reading a file and reverse it to another file? - c

Question

i have this code but every time i try to run it it deletes the source file without giving any output, so how can i fix my problem?
note the question is asking me this:
Write a program that takes two file names from the command line, and copies the reverse of the contents of the first file into the second file, assuming that it is able to open the first file for reading and the second one for writing. If it can’t open the first file for reading, it must neither create nor modify (as the case may be) the second file. This program must use the low-level functions
#include<stdlib.h>
#include<stdio.h>
#include<fcntl.h>
#include<string.h>
#include<sys/stat.h>
#include<sys/types.h>
#include<unistd.h>
int main(int argc, char *argv[])
{
int source, dest, n;
char buf;
int filesize;
int i;
if (argc != 3)
{
fprintf(stderr, "usage %s <source> <dest>\n", argv[0]);
exit(-1);
}
at this par i am trying to use the following format: open("outf", O_WRONLY | O_CREAT | O_TRUNC, 0666)
if ((source = open(argv[1],O_WRONLY | O_CREAT | O_TRUNC, 0666)) < 0)
{ //read permission for user on source
fprintf(stderr, "can't open source\n");
exit(-1);
}
if ((dest = creat(argv[2], 0666)) < 0)
{ //rwx permission for user on dest
fprintf(stderr, "can't create dest");
exit(-1);
}
filesize = lseek(source, (off_t) 0, SEEK_END); //filesize is lastby +offset
printf("Source file size is %d\n", filesize);
for (i = filesize - 1; i >= 0; i--)
{ //read byte by byte from end
lseek(source, (off_t) i, SEEK_SET);
n = read(source, &buf, 1);
if (n != 1)
{
fprintf(stderr, "can't read 1 byte\n");
exit(-1);
}
n = write(dest, &buf, 1);
if (n != 1)
{
fprintf(stderr, "can't write 1 byte\n");
exit(-1);
}
}
write(STDOUT_FILENO, "DONE\n", 5);
close(source);
close(dest);
return 0;
}
thanks

Answer 1

I am sorry to be rude but did you even read the options you are passing to the first open call? O_CREAT | O_TRUNC?
What do you think those options do? Those options are causing your source file to be deleted.

Answer 2

You need to open your input file with the mode O_RDONLY, i.e. ReaD ONLY not O_WRONLY, i.e. WRite ONLY.

Answer 3

Check this:
int checkstatus(ifstream &in)
{
ios::iostate i;
i = in.rdstate();
if(i & ios::eofbit)
return 0;//cout << "EOF encountered\n";
else if(i & ios::failbit)
return 0;//cout<<"Non-Fatal I/O error\n";
else if(i & ios::badbit)
return 0;//cout<<"Fatal I/O error\n";
return 1;
}
int main()
{
ifstream in;
ofstream o;
in.open("test.txt");
o.open("test1.txt",ios::out);
char c;
in.seekg(0,ios::end);
while(checkstatus(in) != 0)
{
in.seekg(-1,ios::cur);
in.get(c);
in.seekg(-1,ios::cur);
if(checkstatus(in) == 0)
break;
cout<<c;
o.put(c);
}
in.close();
o.close();
return 0;
}

Answer 4

i have made some changes here and it worked very decent but not with very large file!!
#include<stdlib.h>
#include<stdio.h>
#include<fcntl.h>
#include<sys/stat.h>
#include<unistd.h>
int main(int argc, char *argv[])
{
int source, dest, n;
char buf;
int filesize;
int i;
if (argc != 3)
{
fprintf(stderr, "usage %s <source> <dest>\n", argv[0]);
exit(-1);
}
if ((source = open(argv[1], 0666)) < 0)
{ //read permission for user on source
fprintf(stderr, "can't open source\n");
exit(-1);
}
if ((dest = open(argv[2],O_WRONLY | O_CREAT | O_TRUNC, 0666)) < 0)
{ //rwx permission for user on dest
fprintf(stderr, "can't create dest");
exit(-1);
}
filesize = lseek(source, (off_t) 0, SEEK_END); //filesize is lastby +offset
printf("Source file size is %d\n", filesize);
for (i = filesize - 1; i >= 0; i--)
{ //read byte by byte from end
lseek(source, (off_t) i, SEEK_SET);
n = read(source, &buf, 1);
if (n != 1)
{
fprintf(stderr, "can't read 1 byte\n");
exit(-1);
}
n = write(dest, &buf, 1);
if (n != 1)
{
fprintf(stderr, "can't write 1 byte\n");
exit(-1);
}
}
write(STDOUT_FILENO, "DONE\n", 5);
close(source);
close(dest);
return 0;
}
do i need to do something with lseek()?