I'm compiling with a command:
gcc grep.c -std=c99 -g -Winit-self -pedantic -w -o main2 && ./main2 lib text.txt
and I wish to receive warnings for initialized but not used variables and functions.
If you use -Wunused-variable it will warn for unused variables. But I recommend using -Wall -Wextra. Then you will get that for free with a bunch of other stuff.
When it comes to unused functions I refer to this: GCC -Wunused-function not working (but other warnings are working)
You can use the -Wunused-but-set-variable option to warn for these.
test.c:
int main(void)
{
int c = 0;
c = 3;
}
Example:
$ gcc test.c -Wunused-but-set-variable -o test
test.c: In function ‘main’:
test.c:3:9: warning: variable ‘c’ set but not used [-Wunused-but-set-variable]
int c = 0;
^
Related
I'm trying to make the C compiler clang go into ANSI C89 mode but without success.
Here is an example session:
$ cat t.c
#include <stdio.h>
int main(void)
{
puts(__FUNCTION__);
return 0;
}
$ gcc -pedantic -std=c89 -Wall t.c
t.c: In function ‘main’:
t.c:5:7: warning: ISO C does not support ‘__FUNCTION__’ predefined identifier [-Wpedantic]
puts(__FUNCTION__);
^~~~~~~~~~~~
$ clang -pedantic -std=c89 -Wall t.c
$ clang --version
clang version 3.8.1-24 (tags/RELEASE_381/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
As you can see, the clang command completes with no warning. Is there a command option that I'm missing here?
It seems as if this specific warning is not emitted by clang, but apparently -std=c89 does toggle ANSI C89 syntax checking.
For example:
inline int foo(int* restrict p)
{
return *p;
}
Will refuse to compile with -std=c89 -pedantic -Wall:
t.c:1:1: error: unknown type name 'inline'
t.c:1:23: error: expected ')'
int foo(int* restrict p)
But will compile without errors using -std=c99.
The non-standard predefined identifiers warning was introduced with GCC 5 (https://gcc.gnu.org/gcc-5/porting_to.html), and apparently clang did not adapt with it.
I created a basic C project in Xcode and modified the starter code in main.c slightly. I also went into the build settings and told it to use ANSI-C. Here's the code I have:
int main(int argc, const char * argv[])
{
// a statement!
printf("Hello, World!\n");
// shouldn't this cause a compiler error?
// the variable isn't declared at the top of the scope.
int x;
x += 10;
return 0;
}
Obviously, it doesn't do much, but I expected the variable declaration to produce a compiler error (since older versions of C require variable declarations at the beginning of the scope, before other statements). However, Xcode happily compiles it and runs it with neither an error or warning.
I might be making a dumb mistake somewhere, but I'm trying to understand why this code compiles. I've read that C99 and C11 allow you to declare variables anywhere, so this would work, but I explicitly set the project to use ANSI-C. Is this just the way Apple's LLVM compiler works? Or am I missing something elsewhere?
TL;DR You need to add -pedantic (or -Wdeclaration-after-statement) to -ansi to get the warning you want.
Somewhat to my surprise, both clang (from Apple XCode 7.2) and gcc (from GCC 5.3.0, which I built), accept the code when compiled with either -std=c90 or -ansi even though it is not strictly compliant with C90.
However, both complain when told to be -pedantic.
$ clang -ansi -c xyz.c
$ clang -std=c90 -c xyz.c
$ gcc -std=c90 -c xyz.c
$ which gcc
/opt/gcc/v5.3.0/bin/gcc
$ gcc -std=c90 -pedantic -c xyz.c
xyz.c: In function ‘main’:
xyz.c:7:5: warning: ISO C90 forbids mixed declarations and code [-Wdeclaration-after-statement]
int x;
^
$ clang -pedantic -std=c90 -c xyz.c
xyz.c:7:9: warning: ISO C90 forbids mixing declarations and code [-Wdeclaration-after-statement]
int x;
^
1 warning generated.
$ clang -pedantic -ansi -c xyz.c
xyz.c:7:9: warning: ISO C90 forbids mixing declarations and code [-Wdeclaration-after-statement]
int x;
^
1 warning generated.
$
The file xyz.c is your source code with the comments stripped, #include <stdio.h> added at the top, and int main(void) in place of int main(int argc, char **argv) since the code doesn't use the arguments.
Note that your code has undefined behaviour; incrementing an uninitialized variable is a bad idea.
How can I let gcc and clang generate warnings for header files included by header files in non-current directories?
I'm using gcc 4.9.2 and clang 3.6.0.
For example, assume that ./include_a.c includes ./dir/a.h, ./dir/a.h includes ./b.h, and ./b.h is expected to generate a warning for -Wconversion; then, gcc and clang with -Wconversion DO NOT generate the expected warning when they compile include_a.c. With -Wconversion -Wsystem-headers, the expected warning is generated, but it often comes with many useless warnings for system headers. When ./b.h is directly included from a source file in the current directory (such as ./include_b.c), the expected warning is generated without -Wsystem-headers.
The following shell script reproduces this example (the case of clang is omitted):
#!/bin/sh
mkdir dir
echo '#include "b.h"' >dir/a.h
echo 'void f() {int a = 0; char b = a;}' >b.h
echo '#include "dir/a.h"' >include_a.c
echo '#include "b.h"' >include_b.c
set -x
gcc -c include_a.c -Wconversion # DOES NOT generate a warning
gcc -c include_a.c -Wconversion -Wsystem-headers # generate a warning
gcc -c include_b.c -Wconversion # generate a warning
Output:
+ gcc -c include_a.c -Wconversion
+ gcc -c include_a.c -Wconversion -Wsystem-headers
In file included from dir/a.h:1:0,
from include_a.c:1:
./b.h: In function ‘f’:
./b.h:1:31: warning: conversion to ‘char’ from ‘int’ may alter its value [-Wconversion]
void f() {int a = 0; char b = a;}
^
+ gcc -c include_b.c -Wconversion
In file included from include_b.c:1:0:
b.h: In function ‘f’:
b.h:1:31: warning: conversion to ‘char’ from ‘int’ may alter its value [-Wconversion]
void f() {int a = 0; char b = a;}
^
This behavior seems not to be a bug because gcc and clang perform in the same way, but how can I obtain warnings from all the my source files?
I have the next code :
test.c
#include "a1.h"
int main() {
int a = 8;
foo(a);
return a;
}
a1.h
void foo (int a);
a1.c
int f = 0;
void foo (int a, int b){
f=5+a+b;
return;
}
Pay attention that in a1.c foo has 1 more parameter than the prototype defined in a1.h.
The compiler isn't issue a warning or an error and so as coverity :
make all
Building file: ../src/a1.c
Invoking: GCC C Compiler
gcc -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/a1.d" -MT"src/a1.d" -o "src/a1.o" "../src/a1.c"
Finished building: ../src/a1.c
Building file: ../src/test.c
Invoking: GCC C++ Compiler
gcc -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/test.d" -MT"src/test.d" -o "src/test.o" "../src/test.c"
Finished building: ../src/test.c
Building target: test
Invoking: GCC C++ Linker
gcc -o "test" ./src/a1.o ./src/test.o
Finished building target: test
How can I defend myself in those cases ? I know that if I will add #include "a1.h" in the a1.c file I will get an error but is there a way to get an error without the "include " ?
Compiler isn't issuing a warning because it does not know that foo(int) from a1.h header and foo(int,int) from a1.c file is the same function. C++ allows functions to be overloaded, so both functions could potentially coexist. That is why C++ compiler cannot detect this problem, so you need to wait until the linking stage.
If you were compiling using C, not C++, you could have the compiler detect this condition simply by including a1.h at the top of a1.c file.
You're overloading foo. The version with only one parameter is never defined, hence you should get a linker error when using it.
How can I defend myself in those cases ?
You can't defend yourself from function overloading. Just make sure that you've got the same signature in both the header as the source file.
I’m using the following flags, but still I m not able to get this warning:
pointer of type void * used in arithmetic
Flags used:
-O2 -Werror -Wall -Wno-main -Wno-format-zero-length -Wpointer-arith -Wmissing-prototypes -Wstrict-prototypes -Wswitch -Wshadow -Wcast-qual -Wwrite-strings -Wno-sign-compare -Wno-pointer-sign -Wno-attributes -fno-strict-aliasing
-Wpointer-arith should catch this type of warning, but I’m not able to get this warning:
pointer of type void * used in arithmetic
Which specific cflag should be used to get this warning?
Edit: my mistake, it is there as part of a macro check which is not defined. :( By defining that macro, I’m able to get that error.
You're right. -Wpointer-arith should give you a warning as per the documentation.
I have just tried the following program (with intentional error):
~/code/samples$ cat foo.c
#include <stdio.h>
int main (int argc, char **argv)
{
void * bar;
void * foo;
foo = bar + 1;
return 0;
}
I have compiled the program with just the -Wpointer-arith option, and all your options as listed above. Both attempts threw up the desired warning. I am using gcc version 4.3.4 (Debian 4.3.4-6).:
~/code/samples$ gcc -Wpointer-arith foo.c
foo.c: In function ‘main’:
foo.c:6: warning: pointer of type ‘void *’ used in arithmetic
and
~/code/samples$ gcc -O2 -Werror -Wall -Wno-main -Wno-format-zero-length -Wpointer-arith -Wmissing-prototypes -Wstrict-prototypes -Wswitch -Wshadow -Wcast-qual -Wwrite-strings -Wno-sign-compare -Wno-pointer-sign -Wno-attributes -fno-strict-aliasing foo.c
cc1: warnings being treated as errors
foo.c: In function ‘main’:
foo.c:6: error: pointer of type ‘void *’ used in arithmetic
The compiler does throw up the warning if you give it the 'right' code. So, I would recommend you examine why it is you expect this warning. Maybe the code you're compiling has changed?
One possible clue I can give you: foo = bar + 1; in the code above triggers the warning. But foo = bar ++; will not (You get a different warning). So if your code uses increment (or decrement) operators on pointers, it will probably not trigger the warning.
I know this is not a direct answer, but I hope this helps you focus your investigation.
With gcc 4.2.1 on OS X, I get this warning:
p.c:7: warning: wrong type argument to increment
for the following program:
#include <stdio.h>
int main(void)
{
int i[] = { 42 };
void *p = i;
printf("%p\n", p++);
return 0;
}
I am compiling it as:
$ gcc -Wpointer-arith p.c
Can you post your program, or post the result of compiling the above?