NSMutableData dataWithLength example in Objective C (iOS).

NSMutableData dataWithLength

Creates and returns an NSMutableData object containing a given number of zeroed bytes.

+ (id)dataWithLength:(NSUInteger)length

Parameters
length
The number of bytes the new data object initially contains.

Return Value of [NSMutableData dataWithLength]
A new NSMutableData object of length bytes, filled with zeros.

NSMutableData dataWithLength example.

 long long length = 1024ull * 1024ull * 1024ull * 2ull; // 2 GB

 db = [NSMutableData dataWithLength:length];

 char *array = [db mutableBytes];

 for(long long i = 0; i < length - 1; i++) {
      array[i] = i % 256;
 }

Example of [NSMutableData dataWithLength].

NSMutableData* mutableData = [NSMutableData dataWithLength: someLength];
void* bitmapData = [mutableData mutableBytes];
CGContextRef context = CGBitmapContextCreate(bitmapData,...);
// ...use context
CGContextRelease(context);

NSMutableData dataWithLength example.

- (NSData *)gzipInflate:(NSData*)data
{
    if ([data length] == 0) return data;

    unsigned full_length = [data length];
    unsigned half_length = [data length] / 2;

    NSMutableData *decompressed = [NSMutableData dataWithLength: full_length + half_length];
    BOOL done = NO;
    int status;

    z_stream strm;
    strm.next_in = (Bytef *)[data bytes];
    strm.avail_in = [data length];
    strm.total_out = 0;
    strm.zalloc = Z_NULL;
    strm.zfree = Z_NULL;

    if (inflateInit2(&strm, (15+32)) != Z_OK) return nil;
    while (!done)
    {
        // Make sure we have enough room and reset the lengths.
        if (strm.total_out >= [decompressed length])
            [decompressed increaseLengthBy: half_length];
        strm.next_out = [decompressed mutableBytes] + strm.total_out;
        strm.avail_out = [decompressed length] - strm.total_out;

        // Inflate another chunk.
        status = inflate (&strm, Z_SYNC_FLUSH);
        if (status == Z_STREAM_END) done = YES;
        else if (status != Z_OK) break;
    }
    if (inflateEnd (&strm) != Z_OK) return nil;

    // Set real length.
    if (done)
    {
        [decompressed setLength: strm.total_out];
        return [NSData dataWithData: decompressed];
    }
    else return nil;
}

End of NSMutableData dataWithLength example article.