C non-blocking keyboard input

Question

I m trying to write a program in C  on Linux  that loops until the user presses a key  but shouldn t require a keypress to continue each loop   Is there a simple way to do this  I figure I could possibly do it with select   but that seems like a lot of work   Alternatively  is there a way to catch a ctrl-c keypress to do cleanup before the program closes instead of non-blocking io

User · Answer

There is no portable way to do this  but select   might be a good way  See http   c-faq com osdep readavail html for more possible solutions

User · Answer

As already stated  you can use sigaction to trap ctrl-c  or select to trap any standard input   Note however that with the latter method you also need to set the TTY so that it s in character-at-a-time rather than line-at-a-time mode   The latter is the default - if you type in a line of text it doesn t get sent to the running program s stdin until you press enter   You d need to use the tcsetattr   function to turn off ICANON mode  and probably also disable ECHO too   From memory  you also have to set the terminal back into ICANON mode when the program exits   Just for completeness  here s some code I ve just knocked up  nb  no error checking   which sets up a Unix TTY and emulates the DOS  lt conio h gt  functions kbhit   and getch      include  lt stdlib h gt   include  lt string h gt   include  lt unistd h gt   include  lt sys select h gt   include  lt termios h gt   struct termios orig termios   void reset terminal mode         tcsetattr 0  TCSANOW   amp orig termios      void set conio terminal mode         struct termios new termios          take two copies - one for now  one for later        tcgetattr 0   amp orig termios       memcpy  amp new termios   amp orig termios  sizeof new termios            register cleanup handler  and set the new terminal mode        atexit reset terminal mode       cfmakeraw  amp new termios       tcsetattr 0  TCSANOW   amp new termios      int kbhit         struct timeval tv     0L  0L        fd set fds      FD ZERO  amp fds       FD SET 0   amp fds       return select 1   amp fds  NULL  NULL   amp tv      int getch         int r      unsigned char c      if   r   read 0   amp c  sizeof c     lt  0            return r        else           return c           int main int argc  char  argv          set conio terminal mode         while   kbhit                 do some work               void getch       consume the character

User · Answer

The curses library can be used for this purpose  Of course  select   and signal handlers can be used too to a certain extent

User · Answer

As already stated  you can use sigaction to trap ctrl-c  or select to trap any standard input   Note however that with the latter method you also need to set the TTY so that it s in character-at-a-time rather than line-at-a-time mode   The latter is the default - if you type in a line of text it doesn t get sent to the running program s stdin until you press enter   You d need to use the tcsetattr   function to turn off ICANON mode  and probably also disable ECHO too   From memory  you also have to set the terminal back into ICANON mode when the program exits   Just for completeness  here s some code I ve just knocked up  nb  no error checking   which sets up a Unix TTY and emulates the DOS  lt conio h gt  functions kbhit   and getch      include  lt stdlib h gt   include  lt string h gt   include  lt unistd h gt   include  lt sys select h gt   include  lt termios h gt   struct termios orig termios   void reset terminal mode         tcsetattr 0  TCSANOW   amp orig termios      void set conio terminal mode         struct termios new termios          take two copies - one for now  one for later        tcgetattr 0   amp orig termios       memcpy  amp new termios   amp orig termios  sizeof new termios            register cleanup handler  and set the new terminal mode        atexit reset terminal mode       cfmakeraw  amp new termios       tcsetattr 0  TCSANOW   amp new termios      int kbhit         struct timeval tv     0L  0L        fd set fds      FD ZERO  amp fds       FD SET 0   amp fds       return select 1   amp fds  NULL  NULL   amp tv      int getch         int r      unsigned char c      if   r   read 0   amp c  sizeof c     lt  0            return r        else           return c           int main int argc  char  argv          set conio terminal mode         while   kbhit                 do some work               void getch       consume the character

User · Answer

There is no portable way to do this  but select   might be a good way  See http   c-faq com osdep readavail html for more possible solutions

User · Answer

If you are happy just catching Control-C  it s a done deal   If you really want non-blocking I O but you don t want the curses library  another alternative is to move lock  stock  and barrel to the AT amp T sfio library   It s nice library patterned on C stdio but more flexible  thread-safe  and performs better    sfio stands for safe  fast I O

User · Answer

On UNIX systems  you can use sigaction call to register a signal handler for SIGINT signal which represents the Control C key sequence  The signal handler can set a flag which will be checked in the loop making it to break appropriately

User · Answer

select   is a bit too low-level for convenience   I suggest you use the ncurses library to put the terminal in cbreak mode and delay mode  then call getch    which will return ERR if no character is ready   WINDOW  w   initscr    cbreak    nodelay w  TRUE     At that point you can call getch without blocking

User · Answer

There is no portable way to do this  but select   might be a good way  See http   c-faq com osdep readavail html for more possible solutions

User · Answer

If you are happy just catching Control-C  it s a done deal   If you really want non-blocking I O but you don t want the curses library  another alternative is to move lock  stock  and barrel to the AT amp T sfio library   It s nice library patterned on C stdio but more flexible  thread-safe  and performs better    sfio stands for safe  fast I O

User · Answer

The curses library can be used for this purpose  Of course  select   and signal handlers can be used too to a certain extent

User · Answer

There is no portable way to do this  but select   might be a good way  See http   c-faq com osdep readavail html for more possible solutions

User · Answer

If you are happy just catching Control-C  it s a done deal   If you really want non-blocking I O but you don t want the curses library  another alternative is to move lock  stock  and barrel to the AT amp T sfio library   It s nice library patterned on C stdio but more flexible  thread-safe  and performs better    sfio stands for safe  fast I O

User · Answer

Here s a function to do this for you  You need termios h which comes with POSIX systems    include  lt termios h gt  void stdin set int cmd        struct termios t      tcgetattr 1  amp t       switch  cmd        case 1              t c lflag  amp    ICANON              break      default              t c lflag    ICANON              break            tcsetattr 1 0  amp t       Breaking this down  tcgetattr gets the current terminal information and stores it in t  If cmd is 1  the local input flag in t is set to non-blocking input  Otherwise it is reset  Then tcsetattr changes standard input to t   If you don t reset standard input at the end of your program you will have problems in your shell

User · Answer

select   is a bit too low-level for convenience   I suggest you use the ncurses library to put the terminal in cbreak mode and delay mode  then call getch    which will return ERR if no character is ready   WINDOW  w   initscr    cbreak    nodelay w  TRUE     At that point you can call getch without blocking

User · Answer

As already stated  you can use sigaction to trap ctrl-c  or select to trap any standard input   Note however that with the latter method you also need to set the TTY so that it s in character-at-a-time rather than line-at-a-time mode   The latter is the default - if you type in a line of text it doesn t get sent to the running program s stdin until you press enter   You d need to use the tcsetattr   function to turn off ICANON mode  and probably also disable ECHO too   From memory  you also have to set the terminal back into ICANON mode when the program exits   Just for completeness  here s some code I ve just knocked up  nb  no error checking   which sets up a Unix TTY and emulates the DOS  lt conio h gt  functions kbhit   and getch      include  lt stdlib h gt   include  lt string h gt   include  lt unistd h gt   include  lt sys select h gt   include  lt termios h gt   struct termios orig termios   void reset terminal mode         tcsetattr 0  TCSANOW   amp orig termios      void set conio terminal mode         struct termios new termios          take two copies - one for now  one for later        tcgetattr 0   amp orig termios       memcpy  amp new termios   amp orig termios  sizeof new termios            register cleanup handler  and set the new terminal mode        atexit reset terminal mode       cfmakeraw  amp new termios       tcsetattr 0  TCSANOW   amp new termios      int kbhit         struct timeval tv     0L  0L        fd set fds      FD ZERO  amp fds       FD SET 0   amp fds       return select 1   amp fds  NULL  NULL   amp tv      int getch         int r      unsigned char c      if   r   read 0   amp c  sizeof c     lt  0            return r        else           return c           int main int argc  char  argv          set conio terminal mode         while   kbhit                 do some work               void getch       consume the character

User · Answer

Another way to get non-blocking keyboard input is to open the device file and read it     You have to know the device file you are looking for  one of  dev input event    You can run cat  proc bus input devices to find the device you want   This code works for me  run as an administrator       include  lt stdlib h gt     include  lt stdio h gt     include  lt unistd h gt     include  lt fcntl h gt     include  lt errno h gt     include  lt linux input h gt     int main int argc  char   argv            int fd  bytes        struct input event data         const char  pDevice     dev input event2             Open Keyboard       fd   open pDevice  O RDONLY   O NONBLOCK         if fd    -1                    printf  ERROR Opening  s n   pDevice             return -1                 while 1                       Read Keyboard Data           bytes   read fd   amp data  sizeof data              if bytes  gt  0                            printf  Keypress value  x  type  x  code  x n   data value  data type  data code                         else                              Nothing read               sleep 1                              return 0

User · Answer

On UNIX systems  you can use sigaction call to register a signal handler for SIGINT signal which represents the Control C key sequence  The signal handler can set a flag which will be checked in the loop making it to break appropriately

User · Answer

You probably want kbhit       Example will loop until a key is pressed  include  lt conio h gt   include  lt iostream gt   using namespace std   int main         while 1                if kbhit                          break                      this may not work on all environments  A portable way would be to create a monitoring thread and set some flag on getch

User · Answer

The curses library can be used for this purpose  Of course  select   and signal handlers can be used too to a certain extent

User · Answer

You probably want kbhit       Example will loop until a key is pressed  include  lt conio h gt   include  lt iostream gt   using namespace std   int main         while 1                if kbhit                          break                      this may not work on all environments  A portable way would be to create a monitoring thread and set some flag on getch

User · Answer

You probably want kbhit       Example will loop until a key is pressed  include  lt conio h gt   include  lt iostream gt   using namespace std   int main         while 1                if kbhit                          break                      this may not work on all environments  A portable way would be to create a monitoring thread and set some flag on getch

User · Answer

You can do that using select as follow     int nfds   0    fd set readfds    FD ZERO  amp readfds     FD SET 0   amp readfds      set the stdin in the set of file descriptors to be selected      while 1              Do what you want         int count   select nfds   amp readfds  NULL  NULL  NULL        if  count  gt  0          if  FD ISSET 0   amp readfds                  If a character was pressed then we get it and exit              getchar              break                       Not too much work  D

User · Answer

On UNIX systems  you can use sigaction call to register a signal handler for SIGINT signal which represents the Control C key sequence  The signal handler can set a flag which will be checked in the loop making it to break appropriately

User · Answer

As already stated  you can use sigaction to trap ctrl-c  or select to trap any standard input   Note however that with the latter method you also need to set the TTY so that it s in character-at-a-time rather than line-at-a-time mode   The latter is the default - if you type in a line of text it doesn t get sent to the running program s stdin until you press enter   You d need to use the tcsetattr   function to turn off ICANON mode  and probably also disable ECHO too   From memory  you also have to set the terminal back into ICANON mode when the program exits   Just for completeness  here s some code I ve just knocked up  nb  no error checking   which sets up a Unix TTY and emulates the DOS  lt conio h gt  functions kbhit   and getch      include  lt stdlib h gt   include  lt string h gt   include  lt unistd h gt   include  lt sys select h gt   include  lt termios h gt   struct termios orig termios   void reset terminal mode         tcsetattr 0  TCSANOW   amp orig termios      void set conio terminal mode         struct termios new termios          take two copies - one for now  one for later        tcgetattr 0   amp orig termios       memcpy  amp new termios   amp orig termios  sizeof new termios            register cleanup handler  and set the new terminal mode        atexit reset terminal mode       cfmakeraw  amp new termios       tcsetattr 0  TCSANOW   amp new termios      int kbhit         struct timeval tv     0L  0L        fd set fds      FD ZERO  amp fds       FD SET 0   amp fds       return select 1   amp fds  NULL  NULL   amp tv      int getch         int r      unsigned char c      if   r   read 0   amp c  sizeof c     lt  0            return r        else           return c           int main int argc  char  argv          set conio terminal mode         while   kbhit                 do some work               void getch       consume the character

User · Answer

If you are happy just catching Control-C  it s a done deal   If you really want non-blocking I O but you don t want the curses library  another alternative is to move lock  stock  and barrel to the AT amp T sfio library   It s nice library patterned on C stdio but more flexible  thread-safe  and performs better    sfio stands for safe  fast I O

User · Answer

select   is a bit too low-level for convenience   I suggest you use the ncurses library to put the terminal in cbreak mode and delay mode  then call getch    which will return ERR if no character is ready   WINDOW  w   initscr    cbreak    nodelay w  TRUE     At that point you can call getch without blocking

User · Answer

Here s a function to do this for you  You need termios h which comes with POSIX systems    include  lt termios h gt  void stdin set int cmd        struct termios t      tcgetattr 1  amp t       switch  cmd        case 1              t c lflag  amp    ICANON              break      default              t c lflag    ICANON              break            tcsetattr 1 0  amp t       Breaking this down  tcgetattr gets the current terminal information and stores it in t  If cmd is 1  the local input flag in t is set to non-blocking input  Otherwise it is reset  Then tcsetattr changes standard input to t   If you don t reset standard input at the end of your program you will have problems in your shell

User · Answer

The curses library can be used for this purpose  Of course  select   and signal handlers can be used too to a certain extent

User · Answer

On UNIX systems  you can use sigaction call to register a signal handler for SIGINT signal which represents the Control C key sequence  The signal handler can set a flag which will be checked in the loop making it to break appropriately

User · Answer

You can do that using select as follow     int nfds   0    fd set readfds    FD ZERO  amp readfds     FD SET 0   amp readfds      set the stdin in the set of file descriptors to be selected      while 1              Do what you want         int count   select nfds   amp readfds  NULL  NULL  NULL        if  count  gt  0          if  FD ISSET 0   amp readfds                  If a character was pressed then we get it and exit              getchar              break                       Not too much work  D

User · Answer

You probably want kbhit       Example will loop until a key is pressed  include  lt conio h gt   include  lt iostream gt   using namespace std   int main         while 1                if kbhit                          break                      this may not work on all environments  A portable way would be to create a monitoring thread and set some flag on getch

User · Answer

Another way to get non-blocking keyboard input is to open the device file and read it     You have to know the device file you are looking for  one of  dev input event    You can run cat  proc bus input devices to find the device you want   This code works for me  run as an administrator       include  lt stdlib h gt     include  lt stdio h gt     include  lt unistd h gt     include  lt fcntl h gt     include  lt errno h gt     include  lt linux input h gt     int main int argc  char   argv            int fd  bytes        struct input event data         const char  pDevice     dev input event2             Open Keyboard       fd   open pDevice  O RDONLY   O NONBLOCK         if fd    -1                    printf  ERROR Opening  s n   pDevice             return -1                 while 1                       Read Keyboard Data           bytes   read fd   amp data  sizeof data              if bytes  gt  0                            printf  Keypress value  x  type  x  code  x n   data value  data type  data code                         else                              Nothing read               sleep 1                              return 0

User · Answer

select   is a bit too low-level for convenience   I suggest you use the ncurses library to put the terminal in cbreak mode and delay mode  then call getch    which will return ERR if no character is ready   WINDOW  w   initscr    cbreak    nodelay w  TRUE     At that point you can call getch without blocking

[c] C non-blocking keyboard input

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