);
/* Free buffer, if allocated. free(NULL) is safe. */
free(data);
/* Close file, and return with cause. */
do {
result = close(fd);
} while (result == -1 && errno == EINTR);
return errno = cause;
}
For portability, all the return parameters are 64-bit unsigned integers,
and specify the corresponding sizes in bytes. Note that (*storedptr)+(*sparseptr) defines the total number of bytes rounded up to next multiple of (*blocksizeptr). (*zeroesptr) only includes explicitly stored zeroes, not sparse holes. Again, think of (*zeroesptr) as the number of unnecessarily stored zeroes.
I used rm -f test ; dd if=/dev/zero of=test bs=10000 seek=3 count=1 to generate a test file with a 30,000 byte hole followed by 10,000 zeroes. examine() returns size=40000, blocksize=4096, stored=12288, sparse=28672, zeroed=12288, which seems correct to me.
Questions?
