Question

Systems Programming - File Operations in Linux using C

Preventing copying the same file

1. What does the standard cp do if you try to copy a file onto itself?

   $ cp file1 file1
   cp: 'file1' and 'file1' are the same file
   

2. Modify cp1.c to handle the situation and include comments.

/**
* @file cp1.c
* @brief Uses read and write with tunable buffer size
* usage: cp1 src dest

*/
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>

#define BUFFERSIZE 4096
#define COPYMODE 0644

void oops(char *, char *);

int main(int argc, char *argv[])
{
int infd, outfd, nChars;
char buf[BUFFERSIZE];

// check args
if (argc != 3)
{
fprintf(stderr, "usage: %s source destination\n", *argv);
exit(1);
}

// open files
if ((infd = open(argv[1], O_RDONLY)) == -1)
{
oops("Cannot open ", argv[1]);
}

if ((outfd = creat(argv[2], COPYMODE)) == -1)
{
oops("Cannot open ", argv[2]);
}

// copy files
while ((nChars = read(infd, buf, BUFFERSIZE)) > 0)
{
if (write(outfd, buf, nChars) != nChars)
{
oops("Write error to ", argv[2]);
}
}
if (nChars == -1)
{
oops("Read error from ", argv[1]);
}

if (close(infd) == -1 || close(outfd) == -1)
{
oops("Error closing files", "");
}

return 0;
}

/**
* @brief Displays error to the standard error and
* prints error by the system call using perror().
* Program terminates when this function is called.
*
* @param s1 type of error
* @param s2 file that cause the error
*/
void oops(char *s1, char *s2)
{
fprintf(stderr, "Error: %s ", s1);
perror(s2);
exit(1);
}

Answer 1

Following is the modified cp1.c file to handle the situation.

Here "strcmp()" function is used to compare the arguments. If both the arguments are same then the error can be thrown.

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
#include<string.h>
#define BUFFERSIZE 4096
#define COPYMODE 0644

void oops(char *, char *);

int main(int argc, char *argv[])
{
int infd, outfd, nChars;
char buf[BUFFERSIZE];

// check args
if (argc != 3)
{
fprintf(stderr, "usage: %s source destination\n", *argv);
exit(1);
}

// compare the arguments using strcmp and verify if they are same
// if both are same then the error will be displayed.
if (strcmp(argv[1],argv[2])==0)
{
fprintf(stderr,"%s and %s are same file\n",argv[1],argv[2]);
exit(1);
}

// open files
if ((infd = open(argv[1], O_RDONLY)) == -1)
{
oops("Cannot open ", argv[1]);
}

if ((outfd = creat(argv[2], COPYMODE)) == -1)
{
oops("Cannot open ", argv[2]);
}

// copy files
while ((nChars = read(infd, buf, BUFFERSIZE)) > 0)
{
if (write(outfd, buf, nChars) != nChars)
{
oops("Write error to ", argv[2]);
}
}
if (nChars == -1)
{
oops("Read error from ", argv[1]);
}

if (close(infd) == -1 || close(outfd) == -1)
{
oops("Error closing files", "");
}

return 0;
}

/**
* @brief Displays error to the standard error and
* prints error by the system call using perror().
* Program terminates when this function is called.
*
* @param s1 type of error
* @param s2 file that cause the error
*/
void oops(char *s1, char *s2)
{
fprintf(stderr, "Error: %s ", s1);
perror(s2);
exit(1);

}

Output

./a.out file1 file1

file1 and file1 are same file