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Top Forums Programming Clear standard input buffer for C program in Linux Post 302912455 by Corona688 on Friday 8th of August 2014 11:09:09 AM
Old 08-08-2014
It certainly is showing error but you're not looking for it. That's what scanf()'s return value is for, it returns 0 if it reads no values, 1 if it found 1 value, 2 for 2, etc. Each %... code counts as one "value".

The buffering thing is a known issue with scanf(). It's a "feature" -- it stops scanning at the first bad character -- which is pretty useless if you're not building a compiler.

fflush should work, if you tell it which stream to flush, which you are not. But the proper way to avoid these buffer problems is to not use scanf().

You can still use sscanf though, which is just as good without the problems! It scans a string, and has no buffer to leave things in.

Here's the usual way to do it:

Code:
while(ans=='y')
{
        char line[512];
        float val;
        // Quit the loop if the file hits EOF
        if(fgets(line, 512, stdin) == NULL) break;

        // sscanf returns 1 if it reads one value, 2 if it found 2 values, etc.
        if(sscanf(line, "%f", &val) != 1)
        {
                fprintf(stderr, "'%s' is not a number\n", line);
                continue;  // Skip back to top of 'while'
        }

        // Quit the loop if the file hits EOF
        if(fgets(line, 512, stdin) == NULL) break;
        if(sscanf(line, "%c", &ans) != 1)
        {
                fprintf(stderr, "Couldn't read Y/N\n");
                continue;
        }
}


Last edited by Corona688; 08-08-2014 at 12:18 PM..
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GETS(3) 						     Linux Programmer's Manual							   GETS(3)

NAME
fgetc, fgets, getc, getchar, gets, ungetc - input of characters and strings SYNOPSIS
#include <stdio.h> int fgetc(FILE *stream); char *fgets(char *s, int size, FILE *stream); int getc(FILE *stream); int getchar(void); char *gets(char *s); int ungetc(int c, FILE *stream); DESCRIPTION
fgetc() reads the next character from stream and returns it as an unsigned char cast to an int, or EOF on end of file or error. getc() is equivalent to fgetc() except that it may be implemented as a macro which evaluates stream more than once. getchar() is equivalent to getc(stdin). gets() reads a line from stdin into the buffer pointed to by s until either a terminating newline or EOF, which it replaces with ''. No check for buffer overrun is performed (see BUGS below). fgets() reads in at most one less than size characters from stream and stores them into the buffer pointed to by s. Reading stops after an EOF or a newline. If a newline is read, it is stored into the buffer. A '' is stored after the last character in the buffer. ungetc() pushes c back to stream, cast to unsigned char, where it is available for subsequent read operations. Pushed - back characters will be returned in reverse order; only one pushback is guaranteed. Calls to the functions described here can be mixed with each other and with calls to other input functions from the stdio library for the same input stream. For non-locking counterparts, see unlocked_stdio(3). RETURN VALUE
fgetc(), getc() and getchar() return the character read as an unsigned char cast to an int or EOF on end of file or error. gets() and fgets() return s on success, and NULL on error or when end of file occurs while no characters have been read. ungetc() returns c on success, or EOF on error. CONFORMING TO
ANSI - C, POSIX.1 BUGS
Never use gets(). Because it is impossible to tell without knowing the data in advance how many characters gets() will read, and because gets() will continue to store characters past the end of the buffer, it is extremely dangerous to use. It has been used to break computer security. Use fgets() instead. It is not advisable to mix calls to input functions from the stdio library with low - level calls to read() for the file descriptor associ- ated with the input stream; the results will be undefined and very probably not what you want. SEE ALSO
read(2), write(2), ferror(3), fopen(3), fread(3), fseek(3), puts(3), scanf(3), unlocked_stdio(3) GNU
1993-04-04 GETS(3)
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