How can you create a large file in iOS quickly

Question

I need to be able to create a large \"dummy\" file (around 1 GB) quickly. I am currently looping over a byte array and appending it to the file, but this results in may sepa

Answer 1

If you want to stay in Objective-C, use NSFileHandle like you do in your example:

NSFileHandle *file = [NSFileHandle fileHandleForWritingAtPath:dummyFilePath];
if (file == nil) {
    [[NSFileManager defaultManager] createFileAtPath:dummyFilePath contents:nil attributes:nil];
    file = [NSFileHandle fileHandleForWritingAtPath:dummyFilePath];
}    
[file truncateFileAtOffset:sizeInMB * 1024 * 1024];
[file closeFile];

Note that this is sloooow. Writing a 1GB file takes over 30 seconds for me.

Another way to do it would be to do a write:

[file seekToFileOffset:sizeInMB * 1024 * 1024];
const unsigned char bytes[] = {1};
[file writeData:[NSData dataWithBytes:bytes length:sizeof(bytes)]]; 

This was even slower for me, taking 55 seconds for the 1GB.

Answer 2

You can check this answer. It says that you can lseek() past the end of file (EOF) then write something. The gap between the initial end of file and the bytes you wrote will return 0's if queried.

Answer 3

You can use truncate to set the length of a file directly:

int success = truncate("/path/to/file", 1024 * 1024 * 1024);
if (success != 0) {
    int error = errno;
    /* handle errors here. See 'man truncate' */
}