Generating file of a certain size

**ukee** · 10-17-2016

Not sure if this is a good way of generating a 15MB file from opening a file and writing to a new file. The "file.in" is just a dummy file of size 50mb created from

Code:

dd if=/dev/zero of=yourfilename.test bs=52428800 count=1

.

Is there a more optimized way of generating a file of size 15MB given the code below? I use the code below and it does generate a file of size 15.7mb from the "file.in".

Here's my code:

Code:

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

int main() 
{ 
    char buffer[1024]; 
    int in, out, outread; 
    int temp = 0;
    in = open("file.in", O_RDONLY);
    out = open("file.out", O_WRONLY | O_CREAT, S_IRUSR | S_IWUSR);
    while((outread = read(in, buffer, sizeof(buffer))) > 0)
    {
        if(temp == 15728640) return 0; // returns when size is 15mb
        else write(out, buffer, outread);
        temp+=outread;
    }

    close[in]; 
    close[out]; 

    return 0; 
}

**laserlight** · 10-17-2016

This is obviously wrong:

Code:

close[in];
close[out];

It should have been:

Code:

close(in);
close(out);

Instead of using a magic number, perhaps it would be better to:

Code:

#define SIZE_REQUIRED (15 * 1024 * 1024) /* 15 MB */

temp is a common abbreviation for "temporary", but here that's not the most interesting thing about it, so I would rename it to total_bytes_read, or something like that. Your loop could then be changed to:

Code:

while(total_bytes_read < SIZE_REQUIRED && (outread = read(in, buffer, sizeof(buffer))) > 0)
{
    write(out, buffer, outread);
    total_bytes_read += outread;
}

As for efficiency: it seems pretty reasonable. Maybe having the buffer size be BUFSIZ would help (if 15728640 is perfectly divisible by BUFSIZ), or maybe not.

It would also be a good idea to check the return value of open.

**ukee** · 10-17-2016

Sorry for this

Code:

close[in];
close[out];

wasn't paying attention when I typed that out.

I've also changed my codes accordingly based on your input, thanks again!

**Salem** · 10-18-2016

> wasn't paying attention when I typed that out.
So why didn't you post some code you actually tried, instead of just winging it?
Otherwise, we just waste time pointing out the bleedingly obvious stupid mistakes.

The constant BUFSIZ in stdio.h is as good a choice as any for the size of block file access.

Disk seek times are millions of times slower than DRAM access times, so worrying about whether 1K or 2K buffers will make a difference isn't worth the effort.

Also, filling a large files with nothing but zeros may be detected by the OS (or filesystem), and you end up with a sparse file. This might be superficially a lot quicker, since it involves a lot less writing to disk.

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