i have the following bit of C code which reads from a pipe and then should block but it never blocks
int pipe_fd;
int res;
int open_mode = O_RDONLY;
char buf[100];
int bytes_read = 0;
memset (buf, '\0', sizeof(buf));
pipe_fd = open(FIFO_NAME, open_mode);
if (access(FIFO_NAME, F_OK) == -1)
{
res = mkfifo(FIFO_NAME, 0777);
if (res != 0)
{
fprintf (stderr, "Could not create fifo %s\n", FIFO_NAME);
exit (EXIT_FAILURE);
}
}
for(;;)
{
do
{
res = read(pipe_fd, buf, sizeof(buf));
bytes_read += res;
}while (res > 0);
// process data then go back and block
............
}
It is sent a simple buffer by some code in a bash script like this './test 1'
#!/bin/bash
pipe=/tmp/pipe
if [[ ! -p $pipe ]]; then
echo "Reader not running"
exit 1
fi
if [[ "$1" ]]; then
echo "some string" >$pipe
else
echo "q" >$pipe
fi
I run the C code program in gdb and initially it does block on the read but as soon as i call the bash script the C code no longer blocks , it does successfully read the data from the buffer and then each time it reads there are 0 bytes read so not sure why its no longer blocking. The 'some string' data is correctly received at the other side.
I just need it to sit there waiting for data process it and then go back and wait for more
I run the C code program in gdb and initially it does block on the read but as soon as i call the bash script the C code no longer blocks , it does successfully read the data from the buffer and then each time it reads there are 0 bytes read so not sure why its no longer blocking. The 'some string' data is correctly received at the other side.
0
means EOF. FIFO can be read or written only when there are processes connected to it for both reading and writing. When there are no more writers (your shell scripts terminated) readers are notified about that through read()
returning the EOF.
FIFOs behave that way to be compatible with shell pipe logic e.g.:
$ mkfifo ./tmp1
$ cat < input > ./tmp1 &
$ cat < ./tmp1 > /dev/null
If read()
will not return EOF, the second cat
would block forever.
I just need it to sit there waiting for data process it and then go back and wait for more
In your C program you have to reopen()
the FIFO after read()
returned the EOF first time.
P.S. Found quite nice FIFO summary for your. Check the table on the second page.