Program doesn't wait for user input with scanf("%c",&yn);

Question

This is the basic code to a program I am writing to practise using files in C. I am trying to detect whether the output file already exists and if it does exist I want to ask the user if they would like to overwrite it or not. This is the reason that I have first opened the outfilename file in with fopen(outfilename,"r"); as opposed to fopen(outfilename,"w");.

It detects the case of the file not existing, however, if it does exist it executes the printf("Output file already exists, overwrite (y/n):"); statement but completely ignores the scanf("%c",&yn); statement!

The printf at the end of the program reads "yn=0" if the file doesn't exist and just "yn=" if it does exist. Can anybody help me?

#include <stdio.h>
#include <stdlib.h>
#include <float.h>
#include <string.h>

int main(void) {
    FILE *inf;
    FILE *outf;
    char filename[21],outfilename[21];
    char yn='0';

    printf("Please enter an input filename: ");
    scanf("%s",&filename);

    printf("Please enter an output filename: ");    
    scanf("%s",&outfilename);

    /* Open file for reading */
    inf=fopen (filename,"r");
    outf=fopen(outfilename,"r");

    /*check that input file exists*/
    if (inf!=NULL) {

        /*check that the output file doesn't already exist*/
        if (outf==NULL){
            fclose(outf);
            /*if it doesn't already exist create file by opening in "write" mode*/
            outf=fopen(outfilename,"w");
        } else {
            /*If the file does exist, give the option to overwrite or not*/
            printf("Output file already exists, overwrite (y/n):");
            scanf("%c",&yn);
        }
    }
    printf("\n yn=%c \n",yn);
    return 0;
}

Miler

Don't fclose(outf) if outf == NULL. You can't close a file you failed to open in the first place. – Unciform

i wasnt sure so i just added it in! not very good pracice i realise! – Miler

Answer 1

printf("Please enter an output filename: ");    
scanf("%s",&outfilename);

When you enter the second string and hit the ENTER key, a string and a character are placed in the input buffer, they are namely: the entered string and the newline character.The string gets consumed by the scanf but the newline remains in the input buffer.

Further,

scanf("%c",&yn);

Your next scanf for reading the character just reads/consumes the newline and hence never waits for user input.

Solution is to consume the extra newline by using:

scanf(" %c", &yn);
      ^^^   <------------Note the space

Or by using getchar()

You may want to check out my answer here for a detailed step by step explanation of the problem.

Lapland

definitely, I am new to all of this but I understand that the second scanf reads and stores "\nfile.txt" to char outfilename[21] and that the 3rd scanf statement stores the remaining "\n" to char yn that was in the input buffer after I inputted the outfilename. Cheers for your help – Miler

Answer 2

Use

scanf("%20s",&filename);

and remember that stdin is line buffered and on Linux is following a tty discipline

You could use GNU readline or ncurses if you want more detailed control.

Irresolvable

How is that related to the problem at hand? – Periodontal

Using readline will read a whole line, with editing abilities, which is relevant for reading e.g. the filename and the yn... – Irresolvable

And following the URL link about tty is very relevant to the question. It contains a lot of explanations (notably historical) which explains why reading from stdin inside a terminal is so complex.... – Irresolvable

wouldn't it be scanf("%20s",filename); ? – All

Answer 3

The better way to handle this problem I found is explained here.

It recomends to use an alternative way of handle input and is very well explained.

I use always this function to get user input.

char * read_line (char * buf, size_t length) {
    /**** Copyright de home.datacomm.ch/t_wolf/tw/c/getting_input.html#skip
    Read at most 'length'-1 characters from the file 'f' into
    'buf' and zero-terminate this character sequence. If the
    line contains more characters, discard the rest.
    */
    char *p;
    if ((p = fgets (buf, length, stdin))) {
        size_t last = strlen (buf) - 1;
        if (buf[last] == '\n') {
            /**** Discard the trailing newline */
            buf[last] = '\0';
        } else {
            /**** There's no newline in the buffer, therefore there must be
            more characters on that line: discard them!
            */
            fscanf (stdin, "%*[^\n]");
            /**** And also discard the newline... */
            (void) fgetc (stdin);
        } /* end if */
    } /* end if */
    return p;
} /* end read_line */

---

Old Answer

I fixed this sort of problems with this rule:

// first I get what I want.
c = getchar();
// but after any user input I clear the input buffer
// until the \n character:
while (getchar() != '\n');
// this also discard any extra (unexpected) character.

If you make this after any input, there should be not problem.

Obstacle

Answer 4

scanf("%s", ...) leaves the \n terminating the line in the input. It isn't causing a problem for the next one as scanf("%s", ...) starts by skipping whites. scanf("%c", ...) doesn't and thus you read the \n.

BTW You'll probably meet other problems is you put spaces in your file name (%s doesn't read them) and if you enter too long names (%s has no input length limitations).

One solution for the problem you complained (but not the other one) is to use scanf(" %c", ...) (see the space before %c? scanf is tricky to use) which starts by skipping white spaces.

Periodontal

Answer 5

scanf("%s",&filename);

also remove the &

scanf.c:13: warning: format '%s' expects type 'char ', but argument 2 has type 'char ()[20u]'

Hundredth

i'm not sure i understand your answer – Miler