[hackers] [PATCH] dd: Use sigaction(2) to obviate select(2)

[Eric Pruitt]

By setting the SIGINT handler with sigaction(2), automatic retries of
the splice(2) syscall can be disabled by not setting SA_RESTART. This
makes it possible to use Ctrl+C even if the "if" operand refers to the
controlling terminal. The SIGINT message has also been moved outside the
signal handler since fprintf(3) is not an async-signal-safe function.
---
 dd.c | 61 ++++++++++++++++++++++++++++++++-----------------------------
 1 file changed, 32 insertions(+), 29 deletions(-)

diff --git dd.c dd.c
index cc05d40..2dce98e 100644
--- dd.c
+++ dd.c
@@ -8,7 +8,6 @@
  */
 #include <sys/ioctl.h>
 #include <sys/mount.h>
-#include <sys/select.h>
 #include <sys/stat.h>
 #include <sys/time.h>
 #include <sys/types.h>
@@ -38,6 +37,15 @@ struct dd_config {
 
 static int sigint = 0;
 
+static void
+sig_int(int unused_1, siginfo_t *unused_2, void *unused_3)
+{
+       (void) unused_1;
+       (void) unused_2;
+       (void) unused_3;
+       sigint = 1;
+}
+
 static int
 prepare_copy(struct dd_config *ddc, int *ifd, int *ofd)
 {
@@ -147,7 +155,6 @@ copy_splice(struct dd_config *ddc)
        int ifd, ofd, p[2] = {-1, -1};
        ssize_t r = 0;
        size_t n = 0;
-       fd_set rfd, wfd;
 
        if (prepare_copy(ddc, &ifd, &ofd) < 0)
                return -1;
@@ -165,27 +172,24 @@ copy_splice(struct dd_config *ddc)
 #endif
        n = ddc->bs;
        for (;ddc->b_out != ddc->count && !sigint;) {
-               FD_ZERO(&rfd);
-               FD_ZERO(&wfd);
-               FD_SET(ifd, &rfd);
-               FD_SET(ofd, &wfd);
-               r = select(ifd > ofd ? ifd + 1 : ofd + 1, &rfd, &wfd, NULL, 
NULL);
                if (r < 0)
                        break;
-               if (FD_ISSET(ifd, &rfd) == 1 && FD_ISSET(ofd, &wfd) == 1) {
-                       if (n > ddc->count - ddc->b_out)
-                               n = ddc->count - ddc->b_out;
-                       r = splice(ifd, NULL, p[1], NULL, n, SPLICE_F_MORE);
-                       if (r <= 0)
-                               break;
-                       ++ddc->rec_in;
-                       r = splice(p[0], NULL, ofd, NULL, r, SPLICE_F_MORE);
-                       if (r <= 0)
-                               break;
-                       ddc->b_out += r;
-                       ++ddc->rec_out;
-               }
+               if (n > ddc->count - ddc->b_out)
+                       n = ddc->count - ddc->b_out;
+               r = splice(ifd, NULL, p[1], NULL, n, SPLICE_F_MORE);
+               if (r <= 0)
+                       break;
+               ++ddc->rec_in;
+               r = splice(p[0], NULL, ofd, NULL, r, SPLICE_F_MORE);
+               if (r <= 0)
+                       break;
+               ddc->b_out += r;
+               ++ddc->rec_out;
        }
+
+       if (sigint)
+               fprintf(stderr, "SIGINT! Aborting ...\n");
+
        close(ifd);
        close(ofd);
        close(p[0]);
@@ -227,14 +231,6 @@ print_stat(const struct dd_config *ddc)
 }
 
 static void
-sig_int(int unused)
-{
-       (void) unused;
-       fprintf(stderr, "SIGINT! Aborting ...\n");
-       sigint = 1;
-}
-
-static void
 usage(void)
 {
        eprintf("usage: %s [-h] [if=infile] [of=outfile] [bs[=N]] [seek=N] "
@@ -248,6 +244,7 @@ main(int argc, char *argv[])
        int i = 0;
        char buf[1024];
        struct dd_config config;
+       struct sigaction sa;
 
        argv0 = argv[0];
        memset(&config, 0, sizeof(config));
@@ -286,7 +283,13 @@ main(int argc, char *argv[])
        }
 
        signal(SIGPIPE, SIG_IGN);
-       signal(SIGINT, sig_int);
+
+       sa.sa_flags = SA_SIGINFO;
+       sigemptyset(&sa.sa_mask);
+       sa.sa_sigaction = sig_int;
+
+       if (sigaction(SIGINT, &sa, NULL) == -1)
+               weprintf("sigaction");
 
        if (copy(&config) < 0)
                weprintf("copy:");
-- 
2.11.0