gcc/gcc
2004-03-10 21:57:49 -05:00
..
ada [multiple changes] 2004-03-05 11:58:59 +01:00
config optabs.h (enum optab_index): Add new OTI_exp10 and OTI_exp2. 2004-03-10 22:36:22 +00:00
cp re PR c++/14397 (Access check for wrong copy constructor) 2004-03-09 18:44:02 +00:00
doc invoke.texi (ARM Options): Fix -mpfu typo. 2004-03-10 21:43:35 +00:00
f parse.c (ffe_parse_file): Handle the case that main_input_filename is NULL. 2004-02-29 14:12:20 +00:00
fixinc
ginclude
java jcf-parse.c (java_parse_file): Handle the case that input_filename is NULL. 2004-02-29 14:09:58 +00:00
objc objc-act.c (synth_module_prologue): Const-qualify objc_selector type if using the GNU runtime... 2004-03-06 00:26:42 +00:00
po * fr.po: Update. 2004-02-25 17:26:04 +00:00
testsuite re PR c/14517 (change in behaviour wrt duplicate qualifiers) 2004-03-10 16:45:37 -08:00
treelang
.cvsignore
ABOUT-GCC-NLS
aclocal.m4
alias.c New option -Wstrict-aliasing=2. 2004-03-09 22:02:55 -08:00
alloc-pool.c
alloc-pool.h
attribs.c
basic-block.h basic-block.h (make_eh_edge, [...]): Declare. 2004-02-25 20:00:00 +00:00
bb-reorder.c bb-reorder.c, [...]: Update copyright. 2004-02-27 14:50:47 +00:00
bitmap.c
bitmap.h
bt-load.c
builtin-attrs.def
builtin-types.def
builtins.c optabs.h (enum optab_index): Add new OTI_exp10 and OTI_exp2. 2004-03-10 22:36:22 +00:00
builtins.def
c-aux-info.c
c-common.c c-common.c (handle_alias_attribute): Reject the attribute if current_function_decl is set. 2004-02-25 17:24:37 -08:00
c-common.def
c-common.h
c-config-lang.in
c-convert.c
c-cppbuiltin.c real.c (encode_ibm_extended): Don't bother rounding low double. 2004-03-04 20:53:23 +10:30
c-decl.c re PR c/14517 (change in behaviour wrt duplicate qualifiers) 2004-03-10 16:45:37 -08:00
c-dump.c
c-errors.c
c-format.c
c-incpath.c Add framework support for darwin. 2004-03-04 00:18:54 +00:00
c-incpath.h Add framework support for darwin. 2004-03-04 00:18:54 +00:00
c-lang.c
c-lex.c target-supports.exp (check-iconv-available): New function. 2004-02-27 02:01:10 +00:00
c-objc-common.c
c-opts.c target-def.h (TARGET_OPTF): Delete. 2004-03-04 13:09:37 +10:30
c-parse.in target-supports.exp (check-iconv-available): New function. 2004-02-27 02:01:10 +00:00
c-pch.c
c-ppoutput.c c-decl.c, [...]: Fix comment typos and formatting. 2004-02-27 07:09:43 +00:00
c-pragma.c
c-pragma.h target-supports.exp (check-iconv-available): New function. 2004-02-27 02:01:10 +00:00
c-pretty-print.c
c-pretty-print.h
c-semantics.c
c-tree.h re PR middle-end/13448 (gcc 3.3.2, internal error when -O3 and trying to modify function const arg) 2004-02-29 23:43:29 +00:00
c-typeck.c New option -Wstrict-aliasing=2. 2004-03-09 22:02:55 -08:00
c.opt Add framework support for darwin. 2004-03-04 00:18:54 +00:00
caller-save.c alias.c (rtx_equal_for_memref_p): Use predicates to test rtx classes and new rtx class codes... 2004-03-03 08:35:33 +00:00
calls.c calls.c: Don't reference FINAL_REG_PARM_STACK_SPACE or MAYBE_REG_PARM_STACK_SPACE. 2004-02-26 23:32:09 +00:00
cfg.c
cfganal.c
cfgbuild.c bb-reorder.c, [...]: Update copyright. 2004-02-27 14:50:47 +00:00
cfgcleanup.c rtl.h (mem_expr_equal_p): Function prototype added. 2004-03-06 01:21:28 +00:00
cfghooks.c
cfghooks.h
cfglayout.c basic-block.h (make_eh_edge, [...]): Declare. 2004-02-25 20:00:00 +00:00
cfglayout.h
cfgloop.c
cfgloop.h cfgloop.h (struct loop_desc): Removed. 2004-03-02 20:21:14 +00:00
cfgloopanal.c cfgloop.h (struct loop_desc): Removed. 2004-03-02 20:21:14 +00:00
cfgloopmanip.c
cfgrtl.c reload.c (find_reloads): Reorganize if seqeunce to switch. 2004-03-04 21:31:43 +00:00
cgraph.c
cgraph.h
cgraphunit.c
ChangeLog re PR c/14517 (change in behaviour wrt duplicate qualifiers) 2004-03-10 16:45:37 -08:00
ChangeLog.0
ChangeLog.1
ChangeLog.2
ChangeLog.3
ChangeLog.4
ChangeLog.5
ChangeLog.6
ChangeLog.7
ChangeLog.8
ChangeLog.9
ChangeLog.10 Fix typo: FIRST_PSEOUDO_REGISTER -> FIRST_PSEUDO_REGISTER 2004-03-02 01:14:08 +01:00
ChangeLog.lib
collect2.c system.h (SUNOS4_SHARED_LIBRARIES): Poison. 2004-03-10 00:17:36 +00:00
collect2.h
combine.c simplify-rtx.c (simplify_const_relational_operation): New function renamed from simplify_relational_operation. 2004-03-09 17:06:25 +00:00
common.opt New option -Wstrict-aliasing=2. 2004-03-09 22:02:55 -08:00
conditions.h
config.build
config.gcc re PR target/14480 (gcc/config/rs6000/t-rtems is missing) 2004-03-10 17:23:33 +00:00
config.host
config.in re PR target/14471 (Workaround for errata regarding multiplication bug) 2004-03-08 00:32:37 +00:00
configure configure.ac: Bump AC_PREREQ to 2.59. 2004-03-11 00:27:41 +00:00
configure.ac configure.ac: Bump AC_PREREQ to 2.59. 2004-03-11 00:27:41 +00:00
conflict.c
convert.c
convert.h
COPYING
COPYING.LIB
coretypes.h
coverage.c re PR middle-end/11767 (different code behaviour with -fnon-call-exceptions -fprofile-arcs) 2004-03-02 16:46:06 -08:00
coverage.h
cppcharset.c
cppdefault.c
cppdefault.h
cpperror.c
cppexp.c
cppfiles.c Add framework support for darwin. 2004-03-04 00:18:54 +00:00
cpphash.c
cpphash.h
cppinit.c
cpplex.c
cpplib.c
cpplib.h Add framework support for darwin. 2004-03-04 00:18:54 +00:00
cppmacro.c
cpppch.c
cppspec.c
cpptrad.c
cppucnid.h
cppucnid.pl
cppucnid.tab
crtstuff.c
cse.c simplify-rtx.c (simplify_const_relational_operation): New function renamed from simplify_relational_operation. 2004-03-09 17:06:25 +00:00
cselib.c cfgcleanup.c (thread_jump): Update call of cselib_init. 2004-03-04 16:28:50 +00:00
cselib.h cfgcleanup.c (thread_jump): Update call of cselib_init. 2004-03-04 16:28:50 +00:00
cstamp-h.in
dbxout.c dbxout.c (dbxout_symbol_location): Do not output references to optimized-out constant pool symbols. 2004-03-10 15:12:10 +00:00
dbxout.h
debug.c
debug.h
defaults.h defaults.h (LEGITIMIZE_ADDRESS): Provide a default definition. 2004-03-07 20:23:32 +00:00
df.c
df.h
diagnostic.c bb-reorder.c, [...]: Update copyright. 2004-02-27 14:50:47 +00:00
diagnostic.def
diagnostic.h
dojump.c simplify-rtx.c (simplify_const_relational_operation): New function renamed from simplify_relational_operation. 2004-03-09 17:06:25 +00:00
doloop.c
dominance.c * dominance.c (recount_dominator): Handle postdominators. 2004-03-02 00:28:07 +00:00
dummy-conditions.c
dwarf2.h
dwarf2asm.c
dwarf2asm.h
dwarf2out.c re PR debug/11983 (ICE in dwarf-2 on code using altivec) 2004-03-08 10:12:37 +10:30
dwarf2out.h
dwarf.h
emit-rtl.c rtl.h (mem_expr_equal_p): Function prototype added. 2004-03-06 01:21:28 +00:00
errors.c
errors.h
et-forest.c
et-forest.h
except.c function.c (expand_function_end): Emit a blockage insn before the epilogue when -fnon-call-exceptions is used. 2004-03-10 17:47:45 +00:00
except.h
explow.c bb-reorder.c, [...]: Update copyright. 2004-02-27 14:50:47 +00:00
expmed.c
expr.c expr.c (highest_pow2_factor_for_type): Rename into highest_pow2_factor_for_target. 2004-03-08 06:48:51 +00:00
expr.h
final.c alias.c (rtx_equal_for_memref_p): Use predicates to test rtx classes and new rtx class codes... 2004-03-03 08:35:33 +00:00
fix-header.c Add framework support for darwin. 2004-03-04 00:18:54 +00:00
fixproto
flags.h New option -Wstrict-aliasing=2. 2004-03-09 22:02:55 -08:00
flow.c * flow.c (ior_reg_cond, and_reg_cond): Remove stray ")". 2004-03-03 11:41:47 +00:00
fold-const.c fold-const.c (tree_expr_nonnegative_p): Add more builtin cases. 2004-03-10 18:18:22 +00:00
fp-test.c
FSFChangeLog
FSFChangeLog.10
FSFChangeLog.11
function.c function.c (expand_function_end): Emit a blockage insn before the epilogue when -fnon-call-exceptions is used. 2004-03-10 17:47:45 +00:00
function.h
gbl-ctors.h
gcc.c re PR driver/13577 (multiple -o options with -c cause unrecognized option `-strip') 2004-03-05 13:54:12 +00:00
gcc.h
gccbug.in
gccspec.c
gcov-dump.c
gcov-io.c
gcov-io.h
gcov-iov.c
gcov.c
gcse.c cfgcleanup.c (thread_jump): Update call of cselib_init. 2004-03-04 16:28:50 +00:00
gdbinit.in
gen-protos.c
genattr.c genattr.c (main): Don't define TRADITIONAL_PIPELINE_INTERFACE or DFA_PIPELINE_INTERFACE. 2004-02-28 21:44:22 +00:00
genattrtab.c alias.c (rtx_equal_for_memref_p): Use predicates to test rtx classes and new rtx class codes... 2004-03-03 08:35:33 +00:00
genattrtab.h
genautomata.c
gencheck.c
gencodes.c
genconditions.c
genconfig.c
genconstants.c
genemit.c
genextract.c
genflags.c
gengenrtl.c
gengtype-lex.l
gengtype-yacc.y
gengtype.c
gengtype.h
genmodes.c
genmultilib genmultilib: Change '=' to '-' when translating option names to directory names. 2004-03-04 22:04:38 +00:00
genopinit.c optabs.h (enum optab_index): Add new OTI_exp10 and OTI_exp2. 2004-03-10 22:36:22 +00:00
genoutput.c
genpeep.c
genpreds.c
genrecog.c genattrtab.c: Don't handle MATCH_INSN. 2004-03-02 17:16:53 +00:00
gensupport.c genattrtab.c: Don't handle MATCH_INSN. 2004-03-02 17:16:53 +00:00
gensupport.h
ggc-common.c ggc-common.c (gt_pch_restore): Don't unmap addr unless we are going to call mmap again. 2004-03-05 01:22:58 +00:00
ggc-none.c ggc-common.c (ggc_alloc_cleared_stat, [...]): Rename from ...; make statistics transparent. 2004-03-03 11:25:51 +00:00
ggc-page.c ggc-common.c (ggc_alloc_cleared_stat, [...]): Rename from ...; make statistics transparent. 2004-03-03 11:25:51 +00:00
ggc-zone.c * ggc-zone.c (ggc_pch_write_object): Don't align file pointer. 2004-03-04 04:25:12 +00:00
ggc.h ggc-common.c (ggc_alloc_cleared_stat, [...]): Rename from ...; make statistics transparent. 2004-03-03 11:25:51 +00:00
glimits.h
global.c
gmon.c
graph.c
graph.h
gstab.h
gsyms.h
gsyslimits.h
gthr-aix.h
gthr-dce.h
gthr-gnat.c
gthr-gnat.h
gthr-posix.c
gthr-posix.h
gthr-rtems.h
gthr-single.h
gthr-solaris.h
gthr-vxworks.h
gthr-win32.h
gthr.h
haifa-sched.c c-decl.c, [...]: Fix comment typos and formatting. 2004-02-27 07:09:43 +00:00
hard-reg-set.h
hashtable.c
hashtable.h
hooks.c Add framework support for darwin. 2004-03-04 00:18:54 +00:00
hooks.h Add framework support for darwin. 2004-03-04 00:18:54 +00:00
host-default.c
hosthooks-def.h
hosthooks.h
hwint.h
ifcvt.c ifcvt.c (noce_try_sign_mask): New function to transform "x = (y < 0) ? z ... 2004-03-09 23:15:15 +00:00
input.h
insn-addr.h
integrate.c simplify-rtx.c (simplify_const_relational_operation): New function renamed from simplify_relational_operation. 2004-03-09 17:06:25 +00:00
integrate.h
intl.c
intl.h
jump.c alias.c (rtx_equal_for_memref_p): Use predicates to test rtx classes and new rtx class codes... 2004-03-03 08:35:33 +00:00
langhooks-def.h ggc-common.c (ggc_alloc_cleared_stat, [...]): Rename from ...; make statistics transparent. 2004-03-03 11:25:51 +00:00
langhooks.c ggc-common.c (ggc_alloc_cleared_stat, [...]): Rename from ...; make statistics transparent. 2004-03-03 11:25:51 +00:00
langhooks.h
LANGUAGES
lcm.c
libfuncs.h
libgcc2.c
libgcc2.h
libgcc-darwin.ver
libgcc-std.ver
libgcov.c
limitx.h
limity.h
line-map.c
line-map.h
lists.c
local-alloc.c cse.c (cse_end_of_basic_block): Make static. 2004-03-05 10:17:40 +00:00
longlong.h
loop-init.c
loop-iv.c alias.c (rtx_equal_for_memref_p): Use predicates to test rtx classes and new rtx class codes... 2004-03-03 08:35:33 +00:00
loop-unroll.c
loop-unswitch.c
loop.c cse.c (cse_end_of_basic_block): Make static. 2004-03-05 10:17:40 +00:00
loop.h
machmode.def
machmode.h
main.c
Makefile.in Makefile.in (site.exp): Add libiconv variable definition. 2004-03-09 01:53:28 +00:00
mips-tdump.c
mips-tfile.c
mkconfig.sh
mkdeps.c
mkdeps.h
mkheaders.in
mklibgcc.in
mkmap-flat.awk
mkmap-symver.awk
mode-classes.def
ONEWS
optabs.c optabs.h (enum optab_index): Add new OTI_exp10 and OTI_exp2. 2004-03-10 22:36:22 +00:00
optabs.h optabs.h (enum optab_index): Add new OTI_exp10 and OTI_exp2. 2004-03-10 22:36:22 +00:00
opts.c New option -Wstrict-aliasing=2. 2004-03-09 22:02:55 -08:00
opts.h
opts.sh
output.h passes.c: New file. 2004-02-27 00:54:31 -08:00
params.c
params.def common.opt: Add description of the new -fgcse-after-reload flag. 2004-03-03 11:32:45 -05:00
params.h common.opt: Add description of the new -fgcse-after-reload flag. 2004-03-03 11:32:45 -05:00
passes.c common.opt: Add description of the new -fgcse-after-reload flag. 2004-03-03 11:32:45 -05:00
postreload.c cfgcleanup.c (thread_jump): Update call of cselib_init. 2004-03-04 16:28:50 +00:00
predict.c alias.c (rtx_equal_for_memref_p): Use predicates to test rtx classes and new rtx class codes... 2004-03-03 08:35:33 +00:00
predict.def
predict.h
prefix.c
prefix.h
pretty-print.c
pretty-print.h
print-rtl.c
print-tree.c
profile.c bb-reorder.c, [...]: Update copyright. 2004-02-27 14:50:47 +00:00
protoize.c
ra-build.c bb-reorder.c, [...]: Update copyright. 2004-02-27 14:50:47 +00:00
ra-colorize.c
ra-debug.c alias.c (rtx_equal_for_memref_p): Use predicates to test rtx classes and new rtx class codes... 2004-03-03 08:35:33 +00:00
ra-rewrite.c
ra.c
ra.h
read-rtl.c bb-reorder.c, [...]: Update copyright. 2004-02-27 14:50:47 +00:00
README-fixinc
README.Portability
real.c real.c (encode_ibm_extended): Don't bother rounding low double. 2004-03-04 20:53:23 +10:30
real.h
recog.c alias.c (rtx_equal_for_memref_p): Use predicates to test rtx classes and new rtx class codes... 2004-03-03 08:35:33 +00:00
recog.h recog.c (validate_replace_src): Remove. 2004-02-26 23:39:29 +00:00
reg-stack.c alias.c (rtx_equal_for_memref_p): Use predicates to test rtx classes and new rtx class codes... 2004-03-03 08:35:33 +00:00
regclass.c alias.c (rtx_equal_for_memref_p): Use predicates to test rtx classes and new rtx class codes... 2004-03-03 08:35:33 +00:00
regmove.c alias.c (rtx_equal_for_memref_p): Use predicates to test rtx classes and new rtx class codes... 2004-03-03 08:35:33 +00:00
regrename.c alias.c (rtx_equal_for_memref_p): Use predicates to test rtx classes and new rtx class codes... 2004-03-03 08:35:33 +00:00
regs.h
reload1.c cse.c (cse_end_of_basic_block): Make static. 2004-03-05 10:17:40 +00:00
reload.c reload.c (find_reloads): Reorganize if seqeunce to switch. 2004-03-04 21:31:43 +00:00
reload.h
reorg.c
resource.c
resource.h
rtl-error.c
rtl.c ggc-common.c (ggc_alloc_cleared_stat, [...]): Rename from ...; make statistics transparent. 2004-03-03 11:25:51 +00:00
rtl.def alias.c (rtx_equal_for_memref_p): Use predicates to test rtx classes and new rtx class codes... 2004-03-03 08:35:33 +00:00
rtl.h simplify-rtx.c (simplify_const_relational_operation): New function renamed from simplify_relational_operation. 2004-03-09 17:06:25 +00:00
rtlanal.c rtlanal.c: Include target.h and output.h. 2004-03-05 10:32:54 +00:00
sbitmap.c
sbitmap.h
scan-decls.c
scan-types.sh
scan.c
scan.h
sched-deps.c re PR rtl-optimization/14282 (ICE in sh-elf-gcc with -m4-single-only and -O2) 2004-03-04 14:03:02 -05:00
sched-ebb.c
sched-int.h
sched-rgn.c alias.c (rtx_equal_for_memref_p): Use predicates to test rtx classes and new rtx class codes... 2004-03-03 08:35:33 +00:00
sched-vis.c
sdbout.c
sdbout.h
SERVICE
sibcall.c
simplify-rtx.c simplify-rtx.c (simplify_const_relational_operation): New function renamed from simplify_relational_operation. 2004-03-09 17:06:25 +00:00
sort-protos
sreal.c
sreal.h
stab.def
stack.h
statistics.h ggc-common.c (ggc_alloc_cleared_stat, [...]): Rename from ...; make statistics transparent. 2004-03-03 11:25:51 +00:00
stmt.c hooks.c (hook_tree_tree_identity): New. 2004-03-03 23:55:52 +00:00
stor-layout.c stor-layout.c (layout_type, [...]): Make size FUNCTION_BOUNDARY, not POINTER_SIZE * 2. 2004-03-05 11:19:41 -05:00
stringpool.c
stub-objc.c
sys-protos.h
sys-types.h
system.h system.h (SUNOS4_SHARED_LIBRARIES): Poison. 2004-03-10 00:17:36 +00:00
target-def.h target-def.h (TARGET_OPTF): Delete. 2004-03-04 13:09:37 +10:30
target.h hooks.c (hook_tree_tree_identity): New. 2004-03-03 23:55:52 +00:00
targhooks.c builtins.c (BUILTIN_SETJMP_FRAME_VALUE): Remove. 2004-03-02 22:32:27 +00:00
targhooks.h builtins.c (BUILTIN_SETJMP_FRAME_VALUE): Remove. 2004-03-02 22:32:27 +00:00
timevar.c
timevar.def
timevar.h
tlink.c
toplev.c common.opt: Add description of the new -fgcse-after-reload flag. 2004-03-03 11:32:45 -05:00
toplev.h passes.c: New file. 2004-02-27 00:54:31 -08:00
tracer.c bb-reorder.c, [...]: Update copyright. 2004-02-27 14:50:47 +00:00
tree-dump.c
tree-dump.h
tree-inline.c re PR middle-end/13448 (gcc 3.3.2, internal error when -O3 and trying to modify function const arg) 2004-02-29 23:43:29 +00:00
tree-inline.h
tree-optimize.c
tree.c (build_pointer_type_for_mode): Fix comments and whitespace. 2004-03-10 21:57:49 -05:00
tree.def
tree.h New option -Wstrict-aliasing=2. 2004-03-09 22:02:55 -08:00
tsystem.h
typeclass.h
unroll.c simplify-rtx.c (simplify_const_relational_operation): New function renamed from simplify_relational_operation. 2004-03-09 17:06:25 +00:00
unwind-c.c
unwind-dw2-fde-darwin.c
unwind-dw2-fde-glibc.c bb-reorder.c, [...]: Update copyright. 2004-02-27 14:50:47 +00:00
unwind-dw2-fde.c
unwind-dw2-fde.h
unwind-dw2.c
unwind-pe.h
unwind-sjlj.c
unwind.h
unwind.inc
value-prof.c bb-reorder.c, [...]: Update copyright. 2004-02-27 14:50:47 +00:00
value-prof.h
var-tracking.c Fix typo. 2004-03-04 14:17:29 +00:00
varasm.c
varray.c
varray.h
version.c Daily bump. 2004-03-11 00:16:48 +00:00
version.h
vmsdbg.h
vmsdbgout.c
web.c
xcoff.h
xcoffout.c
xcoffout.h

Copyright (C) 2000, 2003 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 know 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
---------------

Irix6 "cc -n32" and OSF4 "cc" have problems with constant string
initializers with parens around it, e.g.

const char string[] = ("A string");

This is unfortunate since this is what the GNU gettext macro N_
produces.  You need to find a different way to code it.

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.


free and realloc
----------------

Some implementations crash upon attempts to free or realloc the null
pointer.  Thus if mem might be null, you need to write

  if (mem)
    free (mem);


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.