[Bug c/79082] -Wformat-truncation inconsistent behaviour

[sirl at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79082

--- Comment #6 from Franz Sirl <sirl at gcc dot gnu.org> ---
Created attachment 40566
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40566&action=edit
extended testcase

Hmm, looks like there is an off-by-one bug lurking here?
To clarify my setup, here are the warnings for the attached testcase on
Linux/x86-64 (SLES12SP2) with a bootstrapped r244773 compiler.

First, gcc-trunk -c -Wformat-truncation test-snprintf-2.c -O0:
test-snprintf-2.c: In function 'test':
test-snprintf-2.c:6:23: warning: '%2d' directive output may be truncated
writing between 2 and 11 bytes into a region of size 3 [-Wformat-truncation=]
  snprintf(buffer, 3, "%2d", val % 100); /* 1 */ /* should warn */
                       ^~~
test-snprintf-2.c:6:22: note: using the range [1, -2147483648] for directive
argument
  snprintf(buffer, 3, "%2d", val % 100); /* 1 */ /* should warn */
                      ^~~~~
test-snprintf-2.c:6:2: note: format output between 3 and 12 bytes into a
destination of size 3
  snprintf(buffer, 3, "%2d", val % 100); /* 1 */ /* should warn */
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test-snprintf-2.c:7:23: warning: '%2hhd' directive output may be truncated
writing between 2 and 4 bytes into a region of size 3 [-Wformat-truncation=]
  snprintf(buffer, 3, "%2hhd", val % 100); /* 2 */ /* should warn */
                       ^~~~~
test-snprintf-2.c:7:22: note: using the range [1, -128] for directive argument
  snprintf(buffer, 3, "%2hhd", val % 100); /* 2 */ /* should warn */
                      ^~~~~~~
test-snprintf-2.c:7:2: note: format output between 3 and 5 bytes into a
destination of size 3
  snprintf(buffer, 3, "%2hhd", val % 100); /* 2 */ /* should warn */
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test-snprintf-2.c:9:23: warning: '%2d' directive output may be truncated
writing between 2 and 11 bytes into a region of size 3 [-Wformat-truncation=]
  snprintf(buffer, 3, "%2d", (val < 0) ? -(val % 100): val % 100); /* 3 */ /*
should not warn */
                       ^~~
test-snprintf-2.c:9:22: note: using the range [1, -2147483648] for directive
argument
  snprintf(buffer, 3, "%2d", (val < 0) ? -(val % 100): val % 100); /* 3 */ /*
should not warn */
                      ^~~~~
test-snprintf-2.c:9:2: note: format output between 3 and 12 bytes into a
destination of size 3
  snprintf(buffer, 3, "%2d", (val < 0) ? -(val % 100): val % 100); /* 3 */ /*
should not warn */
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test-snprintf-2.c:10:23: warning: '%2d' directive output may be truncated
writing between 2 and 11 bytes into a region of size 3 [-Wformat-truncation=]
  snprintf(buffer, 3, "%2d", __builtin_abs(val % 100)); /* 4 */ /* should not
warn */
                       ^~~
test-snprintf-2.c:10:22: note: using the range [1, -2147483648] for directive
argument
  snprintf(buffer, 3, "%2d", __builtin_abs(val % 100)); /* 4 */ /* should not
warn */
                      ^~~~~
test-snprintf-2.c:10:2: note: format output between 3 and 12 bytes into a
destination of size 3
  snprintf(buffer, 3, "%2d", __builtin_abs(val % 100)); /* 4 */ /* should not
warn */
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test-snprintf-2.c:12:23: warning: '%2x' directive output may be truncated
writing between 2 and 8 bytes into a region of size 3 [-Wformat-truncation=]
  snprintf(buffer, 3, "%2x", val & 0xff); /* 5 */ /* should not warn */
                       ^~~
test-snprintf-2.c:12:22: note: using the range [1, 4294967295] for directive
argument
  snprintf(buffer, 3, "%2x", val & 0xff); /* 5 */ /* should not warn */
                      ^~~~~
test-snprintf-2.c:12:2: note: format output between 3 and 9 bytes into a
destination of size 3
  snprintf(buffer, 3, "%2x", val & 0xff); /* 5 */ /* should not warn */
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test-snprintf-2.c:13:23: warning: '%03x' directive output may be truncated
writing between 3 and 8 bytes into a region of size 4 [-Wformat-truncation=]
  snprintf(buffer, 4, "%03x", val & 0xfff); /* 6 */ /* should not warn */
                       ^~~~
test-snprintf-2.c:13:22: note: using the range [1, 4294967295] for directive
argument
  snprintf(buffer, 4, "%03x", val & 0xfff); /* 6 */ /* should not warn */
                      ^~~~~~
test-snprintf-2.c:13:2: note: format output between 4 and 9 bytes into a
destination of size 4
  snprintf(buffer, 4, "%03x", val & 0xfff); /* 6 */ /* should not warn */
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

No idea why 8 (and maybe 7?) does not warn on my machine, certainly the buffer
is too small for anything >= 0x1000.


Second, gcc-trunk -c -Wformat-truncation test-snprintf-2.c -O2:
test-snprintf-2.c: In function 'test':
test-snprintf-2.c:6:26: warning: output may be truncated before the last format
character [-Wformat-truncation=]
  snprintf(buffer, 3, "%2d", val % 100); /* 1 */ /* should warn */
                       ~~~^
test-snprintf-2.c:6:2: note: format output between 3 and 4 bytes into a
destination of size 3
  snprintf(buffer, 3, "%2d", val % 100); /* 1 */ /* should warn */
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test-snprintf-2.c:7:28: warning: output may be truncated before the last format
character [-Wformat-truncation=]
  snprintf(buffer, 3, "%2hhd", val % 100); /* 2 */ /* should warn */
                       ~~~~~^
test-snprintf-2.c:7:2: note: format output between 3 and 4 bytes into a
destination of size 3
  snprintf(buffer, 3, "%2hhd", val % 100); /* 2 */ /* should warn */
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
test-snprintf-2.c:9:26: warning: output may be truncated before the last format
character [-Wformat-truncation=]
  snprintf(buffer, 3, "%2d", (val < 0) ? -(val % 100): val % 100); /* 3 */ /*
should not warn */
                       ~~~^
test-snprintf-2.c:9:2: note: format output between 3 and 4 bytes into a
destination of size 3
  snprintf(buffer, 3, "%2d", (val < 0) ? -(val % 100): val % 100); /* 3 */ /*
should not warn */
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Also here, no warning for 8?