clausecker · October 17, 2019 14:41 · Oct 17, 2019
diff --git a/wc.c b/wc.c
@@ -0,0 +1,132 @@
+/* parallel wc(1) demo program */
+/* cc -O3 -fopenmp -o wc wc.c */
+#include <ctype.h>
+#include <unistd.h>
+#include <omp.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <stdbool.h>
+#include <sys/stat.h>
+
+#define BUFSIZE (128 * 1024)
+
+/*
+ * Count chars, words, and lines for one segment of the input file.
+ * Track if the first and last characters of the segment were
+ * spaces.  We count words on a blank-nonblank transition.  If the
+ * first character of a segment is not a blank, it does not count
+ * as a word.
+ */
+struct counter {
+	unsigned long chars, words, lines;
+	bool firstblank, lastblank;
+};
+
+static void
+reduce(struct counter *restrict totals, const struct counter *restrict segments, size_t nthreads)
+{
+	size_t i;
+
+	totals->chars = 0;
+	totals->words = 0;
+	totals->lines = 0;
+	totals->lastblank = true;
+
+	for (i = 0; i < nthreads; i++) {
+		if (segments[i].chars == 0)
+			continue;
+
+		totals->chars += segments[i].chars;
+		totals->words += segments[i].words + (totals->lastblank & !segments[i].firstblank);
+		totals->lines += segments[i].lines;
+		totals->lastblank = segments[i].lastblank;
+	}
+}
+
+static void
+dosegment(struct counter *segments, int fd, int tid, int nthreads, off_t filesize)
+{
+	struct counter *seg = segments + tid;
+	off_t start, end;
+	size_t want, i;
+	ssize_t got;
+	unsigned long chars = 0, words = 0, lines = 0;
+	char *buf;
+	bool first = true, blank = false, nblank;
+
+	buf = malloc(BUFSIZE);
+	if (buf == NULL) {
+		perror("malloc");
+		exit(EXIT_FAILURE);
+	}
+
+	start = filesize * ((double)tid / nthreads);
+	end = filesize * ((tid + 1.0) / nthreads);
+
+	while (start < end) {
+		want = end - start < BUFSIZE ? end - start : BUFSIZE;
+		got = pread(fd, buf, want, start);
+		if (got < 0) {
+			perror("pread");
+			exit(EXIT_FAILURE);
+		}
+
+		if (got == 0) {
+			fprintf(stderr, "file truncated during reading\n");
+			exit(EXIT_FAILURE);
+		}
+
+		if (first) {
+			seg->firstblank = isspace(buf[0]);
+			first = 0;
+		}
+
+		chars += got;
+		start += got;
+		for (i = 0; i < got; i++) {
+			lines += buf[i] == '\n';
+			nblank = isspace(buf[i]);
+			words += blank & !nblank;
+			blank = nblank;
+		}
+	}
+
+	seg->chars = chars;
+	seg->lines = lines;
+	seg->words = words;
+	seg->lastblank = blank;
+}
+
+extern int
+main(void)
+{
+	struct stat st;
+	struct counter totals, *segments;
+	size_t nthreads;
+
+	/* check if we can seek the input */
+	if (fstat(STDIN_FILENO, &st) != 0) {
+		perror("fstat");
+		return (EXIT_FAILURE);
+	}
+
+	if (!S_ISREG(st.st_mode)) {
+		fprintf(stderr, "not a regular file\n");
+		return (EXIT_FAILURE);
+	}
+
+	nthreads = omp_get_max_threads();
+	segments = calloc(nthreads, sizeof *segments);
+	if (segments == NULL) {
+		perror("calloc");
+		return (EXIT_FAILURE);
+	}
+
+#	pragma omp parallel
+	dosegment(segments, STDIN_FILENO, omp_get_thread_num(), omp_get_num_threads(), st.st_size);
+
+	reduce(&totals, segments, nthreads);
+	printf("%lu %lu %lu\n", totals.lines, totals.words, totals.chars);
+
+	return (EXIT_SUCCESS);
+}