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Scanf problem under LINUX...
Post 19725 by robotronic on Tuesday 16th of April 2002 07:42:39 AM
04-16-2002
Scanf problem under LINUX...
I have a problem reading characters from keyboard with the scanf function.
Here there is a little piece of code:
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
/* The last 3 libraries are included because in the real program I use some operations with sockets (socket, connect, getsockname, send, recv...) but I'm sure that the problem isn't here... */
int main()
{
char Ch = '0', Ch_Seq[64], inbuf[2];
system("clear");
printf("Insert a character ('q' = Exit): ");
fflush(0);
scanf("%c", &Ch);
while (Ch != 'q')
{
printf("Insert a string: ");
fflush(0);
scanf("%s", Ch_Seq);
/* There isn't length control, I know. Suppose to enter a valid string. */
/* Asks for a new character */
printf("\nInsert a character ('q' = Exit): ");
fflush(0);
scanf("%c", &Ch);
}
printf("End of program.\n\n");
}
This program (except for socket operations, obviously) works perfectly under MS-DOS, but when I try to run it under Linux (Mandrake 8.0) the second scanf functions in the while loop fails, because it reads the enter character of the previous scanf (which works correctly and stores the string in Ch_Seq).
For this reason the program can't never read the 'q' character, and enters in an infinite loop.
I have noticed that the only scanf that works correctly is the first, the one before the program enters in the while loop. Infact, if I run the program and I immediately enter a 'q' character, the program ends, writing "End of program.".
I have tried to use
scanf("%c\n", &Ch);
instead of
scanf("%c", &Ch);
but it doesn't work.
I have also tried with
Ch = getchar();
but the result is the same as described before.
I have temporary solved the problem replacing all the occurencies of
fflush(0);
scanf("%c", &Ch);
with
fflush(stdout);
read(0, inbuf, 2);
Ch = inbuf[0];
With this solution I have noticed a strange thing... If I write
read(stdin, inbuf, 2);
instead of
read(0, inbuf, 2);
it doesn't work!
Why?
However, I want to use anyway the scanf function and understand why under Linux it doesn't work.
Is there someone that could explain me in detail the behaviour of scanf function (and fflush) in Linux and how can I solve the problem?
I need to use scanf also because I want to port under Linux some programs written for MS-DOS that makes large use of this function.
I hope you can help me!
Thanks in advance, and excuse for my poor english!
Here there is a little piece of code:
#include <stdio.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
/* The last 3 libraries are included because in the real program I use some operations with sockets (socket, connect, getsockname, send, recv...) but I'm sure that the problem isn't here... */
int main()
{
char Ch = '0', Ch_Seq[64], inbuf[2];
system("clear");
printf("Insert a character ('q' = Exit): ");
fflush(0);
scanf("%c", &Ch);
while (Ch != 'q')
{
printf("Insert a string: ");
fflush(0);
scanf("%s", Ch_Seq);
/* There isn't length control, I know. Suppose to enter a valid string. */
/* Asks for a new character */
printf("\nInsert a character ('q' = Exit): ");
fflush(0);
scanf("%c", &Ch);
}
printf("End of program.\n\n");
}
This program (except for socket operations, obviously) works perfectly under MS-DOS, but when I try to run it under Linux (Mandrake 8.0) the second scanf functions in the while loop fails, because it reads the enter character of the previous scanf (which works correctly and stores the string in Ch_Seq).
For this reason the program can't never read the 'q' character, and enters in an infinite loop.
I have noticed that the only scanf that works correctly is the first, the one before the program enters in the while loop. Infact, if I run the program and I immediately enter a 'q' character, the program ends, writing "End of program.".
I have tried to use
scanf("%c\n", &Ch);
instead of
scanf("%c", &Ch);
but it doesn't work.
I have also tried with
Ch = getchar();
but the result is the same as described before.
I have temporary solved the problem replacing all the occurencies of
fflush(0);
scanf("%c", &Ch);
with
fflush(stdout);
read(0, inbuf, 2);
Ch = inbuf[0];
With this solution I have noticed a strange thing... If I write
read(stdin, inbuf, 2);
instead of
read(0, inbuf, 2);
it doesn't work!
Why?
However, I want to use anyway the scanf function and understand why under Linux it doesn't work.
Is there someone that could explain me in detail the behaviour of scanf function (and fflush) in Linux and how can I solve the problem?
I need to use scanf also because I want to port under Linux some programs written for MS-DOS that makes large use of this function.
I hope you can help me!
Thanks in advance, and excuse for my poor english!
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LEARN ABOUT SUNOS
wscanw
mvscanw(3XCURSES) X/Open Curses Library Functions mvscanw(3XCURSES) NAME
mvscanw, mvwscanw, scanw, wscanw - convert formatted input from a window SYNOPSIS
cc [ flag... ] file... -I /usr/xpg4/include -L /usr/xpg4/lib -R /usr/xpg4/lib -lcurses [ library... ] c89 [ flag... ] file... -lcurses [ library... ] #include <curses.h> int mvscanw(int y, int x, char *fmt, ...); int mvwscanw(WINDOW *win, int y, int x, char *fmt, ...); int scanw(char *fmt, ...); int wscanw(WINDOW *win, char *fmt, ...); PARAMETERS
y Is the y (row) coordinate of the position of the character to be read. x Is the x (column) coordinate of the position of the character to be read. fmt Is a scanf() format string. win Is a pointer to the window in which the character is to be read. DESCRIPTION
These functions are similar to scanf(3C). Their effect is as though mvwgetstr(3XCURSES) were called to get a multi-byte character string from the current or specified window at the current or specified cursor position, and then sscanf() were used to interpret and convert that string. RETURN VALUES
Upon successful completion, these functions return OK. Otherwise, they return ERR. ERRORS
No errors are defined. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ |MT-Level |Unsafe | +-----------------------------+-----------------------------+ SEE ALSO
getnstr(3XCURSES), libcurses(3XCURSES), printw(3XCURSES), scanf(3C), wcstombs(3C), attributes(5), standards(5) SunOS 5.10 5 Jun 2002 mvscanw(3XCURSES)