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Date: Fri, 7 Apr 2017 14:22:46 +0000
From: Pascal Cuoq <>
To: "" <>
Subject: Undefined behavior in sprintf(dest, "%lld", -1LL)

I am running musl's implementation inside tis-interpreter, a C interpreter that detects a very wide palette of undefined behaviors.

When musl's printf implementation is invoked through the following test driver:

  char dest[100];
  int x = sprintf(dest, "%lld\n", -1LL);

tis-interpreter emits this warning:

src/stdio/vfprintf.c:141: warning: although the type provided to the va_arg macro (unsigned long long) and the actual type of the next variadic argument (long long) are corresponding unsigned - signed variants of the same integer type, the actual value of the argument (signed) is negative thus it cannot be represented in the provided (unsigned) type

The warning of course refers to C11

To get a clearer view, I instrumented the code thus using the debug function tis_printf (we are inside the implementation of printf, it is a bad idea to call printf here):

--- ../musl-1.1.16/src/stdio/vfprintf.c 2017-01-01 04:27:17.000000000 +0100
+++ src/stdio/vfprintf.c 2017-04-07 16:13:19.291592002 +0200
@@ -547,7 +547,7 @@
  if (argpos>=0) goto inval;
  } else {
  if (argpos>=0) nl_type[argpos]=st, arg=nl_arg[argpos];
- else if (f) pop_arg(&arg, st, ap);
+ else if (f) { tis_printf("st:%u LLONG:%u ULLONG:%u\n", st, LLONG, ULLONG); pop_arg(&arg, st, ap); }
  else return 0;

This shows the debug message below:

st:12 LLONG:12 ULLONG:12

On the basis of this result, I would tentatively offer that the undefined behavior that tis-interpreter warns about is real and is caused by line 55 in vfprintf.c:


Because of this approach, when LLONG is passed to the function pop_arg(), the switch case at line 141 is taken:

break; case ULLONG: arg->i = va_arg(*ap, unsigned long long);

This causes UB by consuming a (negative) long long argument from a va_list with va_arg(..., unsigned long long).


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