I am trying to use named pipes for interprocess communication but I'm seeing strange things I cannot understand.
I have this "writer" code:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#define FIFO_NAME "american_maid"
int main(void)
{
char s[300];
int num, fd;
mknod(FIFO_NAME, S_IFIFO | 0666, 0);
printf("waiting for readers...\n");
fd = open(FIFO_NAME, O_WRONLY);
printf ("FD = %d", fd);
printf("got a reader--type some stuff\n");
while (gets(s), !feof(stdin)) {
if ((num = write(fd, s, strlen(s))) == -1)
perror("write");
else
printf("speak: wrote %d bytes\n", num);
}
return 0;
}
Now when I do a "tail -f american_maid" the tail blocks waiting for a writer.
Then I run the writer code and it finds that tail -f is waiting on the other end of the pipe. So far so good.
When I type anything in the writer application, nothing shows up in tail window, HOWEVER, as soon as I close (Ctrl-C) the writer app, ALL that I had typed appear in the tail window.
It's like the writer has to close before tail could show anything. Any ideas what is happening and why it's happening and how I can fix this?
Thanks
There is nothing wrong here. tail wants to extract the last lines. To determine which these are, it is looking for end-of-file in the fifo, which it only sees after the writer has closed the writing end of the fifo.
Related
I am trying to read data sent from the tty/USB0 and print it out with byte format.
Question:
I expect to print out the data once the reading bytes reach 40 However, the time takes much longer than I expect. The read() system call hangs and I believe the data should already be larger than 40. The data will finally be printed out but it should not take so long. Did I make anything wrong in this programming ?
thanks
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <termios.h>
#include <stdio.h>
#define BAUDRATE B9600
#define MODEMDEVICE "/dev/ttyUSB0"
#define FALSE 0
#define TRUE 1
main()
{
int fd,c, res;
struct termios oldtio,newtio;
unsigned char buf[40];
fd = open(MODEMDEVICE, O_RDWR | O_NOCTTY );
if (fd <0) {perror(MODEMDEVICE); exit(-1); }
tcgetattr(fd,&oldtio);
bzero(&newtio, sizeof(newtio));
newtio.c_cflag = BAUDRATE | CS8 | CLOCAL | CREAD;
newtio.c_iflag = IGNPAR | ICRNL;
newtio.c_oflag = 1;
newtio.c_lflag = ICANON;
tcflush(fd, TCIOFLUSH);
tcsetattr(fd,TCSANOW,&newtio);
int i;
while (1) {
res = read(fd,buf,40);
if(res==40){
printf("res reaches 40 \n");
}
printf("res: %d\n",res);
for(i=0;i<res;++i){
printf("%02x ", buf[i]);
}
return;
}
}
--------------------raw mode code------------------------
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <termios.h>
#include <stdio.h>
#define BAUDRATE B9600
#define MODEMDEVICE "/dev/ttyUSB0"
#define _POSIX_SOURCE 1 /* POSIX compliant source */
#define FALSE 0
#define TRUE 1
volatile int STOP=FALSE;
main()
{
int fd,c, res;
struct termios oldtio,newtio;
unsigned char buf[255];
fd = open(MODEMDEVICE, O_RDWR | O_NOCTTY );
if (fd <0) {perror(MODEMDEVICE); exit(-1); }
tcgetattr(fd,&oldtio); /* save current port settings */
bzero(&newtio, sizeof(newtio));
newtio.c_cflag = BAUDRATE | CRTSCTS | CS8 | CLOCAL | CREAD;
newtio.c_iflag = IGNPAR;
newtio.c_oflag = 0;
/* set input mode (non-canonical, no echo,...) */
newtio.c_lflag = 0;
newtio.c_cc[VTIME] = 0;
newtio.c_cc[VMIN] = 40;
tcflush(fd, TCIFLUSH);
tcsetattr(fd,TCSANOW,&newtio);
int i;
while (STOP==FALSE) {
res = read(fd,buf,255);
for( i=0;i<res;++i){
printf("%02x \n", buf[i]);
}
}
tcsetattr(fd,TCSANOW,&oldtio);
}
It now can print out the data once buffer capacity is full ( which is 40 ).
1 question:
When I modified the printf
printf("%02x ", buf[i]); ( remove "\n" )
It will not print out when the buffer is full until more bytes are received. Why this happens?
Thanks
You need to switch the terminal to raw mode to disable line buffering.
Citing this answer:
The terms raw and cooked only apply to terminal drivers. "Cooked" is
called canonical and "raw" is called non-canonical mode.
The terminal driver is, by default a line-based system: characters are
buffered internally until a carriage return (Enter or Return) before
it is passed to the program - this is called "cooked". This allows
certain characters to be processed (see stty(1)), such as Cntl-D,
Cntl-S, Ctrl-U Backspace); essentially rudimentary line-editing. The
terminal driver "cooks" the characters before serving them up.
The terminal can be placed into "raw" mode where the characters are
not processed by the terminal driver, but are sent straight through
(it can be set that INTR and QUIT characters are still processed).
This allows programs like emacs and vi to use the entire screen more
easily.
You can read more about this in the "Canonical mode" section of the
termios(3) manpage.
See e.g. this or this how to achieve that programmatically (did not check the code, but it should be easy to find it).
Alternatively you could use e.g. strace or ltrace to check what stty -F /dev/ttyUSB0 raw does (or read the manual page where it is described).
EDIT>
Regarding printf without a newline -- fflush(stdout); immediately after it should help (another line-buffering is taking place).
You might consider reading this and maybe this.
I'm trying to create a FIFO named pipe using the mknod() command:
int main() {
char* file="pipe.txt";
int state;
state = mknod(file, S_IFIFO & 0777, 0);
printf("%d",state);
return 0;
}
But the file is not created in my current directory. I tried listing it by ls -l . State returns -1.
I found similar questions here and on other sites and I've tried the solution that most suggested:
int main() {
char* file="pipe.txt";
int state;
unlink(file);
state = mknod(file, S_IFIFO & 0777, 0);
printf("%d",state);
return 0;
}
This made no difference though and the error remains. Am I doing something wrong here or is there some sort of system intervention which is causing this problem?
Help.. Thanks in advance
You are using & to set the file type instead of |. From the docs:
The file type for path is OR'ed into the mode argument, and the
application shall select one of the following symbolic
constants...
Try this:
state = mknod(file, S_IFIFO | 0777, 0);
Because this works:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
int main() {
char* file="pipe.txt";
int state;
unlink(file);
state = mknod(file, S_IFIFO | 0777, 0);
printf("state %d\n", state);
return 0;
}
Compile it:
gcc -o fifo fifo.c
Run it:
$ strace -e trace=mknod ./fifo
mknod("pipe.txt", S_IFIFO|0777) = 0
state 0
+++ exited with 0 +++
See the result:
$ ls -l pipe.txt
prwxrwxr-x. 1 lars lars 0 Jul 16 12:54 pipe.txt
I want to debug a console linux application with 2 xterm windows: one window used for gdb and another used for the application (e.g. mc).
What I do now is run 'tty && sleep 1024d' in the second xterm window (this gives me its pseudo-tty name) and then run 'tty ' in gdb to redirect the program to that other xterm window. However, GDB warns that it cannot set a controlling terminal and certain minor functions don't work (e.g. handling window resizing), as 'sleep 1024d' is still running on that xterm window.
Any better way to do it (rather than launching the process from the shell and attaching to it from gdb)?
I have somewhat modified the program given in a related bug to store the filename somewhere
http://sourceware.org/bugzilla/show_bug.cgi?id=11403
here is an example using it:
$ xterm -e './disowntty ~/tty.tmp' & sleep 1 && gdb --tty $(cat ~/tty.tmp) /usr/bin/links
/* tty;exec disowntty */
#include <sys/ioctl.h>
#include <unistd.h>
#include <stdio.h>
#include <limits.h>
#include <stdlib.h>
#include <signal.h>
static void
end (const char *msg)
{
perror (msg);
for (;;)
pause ();
}
int
main (int argc, const char *argv[])
{
FILE *tty_name_file;
const char *tty_filename;
if (argc <= 1)
return 1;
else
tty_filename = argv[1];
void (*orig) (int signo);
setbuf (stdout, NULL);
orig = signal (SIGHUP, SIG_IGN);
if (orig != SIG_DFL)
end ("signal (SIGHUP)");
/* Verify we are the sole owner of the tty. */
if (ioctl (STDIN_FILENO, TIOCSCTTY, 0) != 0)
end ("TIOCSCTTY");
printf("%s %s\n", tty_filename, ttyname(STDIN_FILENO));
tty_name_file = fopen(tty_filename, "w");
fprintf(tty_name_file, "%s\n", ttyname(STDIN_FILENO));
fclose(tty_name_file);
/* Disown the tty. */
if (ioctl (STDIN_FILENO, TIOCNOTTY) != 0)
end ("TIOCNOTTY");
end ("OK, disowned");
return 1;
}
I was curious what kind of buffer sizes write() and read() could handle on Linux/OSX/FreeBSD, so I started playing around with dumb programs like the following:
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
int main( void ) {
size_t s = 8*1024*1024 - 16*1024;
while( 1 ) {
s += 1024;
int f = open( "test.txt", O_CREAT | O_WRONLY | O_TRUNC, S_IRUSR | S_IWUSR | S_IXUSR );
char mem[s];
size_t written = write( f, &mem[0], s );
close( f );
printf( "(%ld) %lu\n", sizeof(size_t), written );
}
return 0;
}
This allowed me to test how close to a seeming "8MB barrier" I could get before segfaulting. Somewhere around the 8MB mark, my program dies, here's an example output:
(8) 8373248
(8) 8374272
(8) 8375296
(8) 8376320
(8) 8377344
(8) 8378368
(8) 8379392
(8) 8380416
(8) 8381440
(8) 8382464
Segmentation fault: 11
This is the same on OSX and Linux, however my FreeBSD VM is not only much faster at running this test, it also can go on for quite a ways! I've successfully tested it up to 511MB, which is just a ridiculous amount of data to write in one call.
What is it that makes the write() call segfault, and how can I figure out the maximum amount that I can possibly write() in a single call, without doing something ridiculous like I'm doing right now?
(Note, all three operating systems are 64-bit, OSX 10.7.3, Ubuntu 11.10, FreeBSD 9.0)
The fault isn't within write(), it's a stack overflow. Try this:
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>
#include <sys/stat.h>
int main( void )
{
void *mem;
size_t s = 512*1024*1024 - 16*1024;
while( 1 )
{
s += 1024;
int f = open( "test.txt", O_CREAT | O_WRONLY | O_TRUNC, S_IRUSR | S_IWUSR | S_IXUSR );
mem = malloc(s);
size_t written = write( f, mem, s );
free(mem);
close( f );
printf( "(%ld) %lu\n", sizeof(size_t), written );
}
return 0;
}
Here is a simple code to fake process name and cmdline on linux:
#include <string.h>
#include <sys/prctl.h>
#include <stdio.h>
#include <unistd.h>
#define NewName "bash"
#define ProcNameMaxLen 16
int main(int argc, char **argv){
int oldlen = strlen(*argv);
char procname[ProcNameMaxLen];
memset(*argv, 0, oldlen);
memccpy(*argv, NewName, 0, oldlen); //modify cmdline
memccpy(procname, NewName, 0, ProcNameMaxLen);
prctl(PR_SET_NAME, procname); //modify procname
sleep(60);
return 0;
}
After run this code I can't view real name by ps,
but something can find in /proc/xxx/exe and /proc/xxx/environ, but so cumbersome.
is there a good way can view real information with all process?
I think this is a big security problem because i usually check process by ps on my server.
way 1: lsof -d txt
Wait more answer...
lsof will tell you the original executable name as it is one of the open files of the malicious process. You can inspect a number of processes using the -p option, or query a single user with the -u option.