Thread: C read and write specific amount of bytes from file and to file

HEY C Board! How is everyone? Binaryperson here wishing everyone a Happy new year!
I want my program to print "partSize" amount of bytes of the original file to the new file.
If the original file contains:

0123456789
0123456789
0123456789
0123456789
0123456789
0123456789

and part size is 2 then

0123456789
0123456789
0123456789

is stored to the new file.
So the program is not writing until end of line, that would be the whole file. It is writing partSize amount of bytes.

Code:

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

int main(int argc, char *argv[])
{
    int createDescriptor;
    int openDescriptorOriginal;
    int closeCreateDescriptor;
int openDescriptorNew;

    //char fileNameOriginal[] = "picture.jpg";
    char fileNameOriginal[] = "original.txt";
    char fileNameNew[] = "new.txt";
    
    int parts;
    int partSize;

    parts=2;

    int bytesRemaining;
    int partNumber;
    char BUFFER[512];
    int readDescriptor;

    int openDescriptorNew;
    
    if ((openDescriptorOriginal = open(fileNameOriginal, O_RDONLY )) == -1)
    {
        printf("Error opening %s", fileNameOriginal);
        exit(EXIT_FAILURE);
    }

    struct stat buf;
    int r = fstat(openDescriptorOriginal, &buf);
    if (r)
    {
        fprintf(stderr, "error: fstat: %s\n", (char *) strerror(errno));
        exit(1);
    }

    int originalFileSize = buf.st_size;
    printf("The file is %.9f Kilobytes large.\n", (double) originalFileSize/1024);

    partSize = ((originalFileSize + parts) + 1)/parts;

    if ((openDescriptorNew = open(fileNameNew, O_CREAT | O_WRONLY )) == -1)
    {
        printf("Error opening %s", openDescriptorNew);
        exit(EXIT_FAILURE);
    }
    
    //I am stuck at this part
    //I want to write the first "partSize" bytes of "fileNameOriginal" to "fileNameNew"
    /*
    while ((readDescriptor = read(openDescriptorOriginal, BUFFER, partSize)) < partSize))
    {
        write(1, BUFFER, readDescriptor);
    }
*/
    if ((closeCreateDescriptor = close(openDescriptorOriginal)) == -1)
    {
        printf("Error closing file.\n");
    }

    return 0;
}

Wow, impressive detective work. What is your point? I've posted my question in three different places and no one has been able to give me any help. Why would I change the question each time I post it?

As for double posting on this forum, that was an accident. I was having trouble posting and I somehow double posted and don't know how to delete the first post.

How do you want me to personalize it?

Is it against this forums rules to post a question that you have posted on another forum/forums?

Hmm... I find your various "XYZDescriptor" variables to be rather confusing. If something is a file descriptor, then a descriptor suffix makes sense. Otherwise, it doesn't. It seems to me that you're working with two files, with corresponding descriptors openDescriptorOriginal and openDescriptorNew. As such, I would expect something like:

Code:

while (total_bytes_read < partSize && (bytes_read = read(openDescriptorOriginal, BUFFER, partSize)) > 0)
{
    total_bytes_read += bytes_read;
    write(openDescriptorNew, BUFFER, bytes_read);
}

So the idea is that you keep track of the total bytes read, then keep reading until that value reaches the partSize. After each read, you write exactly the number of bytes just read.

Thanks to Matticus, I found that this was pretty much answered elsewhere, so I lost interest....

Well, so what do you make of my reply in post #11? It seems like a simple thing to me, but of course if you are looking for answers all over the place, the sum of the replies that you're getting will not be very coherent.

Perhaps the struggle is because you have so many different threads all over the place, that you're lacking a consistent narrative that can explain what is going on.

Pick ONE forum, post your latest effort, and put redirect links on the ends of ALL your other threads to let everyone else who might help know what it going on.

If your chosen forum bombs out, then (and only then) move onto somewhere else. Blasting the net with your question just leaves you confused with disparate answers, and annoys the crap out of people who help when they discover the cross-posts.