Partial write due to SIGINT - c

Question

I have a write_full which should write the buffer fully to standard output even if it was interrupted by a signal.
I have a loop which keep write_full a string until quit is changed by a signal handler. Here's the code:
#include <signal.h>
#include <unistd.h>
#include <errno.h>
volatile sig_atomic_t quit = 0;
void sigint_handler(int s)
{
quit = 1;
}
int write_full(char *buf, size_t len)
{
while (len > 0) {
ssize_t written = write(STDOUT_FILENO, buf, len);
if (written == -1) {
if (errno == EINTR) {
continue;
}
return -1;
}
buf += written;
len -= (size_t)written;
}
return 0;
}
int main(void)
{
struct sigaction act = {
.sa_handler = sigint_handler
};
sigaction(SIGINT, &act, NULL);
while (!quit) {
write_full("loop\n", 5);
}
write_full("cleanup", 7);
return 0;
}
I expect the program to write the "loop" fully before printing "cleanup", but I see an output like this:
loop
loop
loop
l^C
cleanup
Why is this happening? I expect it to be something like this:
loop
loop
loop
l^Coop
cleanup
because the write_full should continue writing the "oop\n" part even after the first write was a short write due to interrupt. I put a breakpoint at the signal handler and stepped and it seems like write is reporting that it has written 4 characters even when it only wrote "l" to the stdout. So instead of writing "oop\n" next, it only writes "\n".
l^C
Program received signal SIGINT, Interrupt.
Breakpoint 1, sigint_handler (s=2) at src/main.c:9
9 quit = 1;
(gdb) next
10 }
(gdb) next
write_full (buf=0x4020a0 "loop\n", len=5) at src/main.c:16
16 if (written == -1) {
(gdb) print written
$1 = 4
Why is this happening? And how can I fix this?

Answer 1

Ctrl-C messes up terminal output. The program writes all it should write, but by default the terminal driver cuts the line after Ctrl-C. This is not under control of your program. The cut happens if the driver sees Ctrl-C in the middle of copying of the complete line buffer to the physical device. Input line buffer is also discarded. This pertains to other signal-generating characters as well.
This is described rather briefly in the stty(1) and termios(3) manual pages.
This behaviour can be disabled with stty noflsh command.
You can also redirect to a file to see the complete output.