How to capture Control+D signal?

As others have already said, to handle Control+D, handle "end of file"s.

Control+D is a piece of communication between the user and the pseudo-file that you see as stdin. It does not mean specifically "end of file", but more generally "flush the input I typed so far". Flushing means that any read() call on stdin in your program returns with the length of the input typed since the last flush. If the line is nonempty, the input becomes available to your program although the user did not type "return" yet. If the line is empty, then read() returns with zero, and that is interpreted as "end of file".

So when using Control+D to end a program, it only works at the beginning of a line, or if you do it twice (first time to flush, second time for read() to return zero).

Try it:

$ catfoo   (type Control-D once)foofoo (read has returned "foo")   (type Control-D again)$

Ctrl+D is not a signal, it's EOF (End-Of-File). It closes the stdin pipe. If read(STDIN) returns 0, it means stdin closed, which means Ctrl+D was hit (assuming there is a keyboard at the other end of the pipe).

A minimalistic example:

#include <unistd.h> #include <stdio.h> #include <termios.h> #include <signal.h> void sig_hnd(int sig){ (void)sig; printf("(VINTR)"); }int main(){  setvbuf(stdout,NULL,_IONBF,0);  struct termios old_termios, new_termios;  tcgetattr(0,&old_termios);  signal( SIGINT, sig_hnd );  new_termios             = old_termios;  new_termios.c_cc[VEOF]  = 3; // ^C  new_termios.c_cc[VINTR] = 4; // ^D  tcsetattr(0,TCSANOW,&new_termios);  char line[256]; int len;  do{    len=read(0,line,256); line[len]='\0';    if( len <0 ) printf("(len: %i)",len);    if( len==0 ) printf("(VEOF)");    if( len >0 ){      if( line[len-1] == 10 ) printf("(line:'%.*s')\n",len-1,line);      if( line[len-1] != 10 ) printf("(partial line:'%s')",line);    }  }while( line[0] != 'q' );  tcsetattr(0,TCSANOW,&old_termios);}

The program change the VEOF char (from Ctrl-D) to Ctrl-C and the VINTR char (from Ctrl-C) to Ctrl-D. If You press Ctrl-D then the terminal driver will send a SIGINT to the signal handler of the program.

Note: pressing VINTR will erase the terminal input buffer so You can not read the characters typed in the line before the VINTR key pressed.