Why are there extra spaces in the file after copying text from one file to another in C?

Question

char c, cp;

FILE *input_file, *output_file;

input_file = fopen("d:\\input.txt","r");
output_file = fopen("d:\\output.txt", "w");

if(input_file==NULL){
    printf("cannot open the input.txt file.");
    exit(0);
}

if(output_file == NULL){
    printf("cannot open the output.txt file.");
    exit(0);
}

cp = fgetc(input_file);
while(cp!=EOF){
    fputc(cp,output_file);
    cp=fgetc(input_file);
}

c = fgetc(output_file);
while(c!=EOF){
    printf("%c",c);
    c=fgetc(output_file);
}

fclose(input_file);
fclose(output_file);
getch();

This is the code that I used while copying a text file. In the input.txt file I have written "Hello how are you".

After executing the code, the text "Hello how are you" is copied to the output.txt file but there are more than a hundred spaces after the copied text. After the code below the program is not working:

cp = fgetc(input_file);
while(cp!=EOF){
    fputc(cp,output_file);
    cp=fgetc(input_file);
}

Code below the above code is not working. What is happening? Please explain in detail. I am a beginner in C.

Try making `c` and `cp` `int`s not `char`s. `EOF` is a magic value, not in the normal range of `char`. — Edd Inglis, Feb 27 '20 at 13:07
after changing c and cp to int, it doesn't solve the problem, till now there are lot's of spaces in output.txt file. Also the program doesn't run completely. — Nilmani Gautam, Feb 27 '20 at 13:20
You should not try to read from a file opened in `"w"` mode... — Serge Ballesta, Feb 27 '20 at 13:21
Why not put your `printf` in the first loop, then ditch the second loop altogether? Try and follow along with what your program thinks it's doing. Even better, do something like `printf( "char is 0x%02x, or '%c'\n", c, c );` — Edd Inglis, Feb 27 '20 at 13:23
@EddInglis `EOF` is very often -1. When `char` is _signed_ (quite often in windows worlds), -1 **is** in the range of `char`. Still `int c, cp` is correct, for other reasons. — chux - Reinstate Monica, Feb 27 '20 at 14:08

Ingo Leonhardt · Accepted Answer · 2020-02-27T14:24:44.247

4

You must define c and cp as int not as char. EOF is defined as an integer value which is distinguishable from any char which may e.g. be read by fgetc(), which returns a value in the unsigned char range or EOF, not necessarily a value in the char range. (credits to @chux).

So while( cp != EOF ) might not become true if cp is a char.

And for the second issue: if you want to read what you have written you must

open output.tx with mode "w+". "w" only permits writing, "w+" allows reading too but like "w" creates the file if it doesn't exist and truncates it if it does. See the man page for further options and details.
call rewind(output_file) or fseek(output_file, 0, SEEK_SET) between reading and writing.

edited Feb 27 '20 at 14:24

answered Feb 27 '20 at 13:27

Ingo Leonhardt

9,435
2
24
33

writing rewind(output_file) this solved my problem – Nilmani Gautam Feb 27 '20 at 13:32
1

@chux-ReinstateMonica yes of course thank you. I've fixed the typo and added your note to the answer – Ingo Leonhardt Feb 27 '20 at 14:19

score 0 · Answer 2 · answered Feb 27 '20 at 13:31

0

you must close your output file to commit changes and be able to read a second time:

cp = fgetc(input_file);
while(cp!=EOF){
    fputc(cp,output_file);
    cp=fgetc(input_file);
}

fclose(output_file);
// .....        
c = fgetc(output_file);

answered Feb 27 '20 at 13:31

Landstalker

1,368
8
9

and don't forget to reopen it – Ingo Leonhardt Feb 27 '20 at 14:04

Why are there extra spaces in the file after copying text from one file to another in C?

2 Answers2