rc: avoid undefined C
There are two bugs in pdec() on INT_MIN:
* wrong output.
`n = 1-n' should be `n = -1-n' when n is INT_MIN.
* infinite loop.
gcc optimizes `if(n>=0)' into `if(true)' because `-INT_MIN' (signed integer overflow) is undefined behavior in C, and gcc assumes the negation of a negative number must be positive. The resulting binary keeps printing '-' forever given INT_MIN.
Try the simplified pdec.c below.
$ gcc pdec.c
$ ./a.out -2147483648
--214748364*
$ gcc pdec.c -O2
$ ./a.out -2147483648
<infinite loop>
$ gcc pdec.c -O2 -D__PATCH__
$ ./a.out -2147483648
-2147483648
=== pdec.c ===
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
#define io void
void pchr(io *f, int c)
{
putchar(c);
}
void pdec(io *f, int n)
{
if(n<0){
#ifndef __PATCH__
n=-n;
if(n>=0){
pchr(f, '-');
pdec(f, n);
return;
}
/* n is two's complement minimum integer */
n = 1-n;
#else
if(n!=INT_MIN){
pchr(f, '-');
pdec(f, -n);
return;
}
/* n is two's complement minimum integer */
n = -(INT_MIN+1);
#endif
pchr(f, '-');
pdec(f, n/10);
pchr(f, n%10+'1');
return;
}
if(n>9)
pdec(f, n/10);
pchr(f, n%10+'0');
}
int main(int argc, char **argv)
{
int n = atoi(argv[1]);
pdec(NULL, n);
putchar('\n');
}
R=rsc
CC=plan9port.codebot
https://codereview.appspot.com/7241055
diff --git a/src/cmd/rc/io.c b/src/cmd/rc/io.c
index f242001..bb8af4a 100644
--- a/src/cmd/rc/io.c
+++ b/src/cmd/rc/io.c
@@ -1,3 +1,4 @@
+#include <limits.h>
#include "rc.h"
#include "exec.h"
#include "io.h"
@@ -119,14 +120,13 @@
pdec(io *f, int n)
{
if(n<0){
- n=-n;
- if(n>=0){
+ if(n!=INT_MIN){
pchr(f, '-');
- pdec(f, n);
+ pdec(f, -n);
return;
}
/* n is two's complement minimum integer */
- n = 1-n;
+ n = -(INT_MIN+1);
pchr(f, '-');
pdec(f, n/10);
pchr(f, n%10+'1');
