make: ensure sufficient space in line buffer

When using fgets(3) to read a line into a buffer it's necessary
to ensure at least two characters are available in the buffer.
Otherwise the read fails.  (At least, it did when pdpmake was
built using MSYS2 on Windows.)

Adds 16 bytes to the 32-bit build.

(GitHub pdpmake issue 44)

1 files changed, 8 insertions, 7 deletions

diff --git a/miscutils/make.c b/miscutils/make.c
index 8394c4e7b..bcba080fe 100644
--- a/miscutils/make.c
+++ b/miscutils/make.c
@@ -1627,23 +1627,24 @@ make_fgets(char *s, int size, FILE *fd)
 static char *
 readline(FILE *fd)
 {
-        char *p, *str;
+        char *p, *str = NULL;
         int pos = 0;
-        int len = 256;
-
-        str = xmalloc(len);
+        int len = 0;
 
         for (;;) {
-                if (make_fgets(str + pos, len - pos, fd) == NULL) {
+                // We need room for at least one character and a NUL terminator
+                if (len - pos > 1 &&
+                                make_fgets(str + pos, len - pos, fd) == NULL) {
                         if (pos)
                                 return str;
                         free(str);
                         return NULL;    // EOF
                 }
 
-                if ((p = strchr(str + pos, '\n')) == NULL) {
+                if (len - pos < 2 || (p = strchr(str + pos, '\n')) == NULL) {
                         // Need more room
-                        pos = len - 1;
+                        if (len)
+                                pos = len - 1;
                         len += 256;
                         str = xrealloc(str, len);
                         continue;