Creating a large Dummy File

问题:

I'm new to this community and also new to iOS development, and I hope that someone can help me with a little problem...

Currently I'm writing a little app for myself, that should create a very large dummy file, with dummy data or words or whatever it makes it big.

The reason is, I would like to click a button and the app should generate a 4 gb file and save it to disk, that no other app could use it, because it isnt free... If I need the 4 gb of free space, I just have to open my little app, and after clicking another button, the file will be deleted, and i got 4 gb of free diskspace...

I know how to write files to disk but I have no idea how to generate a large amount of data in short time, to make it 4 or more gb big???

Does someone have an idea how to generate such a big file with a for loop or something?

回答1:

If you just want to occupy 4GB of disk space, then create a file, seek to 4GB and write a byte. This will create a "sparse file" containing next-to-no data in next-to-no time!

This is C:

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>

...

int fd = open("/path/to/file", O_WRONLY, 0644);
if (fd >= 0)
{
    lseek(fd, 4 * 1024 * 1024 * 1024, SEEK_SET);
    write(fd, &fd, 1);
    close(fd);
}

EDIT Dropped creat() in favour of open() so the file mode can be specified.

Also error checking should be better than shown...

回答2:

You can create an NSMutableData object and use -setLength: to expand it to 4GB. (-setLength will fill with zeros), before writing it out to a file.

I'm not sure how well iOS will cope with an NSMutableData object that large, however.

Edit: Example:

NSString *filename; // The location to save your file
NSMutableData *data = [[NSMutableData alloc] init];
[data setLength:4*1024*1024*1024];
[data writeToFile:fileName atomically:NO];

Bear in mind also that iOS will purge your app's data in certain directories if the device's available storage is low.

回答3:

If you do rough calculation by 1000, not by 1024

 4 gb 
 4000 mb
 4000000 kb 
 4000000000 b 
32000000000 bits

And exactly it will be 34359738368 bits

And if you go on to write that much characters (a b c etc) to a text file, by loop, you will create a file of 4gb.

回答4:

You can just write it in C, incrementally:

// a starting point:
const char path[] = "somewhere";
const size_t NBytesInBuffer = 1024;
/* fill buffer, if you like */
const char buffer[NBytesInBuffer] = {0};
FILE* const file = fopen(path, "w+");
assert(file);
const size_t ElementSize = sizeof(buffer[0]);

uint64_t nBytesWritten = 0;
while (UINT32_MAX > nBytesWritten) {
    nBytesWritten += ElementSize * fwrite(buffer, ElementSize, NBytesInBuffer, file);
}

fclose(file);

Memory consumption will not be an issue here. Of course, it will take some time to write the entire file, as it is I/O bound. You will probably want some real data in there one day, too :)

回答5:

Windows:

Open the command shell - cmd Execute the following command in order to create a file of 1mb

fsutil file createnew ./test.tmp 1048576

Linux:

Use convert and copy command - dd

dd if=/dev/zero of=test.tmp bs=1M count=1

OS X & Solaris:

mkfile 1m test.tmp