C Program to Print the Contents of File in Reverse Order

This C Program reverses the contents of a file and print it.

Here is source code of the C Program to reverse the contents of a file and print it. The C program is successfully compiled and run on a Linux system. The program output is also shown below.

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/* 
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 * C Program to Reverse the Contents of a File and Print it

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 */
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#include <stdio.h>
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#include <errno.h>
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long count_characters(FILE *);
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void main(int argc, char * argv[])
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{
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    int i;
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    long cnt;
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    char ch, ch1;
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    FILE *fp1, *fp2;
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    if (fp1 = fopen(argv[1], "r"))    
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    {
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        printf("The FILE has been opened...\n");

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        fp2 = fopen(argv[2], "w");
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        cnt = count_characters(fp1); // to count the total number of characters inside the source file

        fseek(fp1, -1L, 2);     // makes the pointer fp1 to point at the last character of the file

        printf("Number of characters to be copied %d\n", ftell(fp1));

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        while (cnt)
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        {
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            ch = fgetc(fp1);
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            fputc(ch, fp2);
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            fseek(fp1, -2L, 1);     // shifts the pointer to the previous character

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            cnt--;
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        }
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        printf("\n**File copied successfully in reverse order**\n");

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    }
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    else
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    {
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        perror("Error occured\n");
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    }
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    fclose(fp1);
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    fclose(fp2);
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}
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// count the total number of characters in the file that *f points to

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long count_characters(FILE *f) 
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{
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    fseek(f, -1L, 2);
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    long last_pos = ftell(f); // returns the position of the last element of the file

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    last_pos++;
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    return last_pos;
```
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}
```

$ gcc file12.c
$ cat test2
The function STRERROR returns a pointer to an ERROR MSG STRING whose contents are implementation defined.
THE STRING is not MODIFIABLE and maybe overwritten by a SUBSEQUENT Call to the STRERROR function.
$ a.out test2 test_new
The FILE has been opened..
Number of characters to be copied 203
 
**File copied successfully in reverse order**
$ cat test_new
 
.noitcnuf RORRERTS eht ot llaC TNEUQESBUS a yb nettirwrevo ebyam dna ELBAIFIDOM ton si GNIRTS EHT
.denifed noitatnemelpmi era stnetnoc esohw GNIRTS GSM RORRE na ot retniop a snruter RORRERTS noitcnuf ehT
$ ./a.out test_new test_new_2
The FILE has been opened..
Number of characters to be copied 203
 
**File copied successfully in reverse order**
$ cat test_new_2
The function STRERROR returns a pointer to an ERROR MSG STRING whose contents are implementation defined.
THE STRING is not MODIFIABLE and maybe overwritten by a SUBSEQUENT Call to the STRERROR function.
$ cmp test test_new_2