gcc/gcc
GCC Administrator 57ea089421 Daily bump.
2020-08-27 00:16:27 +00:00
..
ada
analyzer Daily bump. 2020-08-27 00:16:27 +00:00
brig
c Daily bump. 2020-08-26 00:16:32 +00:00
c-family
common x86: Detect Rocket Lake and Alder Lake 2020-08-19 05:03:31 -07:00
config x86: Reject target("no-general-regs-only") 2020-08-26 12:56:18 -07:00
cp Daily bump. 2020-08-27 00:16:27 +00:00
d Daily bump. 2020-08-27 00:16:27 +00:00
doc Fix documentation of -fprofile-exclude-files 2020-08-26 17:07:36 -06:00
fortran Daily bump. 2020-08-27 00:16:27 +00:00
ginclude
go cmd: add -maix32 to gcc calls for aix/ppc 2020-08-26 16:29:03 -07:00
jit Daily bump. 2020-08-23 00:16:25 +00:00
lto
objc
objcp
po
testsuite Daily bump. 2020-08-27 00:16:27 +00:00
ABOUT-GCC-NLS
acinclude.m4
aclocal.m4
addresses.h
adjust-alignment.c
alias.c
alias.h
align.h
alloc-pool.c
alloc-pool.h
array-traits.h
asan.c
asan.h
attribs.c
attribs.h
auto-inc-dec.c
auto-profile.c
auto-profile.h
backend.h
BASE-VER
basic-block.h
bb-reorder.c
bb-reorder.h
bitmap.c
bitmap.h
brig-builtins.def
builtin-attrs.def
builtin-types.def
builtins.c
builtins.def
builtins.h
caller-save.c
calls.c
calls.h
ccmp.c
ccmp.h
cfg-flags.def
cfg.c
cfg.h
cfganal.c
cfganal.h
cfgbuild.c
cfgbuild.h
cfgcleanup.c
cfgcleanup.h
cfgexpand.c
cfgexpand.h
cfghooks.c
cfghooks.h
cfgloop.c
cfgloop.h
cfgloopanal.c
cfgloopmanip.c
cfgloopmanip.h
cfgrtl.c
cfgrtl.h
cgraph.c
cgraph.h
cgraphbuild.c
cgraphclones.c
cgraphunit.c symver: fix attribute matching. 2020-08-26 15:42:31 +02:00
ChangeLog Daily bump. 2020-08-27 00:16:27 +00:00
ChangeLog-1997
ChangeLog-1998
ChangeLog-1999
ChangeLog-2000
ChangeLog-2001
ChangeLog-2002
ChangeLog-2003
ChangeLog-2004
ChangeLog-2005
ChangeLog-2006
ChangeLog-2007
ChangeLog-2008
ChangeLog-2009
ChangeLog-2010
ChangeLog-2011
ChangeLog-2012
ChangeLog-2013
ChangeLog-2014
ChangeLog-2015
ChangeLog-2016
ChangeLog-2017
ChangeLog-2018
ChangeLog-2019
ChangeLog.dataflow
ChangeLog.gimple-classes
ChangeLog.graphite
ChangeLog.jit
ChangeLog.lib
ChangeLog.ptr
ChangeLog.tree-ssa
ChangeLog.tuples
cif-code.def
collect2-aix.c
collect2-aix.h
collect2.c
collect2.h
collect-utils.c
collect-utils.h
color-macros.h
combine-stack-adj.c
combine.c
common.md
common.opt Fix documentation of -fprofile-exclude-files 2020-08-26 17:07:36 -06:00
compare-elim.c
conditions.h
config.build
config.gcc
config.host
config.in
configure
configure.ac
context.c
context.h
convert.c
convert.h
COPYING
COPYING3
COPYING3.LIB
COPYING.LIB
coretypes.h
coroutine-builtins.def
coroutine-passes.cc
coverage.c
coverage.h
cppbuiltin.c
cppbuiltin.h
cppdefault.c
cppdefault.h
cprop.c
cse.c
cselib.c
cselib.h
cstamp-h.in
data-streamer-in.c
data-streamer-out.c
data-streamer.c
data-streamer.h
DATESTAMP Daily bump. 2020-08-27 00:16:27 +00:00
dbgcnt.c
dbgcnt.def
dbgcnt.h
dbxout.c
dbxout.h
dce.c
dce.h
ddg.c
ddg.h
debug.c
debug.h
defaults.h
DEV-PHASE
df-core.c
df-problems.c
df-scan.c
df.h
dfp.c
dfp.h
diagnostic-color.c
diagnostic-color.h
diagnostic-core.h
diagnostic-event-id.h
diagnostic-format-json.cc
diagnostic-metadata.h
diagnostic-path.h
diagnostic-show-locus.c
diagnostic-url.h
diagnostic.c
diagnostic.def
diagnostic.h
digraph.cc
digraph.h
dojump.c
dojump.h
dominance.c
dominance.h
domwalk.c
domwalk.h
double-int.c
double-int.h
dse.c
dump-context.h
dumpfile.c
dumpfile.h
dwarf2asm.c
dwarf2asm.h
dwarf2cfi.c
dwarf2out.c dwarf2out: Fix up dwarf2out_next_real_insn caching [PR96729] 2020-08-26 10:30:15 +02:00
dwarf2out.h
early-remat.c
edit-context.c
edit-context.h
emit-rtl.c Allow try_split to split RTX_FRAME_RELATED_P insns 2020-08-21 17:59:40 +05:30
emit-rtl.h
errors.c
errors.h
escaped_string.h
et-forest.c
et-forest.h
except.c
except.h
exec-tool.in
explow.c
explow.h
expmed.c
expmed.h
expr.c PR middle-end/96665 - memcmp of a constant string not folded 2020-08-18 12:59:09 -06:00
expr.h
fibonacci_heap.c
fibonacci_heap.h
file-find.c
file-find.h
file-prefix-map.c
file-prefix-map.h
final.c
fixed-value.c
fixed-value.h
flag-types.h
flags.h
fold-const-call.c
fold-const-call.h
fold-const.c middle-end: PR tree-optimization/21137: STRIP_NOPS avoids missed optimization. 2020-08-25 10:52:06 +01:00
fold-const.h
fp-test.c
FSFChangeLog
FSFChangeLog.10
FSFChangeLog.11
function-abi.cc
function-abi.h
function-tests.c
function.c
function.h
fwprop.c
gcc-ar.c
gcc-main.c
gcc-plugin.h
gcc-rich-location.c
gcc-rich-location.h
gcc-symtab.h
gcc.c driver: Fix several memory leaks [PR63854] 2020-08-21 14:26:11 +01:00
gcc.h
gcov-counter.def
gcov-dump.c
gcov-io.c
gcov-io.h
gcov-iov.c
gcov-tool.c
gcov.c
gcse-common.c
gcse-common.h
gcse.c
gcse.h
gdbasan.in
gdbhooks.py
gdbinit.in
gen-pass-instances.awk
genattr-common.c
genattr.c
genattrtab.c
genautomata.c
gencfn-macros.c
gencheck.c
genchecksum.c
gencodes.c
genconditions.c
genconfig.c
genconstants.c
genemit.c
genenums.c
generic-match-head.c
generic-match.h
genextract.c
genflags.c
gengenrtl.c
gengtype-lex.l
gengtype-parse.c
gengtype-state.c
gengtype.c
gengtype.h
genhooks.c
genmatch.c
genmddeps.c
genmddump.c
genmodes.c
genmultilib
genopinit.c
genoutput.c
genpeep.c
genpreds.c
genrecog.c
gensupport.c
gensupport.h
gentarget-def.c
ggc-common.c
ggc-internal.h
ggc-none.c
ggc-page.c
ggc-tests.c
ggc.h
gimple-array-bounds.cc
gimple-array-bounds.h
gimple-builder.c
gimple-builder.h
gimple-expr.c
gimple-expr.h
gimple-fold.c Fix PR tree-optimization/96670 - ICE on memchr with an empty initializer. 2020-08-18 12:49:35 -06:00
gimple-fold.h
gimple-isel.cc
gimple-iterator.c
gimple-iterator.h
gimple-laddress.c
gimple-loop-interchange.cc
gimple-loop-jam.c
gimple-loop-versioning.cc
gimple-low.c
gimple-low.h
gimple-match-head.c
gimple-match.h
gimple-predict.h
gimple-pretty-print.c
gimple-pretty-print.h
gimple-ssa-backprop.c
gimple-ssa-evrp-analyze.c
gimple-ssa-evrp-analyze.h
gimple-ssa-evrp.c
gimple-ssa-isolate-paths.c
gimple-ssa-nonnull-compare.c
gimple-ssa-split-paths.c
gimple-ssa-sprintf.c
gimple-ssa-store-merging.c
gimple-ssa-strength-reduction.c
gimple-ssa-warn-alloca.c
gimple-ssa-warn-restrict.c
gimple-ssa-warn-restrict.h
gimple-ssa.h
gimple-streamer-in.c
gimple-streamer-out.c
gimple-streamer.h
gimple-walk.c
gimple-walk.h
gimple.c gimple: Ignore *0 = {CLOBBER} in path isolation [PR96722] 2020-08-25 13:49:40 +02:00
gimple.def
gimple.h
gimplify-me.c
gimplify-me.h
gimplify.c
gimplify.h
glimits.h
godump.c
graph.c
graph.h
graphds.c
graphds.h
graphite-dependences.c
graphite-isl-ast-to-gimple.c
graphite-optimize-isl.c
graphite-poly.c
graphite-scop-detection.c
graphite-sese-to-poly.c
graphite.c
graphite.h
graphviz.cc
graphviz.h
gsstruct.def
gstab.h
gsyms.h
gsyslimits.h
gtm-builtins.def
haifa-sched.c
hard-reg-set.h
hash-map-tests.c
hash-map-traits.h
hash-map.h
hash-set-tests.c
hash-set.h
hash-table.c
hash-table.h
hash-traits.h
highlev-plugin-common.h
hooks.c
hooks.h
host-default.c
hosthooks-def.h
hosthooks.h
hw-doloop.c
hw-doloop.h
hwint.c
hwint.h
ifcvt.c
ifcvt.h
inchash.c
inchash.h
incpath.c
incpath.h
init-regs.c
input.c
input.h
insn-addr.h
insn-notes.def
int-vector-builder.h
internal-fn.c
internal-fn.def
internal-fn.h
intl.c
intl.h
ipa-comdats.c
ipa-cp.c
ipa-devirt.c
ipa-fnsummary.c
ipa-fnsummary.h
ipa-icf-gimple.c
ipa-icf-gimple.h
ipa-icf.c
ipa-icf.h
ipa-inline-analysis.c
ipa-inline-transform.c
ipa-inline.c
ipa-inline.h
ipa-param-manipulation.c
ipa-param-manipulation.h
ipa-polymorphic-call.c
ipa-predicate.c
ipa-predicate.h
ipa-profile.c
ipa-prop.c
ipa-prop.h
ipa-pure-const.c
ipa-ref.c
ipa-ref.h
ipa-reference.c
ipa-reference.h
ipa-split.c
ipa-sra.c
ipa-utils.c
ipa-utils.h
ipa-visibility.c
ipa.c
ira-build.c
ira-color.c
ira-conflicts.c
ira-costs.c
ira-emit.c
ira-int.h
ira-lives.c
ira.c
ira.h
is-a.h
json.cc
json.h
jump.c
langhooks-def.h
langhooks.c
langhooks.h
LANGUAGES
lcm.c
lcm.h
libfuncs.h
limitx.h
limity.h
lists.c
lock-and-run.sh
loop-doloop.c
loop-init.c
loop-invariant.c
loop-iv.c
loop-unroll.c
loop-unroll.h
lower-subreg.c
lower-subreg.h
lra-assigns.c
lra-coalesce.c
lra-constraints.c
lra-eliminations.c
lra-int.h
lra-lives.c
lra-remat.c
lra-spills.c
lra.c
lra.h
lto-cgraph.c
lto-compress.c
lto-compress.h
lto-opts.c
lto-section-in.c
lto-section-names.h
lto-section-out.c
lto-streamer-in.c
lto-streamer-out.c
lto-streamer.c
lto-streamer.h
lto-wrapper.c
machmode.def
machmode.h
main.c
Makefile.in
match.pd match.pd: Simplify copysign (x, -x) to -x [PR96715] 2020-08-25 07:21:26 +02:00
mcf.c
mem-stats-traits.h
mem-stats.h
memmodel.h
memory-block.cc
memory-block.h
mkconfig.sh
mode-classes.def
mode-switching.c
modulo-sched.c
multiple_target.c
omp-builtins.def
omp-expand.c
omp-expand.h
omp-general.c
omp-general.h
omp-low.c
omp-low.h
omp-offload.c
omp-offload.h
omp-simd-clone.c
omp-simd-clone.h
ONEWS
opt-functions.awk
opt-gather.awk
opt-include.awk
opt-problem.cc
opt-problem.h
opt-read.awk
opt-suggestions.c
opt-suggestions.h
optabs-libfuncs.c
optabs-libfuncs.h
optabs-query.c
optabs-query.h
optabs-tree.c
optabs-tree.h
optabs.c
optabs.def
optabs.h
optc-gen.awk
optc-save-gen.awk
opth-gen.awk
optinfo-emit-json.cc
optinfo-emit-json.h
optinfo.cc
optinfo.h
opts-common.c
opts-diagnostic.h
opts-global.c options: Make --help= see overridden values 2020-08-18 21:37:39 -05:00
opts.c
opts.h
ordered-hash-map-tests.cc
ordered-hash-map.h
output.h
params.opt
pass_manager.h
passes.c
passes.def
plugin.c
plugin.def
plugin.h
poly-int-types.h
poly-int.h
postreload-gcse.c
postreload.c
predict.c
predict.def
predict.h
prefix.c
prefix.h
pretty-print.c
pretty-print.h
print-rtl-function.c
print-rtl.c
print-rtl.h
print-tree.c
print-tree.h
profile-count.c
profile-count.h
profile.c
profile.h
range-op.cc
range-op.h
range.cc
range.h
read-md.c
read-md.h
read-rtl-function.c
read-rtl-function.h
read-rtl.c
README.Portability
real.c
real.h
realmpfr.c
realmpfr.h
recog.c Allow try_split to split RTX_FRAME_RELATED_P insns 2020-08-21 17:59:40 +05:30
recog.h Allow try_split to split RTX_FRAME_RELATED_P insns 2020-08-21 17:59:40 +05:30
ree.c
reg-notes.def
reg-stack.c
regcprop.c
regcprop.h
reginfo.c
regrename.c
regrename.h
regs.h
regset.h
regstat.c
reload1.c
reload.c
reload.h
reorg.c reorg.c (fill_slots_from_thread): Improve for TARGET_FLAGS_REGNUM 2020-08-24 03:15:21 +02:00
resource.c
resource.h
rtl-error.c
rtl-error.h
rtl-iter.h
rtl-tests.c
rtl.c
rtl.def Fix a typo in rtl.def 2020-08-25 11:41:24 +01:00
rtl.h
rtlanal.c
rtlhash.c
rtlhash.h
rtlhooks-def.h
rtlhooks.c
rtx-vector-builder.c
rtx-vector-builder.h
run-rtl-passes.c
run-rtl-passes.h
sancov.c
sanitizer.def
sanopt.c
sbitmap.c
sbitmap.h
sched-deps.c
sched-ebb.c
sched-int.h
sched-rgn.c
sel-sched-dump.c
sel-sched-dump.h
sel-sched-ir.c
sel-sched-ir.h
sel-sched.c
sel-sched.h
selftest-diagnostic.c
selftest-diagnostic.h
selftest-rtl.c
selftest-rtl.h
selftest-run-tests.c
selftest.c
selftest.h
sese.c
sese.h
shortest-paths.h
shrink-wrap.c
shrink-wrap.h
signop.h
simplify-rtx.c
sort.cc
sparseset.c
sparseset.h
spellcheck-tree.c
spellcheck-tree.h
spellcheck.c
spellcheck.h
sreal.c
sreal.h
ssa-iterators.h
ssa.h
stab.def
stack-ptr-mod.c
statistics.c
statistics.h
stmt.c
stmt.h
stor-layout.c
stor-layout.h
store-motion.c
streamer-hooks.c
streamer-hooks.h
stringpool.c
stringpool.h
substring-locations.c
substring-locations.h
symbol-summary.h
symtab.c
sync-builtins.def
system.h
target-def.h
target-globals.c
target-globals.h
target-hooks-macros.h
target-insns.def
target.def
target.h
targhooks.c
targhooks.h
timevar.c
timevar.def
timevar.h
toplev.c
toplev.h
tracer.c
tracer.h
trans-mem.c
trans-mem.h
tree-affine.c
tree-affine.h
tree-call-cdce.c
tree-cfg.c
tree-cfg.h
tree-cfgcleanup.c
tree-cfgcleanup.h
tree-chrec.c
tree-chrec.h
tree-complex.c
tree-core.h
tree-data-ref.c
tree-data-ref.h
tree-dfa.c
tree-dfa.h
tree-diagnostic-path.cc
tree-diagnostic.c
tree-diagnostic.h
tree-dump.c
tree-dump.h
tree-eh.c
tree-eh.h
tree-emutls.c
tree-hash-traits.h
tree-hasher.h
tree-if-conv.c
tree-if-conv.h
tree-inline.c
tree-inline.h
tree-into-ssa.c
tree-into-ssa.h
tree-iterator.c
tree-iterator.h
tree-loop-distribution.c
tree-nested.c
tree-nested.h
tree-nrv.c
tree-object-size.c
tree-object-size.h
tree-outof-ssa.c
tree-outof-ssa.h
tree-parloops.c
tree-parloops.h
tree-pass.h
tree-phinodes.c
tree-phinodes.h
tree-predcom.c
tree-pretty-print.c
tree-pretty-print.h
tree-profile.c
tree-scalar-evolution.c
tree-scalar-evolution.h
tree-sra.c sra: Bail out when encountering accesses with negative offsets (PR 96730) 2020-08-25 13:33:44 +02:00
tree-sra.h
tree-ssa-address.c
tree-ssa-address.h
tree-ssa-alias.c
tree-ssa-alias.h
tree-ssa-ccp.c
tree-ssa-ccp.h
tree-ssa-coalesce.c
tree-ssa-coalesce.h
tree-ssa-copy.c
tree-ssa-dce.c
tree-ssa-dce.h
tree-ssa-dom.c Adjust tree-ssa-dom.c for irange API. 2020-08-26 16:03:00 +02:00
tree-ssa-dom.h
tree-ssa-dse.c
tree-ssa-dse.h
tree-ssa-forwprop.c
tree-ssa-ifcombine.c
tree-ssa-live.c
tree-ssa-live.h
tree-ssa-loop-ch.c
tree-ssa-loop-im.c tree-optimization/96548 - fix failure to recompute RPO after CFG change 2020-08-25 14:11:49 +02:00
tree-ssa-loop-ivcanon.c
tree-ssa-loop-ivopts.c
tree-ssa-loop-ivopts.h
tree-ssa-loop-manip.c
tree-ssa-loop-manip.h
tree-ssa-loop-niter.c
tree-ssa-loop-niter.h
tree-ssa-loop-prefetch.c
tree-ssa-loop-split.c
tree-ssa-loop-unswitch.c
tree-ssa-loop.c
tree-ssa-loop.h
tree-ssa-math-opts.c
tree-ssa-operands.c
tree-ssa-operands.h
tree-ssa-phiopt.c
tree-ssa-phiprop.c
tree-ssa-pre.c
tree-ssa-propagate.c
tree-ssa-propagate.h
tree-ssa-reassoc.c
tree-ssa-sccvn.c Add missing vn_reference_t::punned initialization 2020-08-24 09:30:33 +02:00
tree-ssa-sccvn.h
tree-ssa-scopedtables.c
tree-ssa-scopedtables.h
tree-ssa-sink.c
tree-ssa-strlen.c strlen: Fix handle_builtin_string_cmp [PR96758] 2020-08-25 13:47:10 +02:00
tree-ssa-strlen.h
tree-ssa-structalias.c
tree-ssa-tail-merge.c
tree-ssa-ter.c
tree-ssa-ter.h
tree-ssa-threadbackward.c
tree-ssa-threadedge.c
tree-ssa-threadedge.h
tree-ssa-threadupdate.c
tree-ssa-threadupdate.h
tree-ssa-uncprop.c
tree-ssa-uninit.c
tree-ssa.c
tree-ssa.h
tree-ssanames.c
tree-ssanames.h
tree-stdarg.c
tree-stdarg.h
tree-streamer-in.c
tree-streamer-out.c
tree-streamer.c
tree-streamer.h
tree-switch-conversion.c
tree-switch-conversion.h
tree-tailcall.c
tree-vect-data-refs.c SLP: support entire BB. 2020-08-24 10:21:09 +02:00
tree-vect-generic.c
tree-vect-loop-manip.c
tree-vect-loop.c tree-optimization/96698 - fix ICE when vectorizing nested cycles 2020-08-26 16:02:32 +02:00
tree-vect-patterns.c
tree-vect-slp.c SLP: support entire BB. 2020-08-24 10:21:09 +02:00
tree-vect-stmts.c tree-optimization/96698 - fix ICE when vectorizing nested cycles 2020-08-26 16:02:32 +02:00
tree-vector-builder.c
tree-vector-builder.h
tree-vectorizer.c
tree-vectorizer.h tree-optimization/96698 - fix ICE when vectorizing nested cycles 2020-08-26 16:02:32 +02:00
tree-vrp.c Adjust tree-ssa-dom.c for irange API. 2020-08-26 16:03:00 +02:00
tree-vrp.h Adjust tree-ssa-dom.c for irange API. 2020-08-26 16:03:00 +02:00
tree.c
tree.def
tree.h
treestruct.def
tristate.cc
tristate.h
tsan.c
tsan.h
tsystem.h
typeclass.h
typed-splay-tree.c
typed-splay-tree.h
ubsan.c
ubsan.h
unique-ptr-tests.cc
valtrack.c
valtrack.h
value-prof.c
value-prof.h
value-range-equiv.cc
value-range-equiv.h
value-range.cc
value-range.h
var-tracking.c
varasm.c
varasm.h
varpool.c
vec-perm-indices.c
vec-perm-indices.h
vec.c
vec.h
vector-builder.h
version.c
version.h
vmsdbg.h
vmsdbgout.c
vr-values.c Decouple adjust_range_from_scev from vr_values and value_range_equiv. 2020-08-18 21:10:53 +02:00
vr-values.h Decouple adjust_range_from_scev from vr_values and value_range_equiv. 2020-08-18 21:10:53 +02:00
vtable-verify.c
vtable-verify.h
web.c
wide-int-bitmask.h
wide-int-print.cc
wide-int-print.h
wide-int.cc
wide-int.h
xcoff.h
xcoffout.c
xcoffout.h

Copyright (C) 2000-2020 Free Software Foundation, Inc.

This file is intended to contain a few notes about writing C code
within GCC so that it compiles without error on the full range of
compilers GCC needs to be able to compile on.

The problem is that many ISO-standard constructs are not accepted by
either old or buggy compilers, and we keep getting bitten by them.
This knowledge until now has been sparsely spread around, so I
thought I'd collect it in one useful place.  Please add and correct
any problems as you come across them.

I'm going to start from a base of the ISO C90 standard, since that is
probably what most people code to naturally.  Obviously using
constructs introduced after that is not a good idea.

For the complete coding style conventions used in GCC, please read
http://gcc.gnu.org/codingconventions.html


String literals
---------------

Some compilers like MSVC++ have fairly low limits on the maximum
length of a string literal; 509 is the lowest we've come across.  You
may need to break up a long printf statement into many smaller ones.


Empty macro arguments
---------------------

ISO C (6.8.3 in the 1990 standard) specifies the following:

If (before argument substitution) any argument consists of no
preprocessing tokens, the behavior is undefined.

This was relaxed by ISO C99, but some older compilers emit an error,
so code like

#define foo(x, y) x y
foo (bar, )

needs to be coded in some other way.


Avoid unnecessary test before free
----------------------------------

Since SunOS 4 stopped being a reasonable portability target,
(which happened around 2007) there has been no need to guard
against "free (NULL)".  Thus, any guard like the following
constitutes a redundant test:

  if (P)
    free (P);

It is better to avoid the test.[*]
Instead, simply free P, regardless of whether it is NULL.

[*] However, if your profiling exposes a test like this in a
performance-critical loop, say where P is nearly always NULL, and
the cost of calling free on a NULL pointer would be prohibitively
high, consider using __builtin_expect, e.g., like this:

  if (__builtin_expect (ptr != NULL, 0))
    free (ptr);



Trigraphs
---------

You weren't going to use them anyway, but some otherwise ISO C
compliant compilers do not accept trigraphs.


Suffixes on Integer Constants
-----------------------------

You should never use a 'l' suffix on integer constants ('L' is fine),
since it can easily be confused with the number '1'.


			Common Coding Pitfalls
			======================

errno
-----

errno might be declared as a macro.


Implicit int
------------

In C, the 'int' keyword can often be omitted from type declarations.
For instance, you can write

  unsigned variable;

as shorthand for

  unsigned int variable;

There are several places where this can cause trouble.  First, suppose
'variable' is a long; then you might think

  (unsigned) variable

would convert it to unsigned long.  It does not.  It converts to
unsigned int.  This mostly causes problems on 64-bit platforms, where
long and int are not the same size.

Second, if you write a function definition with no return type at
all:

  operate (int a, int b)
  {
    ...
  }

that function is expected to return int, *not* void.  GCC will warn
about this.

Implicit function declarations always have return type int.  So if you
correct the above definition to

  void
  operate (int a, int b)
  ...

but operate() is called above its definition, you will get an error
about a "type mismatch with previous implicit declaration".  The cure
is to prototype all functions at the top of the file, or in an
appropriate header.

Char vs unsigned char vs int
----------------------------

In C, unqualified 'char' may be either signed or unsigned; it is the
implementation's choice.  When you are processing 7-bit ASCII, it does
not matter.  But when your program must handle arbitrary binary data,
or fully 8-bit character sets, you have a problem.  The most obvious
issue is if you have a look-up table indexed by characters.

For instance, the character '\341' in ISO Latin 1 is SMALL LETTER A
WITH ACUTE ACCENT.  In the proper locale, isalpha('\341') will be
true.  But if you read '\341' from a file and store it in a plain
char, isalpha(c) may look up character 225, or it may look up
character -31.  And the ctype table has no entry at offset -31, so
your program will crash.  (If you're lucky.)

It is wise to use unsigned char everywhere you possibly can.  This
avoids all these problems.  Unfortunately, the routines in <string.h>
take plain char arguments, so you have to remember to cast them back
and forth - or avoid the use of strxxx() functions, which is probably
a good idea anyway.

Another common mistake is to use either char or unsigned char to
receive the result of getc() or related stdio functions.  They may
return EOF, which is outside the range of values representable by
char.  If you use char, some legal character value may be confused
with EOF, such as '\377' (SMALL LETTER Y WITH UMLAUT, in Latin-1).
The correct choice is int.

A more subtle version of the same mistake might look like this:

  unsigned char pushback[NPUSHBACK];
  int pbidx;
  #define unget(c) (assert(pbidx < NPUSHBACK), pushback[pbidx++] = (c))
  #define get(c) (pbidx ? pushback[--pbidx] : getchar())
  ...
  unget(EOF);

which will mysteriously turn a pushed-back EOF into a SMALL LETTER Y
WITH UMLAUT.


Other common pitfalls
---------------------

o Expecting 'plain' char to be either sign or unsigned extending.

o Shifting an item by a negative amount or by greater than or equal to
  the number of bits in a type (expecting shifts by 32 to be sensible
  has caused quite a number of bugs at least in the early days).

o Expecting ints shifted right to be sign extended.

o Modifying the same value twice within one sequence point.

o Host vs. target floating point representation, including emitting NaNs
  and Infinities in a form that the assembler handles.

o qsort being an unstable sort function (unstable in the sense that
  multiple items that sort the same may be sorted in different orders
  by different qsort functions).

o Passing incorrect types to fprintf and friends.

o Adding a function declaration for a module declared in another file to
  a .c file instead of to a .h file.