grep: fix -o match with empty string (suggested by Colin Watson <cjwatson@ubuntu.com>)

function                                             old     new   delta
grep_file                                           1216    1251     +35

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

2 files changed, 11 insertions, 3 deletions

diff --git a/findutils/grep.c b/findutils/grep.c
index 688ea6ab2..024f27609 100644
--- a/findutils/grep.c
+++ b/findutils/grep.c
@@ -461,15 +461,19 @@ static int grep_file(FILE *file)
                                                 if (found)
                                                         print_line(gl->pattern, strlen(gl->pattern), linenum, ':');
                                         } else while (1) {
+                                                unsigned start = gl->matched_range.rm_so;
                                                 unsigned end = gl->matched_range.rm_eo;
+                                                unsigned len = end - start;
                                                 char old = line[end];
                                                 line[end] = '\0';
-                                                print_line(line + gl->matched_range.rm_so,
-                                                                end - gl->matched_range.rm_so,
-                                                                linenum, ':');
+                                                /* Empty match is not printed: try "echo test | grep -o ''" */
+                                                if (len != 0)
+                                                        print_line(line + start, len, linenum, ':');
                                                 if (old == '\0')
                                                         break;
                                                 line[end] = old;
+                                                if (len == 0)
+                                                        end++;
 #if !ENABLE_EXTRA_COMPAT
                                                 if (regexec(&gl->compiled_regex, line + end,
                                                                 1, &gl->matched_range, REG_NOTBOL) != 0)
diff --git a/testsuite/grep.tests b/testsuite/grep.tests
index 520a1840f..ffce033e6 100755
--- a/testsuite/grep.tests
+++ b/testsuite/grep.tests
@@ -98,5 +98,9 @@ testing "grep -o does not loop forever" \
         'grep -o "[^/]*$"' \
         "test\n" \
         "" "/var/test\n"
+testing "grep -o does not loop forever on zero-length match" \
+        'grep -o "" | head -n1' \
+        "" \
+        "" "test\n"
 
 exit $FAILCOUNT