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

问题

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;
}

回答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.

回答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.

回答3:

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.

回答4:

scanf("%s",&filename);

also remove the &

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

回答5:

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.

来源：https://stackoverflow.com/questions/8464620/program-doesnt-wait-for-user-input-with-scanfc-yn