Finding off_t size

Question

We have been using a trick copied from the autotools to determine off_t size and whether or not we needed to define _FILE_OFFSET_BITS=64. However this trick seems to fails in recent gcc (>= 4.6). Here is the code:

#include <sys/types.h>

int main(int argc, char **argv)
{
  /* Cause a compile-time error if off_t is smaller than 64 bits */
#define LARGE_OFF_T (((off_t) 1 << 62) - 1 + ((off_t) 1 << 62))
  int off_t_is_large[ (LARGE_OFF_T % 2147483629 == 721 && LARGE_OFF_T % 2147483647 == 1) ? 1 : -1 ];  
  return 0;
}

From my debian/amd64 & debian/ppc installation here is what I get (instead of compile time error):

$ gcc-4.6 -O0 -m32 -o valid.o valid.c 
valid.c: In function ‘main’:
valid.c:7:3: warning: left shift count >= width of type [enabled by default]
valid.c:7:3: warning: left shift count >= width of type [enabled by default]
valid.c:7:3: warning: left shift count >= width of type [enabled by default]
valid.c:7:3: warning: left shift count >= width of type [enabled by default]

So my question, can I simply replaced this code with the following one:

#include <sys/types.h>

int main(int argc, char **argv)
{
  /* Cause a compile-time error if off_t is smaller than 64 bits */
  int off_t_is_large[ sizeof(off_t) >= 8 ? 1 : -1 ];  
  return 0;
}

(extra) question: is this a regression in gcc, or was the initial code just relying on a broken feature ?

Note: Suspect `(off_t) 1 << 62` is implementation defined behavior when the size of `off_t` <= 62 bits. Thus not a reliable formula. Maybe use `(off_t) (1LL << 62)`? — chux - Reinstate Monica, Mar 26 '14 at 15:57

score 3 · Accepted Answer · edited Mar 27 '14 at 09:32

3

The sub-expression (off_t) 1 << 62) runs afoul of 6.5.7p3:

    "[...] If the value of the right operand is [...] greater 
    than or equal to the width of the promoted left operand, 
    the behavior is undefined."

ref

edited Mar 27 '14 at 09:32

wRAR

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answered Mar 27 '14 at 08:04

malat

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Is `#define LARGE_OFF_T ((((off_t)1 << 31) << 31) - 1 + (((off_t)1 << 31) << 31))` valid, then? In my understanding, the individual shifts are. (`sizeof` is dependent on the amount of bits in a char and can also fail.) – mirabilos Oct 22 '16 at 23:51

Finding off_t size

1 Answers1