CODEWITHSUNDEEP
iosobjective-c
lehn0058
lehn0058

Reputation: 20237

How can you create a large file in iOS quickly

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 separate disk hits:

NSFileHandle *handle = [NSFileHandle fileHandleForWritingAtPath:self.path];
[handle seekToEndOfFile];

while (shouldContinue)
{
     NSData *dummyData = [self buildDummyData];
     [handle writeData:dummyData];
}

But this takes around 7 seconds to complete. I would like it to complete in less than 1 second. Is there a way to create a file and allocate its size without needing to append bytes to it?

Upvotes: 2

Views: 2631

Answers (3)

bcattle
bcattle

Reputation: 12819

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.

Upvotes: 1

Putz1103
Putz1103

Reputation: 6211

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.

Upvotes: 2

Catfish_Man
Catfish_Man

Reputation: 41801

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' */
}

Upvotes: 7

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