All Questions
597 questions
2
votes
0
answers
97
views
How can I force the compiler to keep my parameters in the same registers when returning?
I'm trying to write some weird code that I'm planning on using for a very simple JIT. It's a bunch of small bytecode handler functions that I'll use in the generated executable that will look ...
- 192
2
votes
1
answer
119
views
Math error conditions (C99, C11, etc.) in GCC
With ISO C99 onward we have a few macros (or constants) that help understanding how some math errors (see <math.h> related) are signaled. However, to me it looks like they haven't been ...
- 403
1
vote
2
answers
105
views
What is the rationale for 0-size arrays?
Both GCC and Clang support (as an extension) 0-size (also called "0-length") arrays.
What is the rationale for 0-size arrays (example int x[0])?
- 6,255
2
votes
1
answer
92
views
Can I access the first struct element without knowing what other elements are in it?
I suspect this is UB, according to ANSI, but on my system (x64 Linux), even with -Wall -Wextra -Werror -O3, both gcc and clang produce the expected result here:
#include <stdio.h>
typedef ...
- 12.4k
2
votes
3
answers
233
views
How to tell the compiler a function argument will never be zero?
Imagine I have a dummy C function like this:
void Dummy(uint64* dest, const size_t count)
{
for (size_t ii = 0; ii < count; ii += 8) {
*dest++ = (uint64)dest;
}
}
If you look at what the ...
- 1,168
1
vote
1
answer
131
views
How to force gcc to do printf checks on non-literal format strings?
const char* const non_literal_string = "Hello %d";
void my_print()
{
/*
* I would like GCC to throw a warning when compiling this line because the
* argument is not of int ...
- 352
5
votes
2
answers
140
views
Why -Wunused-value does not catch a statement `true;`?
Given following C code:
#include <stdbool.h>
int main (void)
{
true;
return 0;
}
I expect that -Wunused-value would cause a warning at line 5: true;
because this statement does ...
- 711
1
vote
1
answer
211
views
Is it possible to build sqlite extensions on Apple Silicon using clang?
I am attempting to build the percentile sqlite extension on an M1 laptop using the instructions on the page:
gcc -g -fPIC -shared percentile.c -o percentile.so
However, that results in the following ...
- 75.1k
-1
votes
1
answer
55
views
Using __atribute__((noreturn)) on loops?
Is there a compiler attributes (clang / gcc) that does something like noreturn but for loop?
Something like:
__atribute__((loopsnoreturn)) for (int i=0; i<12; i++)
{
if(i==5) return 0; //< ...
- 744
0
votes
2
answers
232
views
Use libgcc/compiler-rt altrenative with GCC/Clang
I am writing small low-level runtime library for fun.
According to the libgcc website, low-level runtime library are libraries that compilers generates calls to its routines whenever it needs to ...
- 29
0
votes
0
answers
119
views
Remove unused data from bss restored section (c)
I've noticed that when using -ffunction-sections -fdata-sections and -Wl,--gc-sections, unused restored BSS sections are still mapped into memory, even though I expect them to be stripped. Why are ...
- 5,097
12
votes
2
answers
2k
views
Logical AND (&&) does not short-circuit correctly in #if
For code:
#if defined(FOO) && FOO(foo)
#error "FOO is defined."
#else
#error "FOO is not defined."
#endif
MSVC 19.38 prints:
<source>(1): warning C4067: ...
- 7,315
3
votes
0
answers
106
views
Did gcc and clang accidentally drop C99 sequence point conformance?
Background:
C99 6.5 §2 said:
Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value ...
- 212k
5
votes
3
answers
225
views
How to make compiler warning about the enum type mismatch in switch and case?
For the following code, I hope there is a warning because a is type enum A, but the case B1 and B2 are type enum B; but I couldn't find a way to make gcc/clang warn about it.
Any suggestion on how to ...
- 1,075
1
vote
1
answer
239
views
GCC "labels as values" – intended usage
I recently heard about the Labels as Values extension to C/C++ in GCC and other compilers. I was thinking about how I'd use that to write a threaded interpreter (where a virtual machine program is ...
- 1,229
3
votes
2
answers
149
views
Compiler doesn't issue warning on `uint64_t` under strict C89 mode
I'm trying to write strict ISO C89-compliant code. Since long long is not standard and is often implemented as compiler extensions before C99, the compiler should warn me it when I use it. However, ...
- 65
1
vote
1
answer
191
views
Using FSIN through inline-assembly
I want to use x87's FSIN through gcc's / clang's inline assembly. How would a sin()-function using an __asm__-block internally for that look like with a 64 bit double parameter ? Using __builtin_sin() ...
0
votes
1
answer
59
views
On RPi 4 GCC and CLANG use 16-bit loads
This is simplified from a larger example.
In the C source, I have:
uint32_t xx = oxdeadbeef ;
I compiled with gcc -O or clang -O
Looking at 'objdump -d a.out' on the RPi 4, I see
9ac: 5297dde8 ...
- 41
1
vote
2
answers
93
views
Does C standard define the calling convention?
Does the C standard define the calling convention? I find the calling convention cdecl stands for C declaration, but i can’t find anything about it in the C stardard draft.
7
votes
2
answers
175
views
In standard C, is the expression "i = (i = 1) + 1;" well defined?
In standard C, is the expression i = (i = 1) + 1; well defined?
What's the difference between it and i = ++i + 1; from the perspective of sequence points?
Someone told me there is a sequence point ...
1
vote
0
answers
90
views
Differences in Compilation Results Between GCC and Clang
I encountered a situation where my C code compiles successfully with GCC but generates errors when compiled with Clang.
Here is my C code:
#include <stdio.h>
#include <stdlib.h>
#include &...
- 63
0
votes
0
answers
97
views
Why there is segment fault when compile with clang15
Given the following code, built on x86 clang, and run, it will segment fault.
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <string.h>
#include <sys/...
- 2,907
1
vote
0
answers
95
views
Diff in `-Bsymbolic` behavior between gcc and clang?
Could it be that clang and gcc somehow implement -Bsymbolic differently?
\\ exe.c
void f();
typedef void (*fptr)();
void compareFuncPtrs(fptr);
int main()
{
compareFuncPtrs(f);
return 0;
}
\...
- 16.1k
0
votes
0
answers
66
views
How compile program with openMP functions?
I tried to run the code related to OpenMP and parallelization on mac m1, but I ran into a number of problems.
The usual connection of the header file <omp.h> didn't work in VSC, so I installed ...
1
vote
0
answers
82
views
C pointers and -O3 optimized semantic anomalies [duplicate]
Code
#include <stdint.h>
#include <stdio.h>
void func(uint64_t* pInst)
{
*pInst = 0xffffffffff;
*pInst = *(uint32_t*)&(*pInst);
uint64_t Ret = *pInst;
printf ("...
0
votes
0
answers
109
views
Preprocessing C code to only use C23 attribute notation
Is there a way for GCC/Clang/a C pretty-printer tool to preprocess/pretty-print a source file, using only the "new" (C23) attribute notation? In other words, transform this code:
int a ...
- 8,893
-1
votes
1
answer
134
views
Use gcc in mac instead of clang
I have a mac with Ventura 13.5.1 (intel proc). I need to usc gcc instead of clang in mac, how i can do it?
Now i have smth like this on my mac:
λ ~/ gcc -v
Apple clang version 14.0.3 (clang-1403.0.22....
- 3
-1
votes
1
answer
162
views
Have two C compilers
I wanted to set up C language on VSCode, and ended up with 2 compilers (clang and gcc) how can I delete one of them ?
P.S: I'm using M2 Mac, had to notice that
I want a solution to my problem, maybe ...
- 11
0
votes
2
answers
219
views
Consolidating GNU C's and C23's deprecated function attribute
The problem I am facing when using the below macro
// Assume that __GNUC__ or __clang__ is defined.
#if defined(__has_c_attribute)
#if __has_c_attribute(deprecated)
#define ...
- 7,315
0
votes
2
answers
178
views
buffer overflow attack works when compiled using clang but not when compiled using gcc
char shellcode[] = "\xeb\x18\x5e\x31\xc0\x89\x76\x08\x88\x46\x07\x89\x46\x0c\xb0\x0b\x89\xf3\x8d\x4e\x08\x8d\x56\x0c\xcd\x80\xe8\xe3\xff\xff\xff/bin/sh ";
char large_string[128];
...
- 21
0
votes
2
answers
331
views
Is there a way to pass ownership of a pointer in C (like in Rust)?
I was looking for a way in C to prevent the usage of a pointer after it's ownership is passed to another function, where I can specify explicitly which parameter of a function should do that. Is there ...
2
votes
2
answers
99
views
C preprocessor macro and warning suppression for custom variadic-argument function?
I am attempting to create a class of functions which can be called with any number and any type of arguments--similar to printf, but with one key difference: I would like to cast all arguments to ...
- 475
13
votes
1
answer
3k
views
Possible Bug in GCC Sanitizers?
I always compile my programs with gcc and during development always with:
gcc -g -Wall -Wextra -std=c2x -fsanitize=address,undefined
Today I noticed that the address sanitizer randomly detects SEGVs ...
- 131
2
votes
0
answers
83
views
Parsing C code to generate a flattened equivalent
I need to parse C code to generate a "flattened" Prolog readable form for static analysis purposes.
e.g.
int map (int den, int a) {
den = den*(a+a);
if (den == 90) return -1/(den-90);
...
- 95
1
vote
0
answers
94
views
Is there a GCC/Clang attribute to mark which fields in a struct that a given mutex covers?
I'm thinking about how I would theoretically implement a multi threaded queue from first principles, using only glibc on POSIX.
Obviously, this is going to require a mutex (or some other locking/...
- 886
4
votes
2
answers
549
views
Why was the C compiler unable to compile code with an inline function?
It looks extremely weird, but I was unable to compile following simple code:
inline void inline_func() {}
static inline void static_inline_func() {}
void some_func()
{
// THERE IS A PROBLEM with &...
- 491
1
vote
2
answers
2k
views
Why can I declare a variable after a label in GCC, but not Clang?
I found some code that compiles under gcc but not under clang:
#include<stdio.h>
int main(int argc, char **argv) {
int test = 0;
if (test == 0) {
goto print_five;
} else {
...
- 112
1
vote
2
answers
383
views
What kind of warnings is -isystem in gcc supposed to ignore?
Several blogs, tutorials and GCC's documentation have lead me to believe -isystem can be used to ignore warnings in header files. A simple test suggests otherwise.
#include <liburing.h>
int ...
- 1,199
0
votes
1
answer
142
views
How to tell GCC/Clang optimizer to generate specific sequence of operations
I have a loop that needs to execute sequences of operations in a specific order. What I am doing here is manually unrolling the loop a number of times:
loop
{
delta = get_delta();
sum1 += ...
user18763589
0
votes
3
answers
106
views
Arithmetic which gives the lowest 64 bits on overflow
In C unsigned integer arithmetic wraps around on overflow and signed arithmetic is undefined on overflow. I would like to have 64 bit integer arithmetic (+,-,*) so that on overflow the result is the ...
- 21.2k
5
votes
1
answer
209
views
Should GCC/Clang optimize this redundant load via an array of restrict-qualified pointers?
I am looking into optimizations permitted for a compiler by adding the C99 restrict type qualifier to the type parameter of an array.
For example, consider the function f below:
int f(int* restrict a[...
- 51
0
votes
1
answer
142
views
Why clang and gcc output different results under `-std=c89` option?
I came across this quiz, so want to verify it myself:
#include <stdio.h>
void
print_sum(a, b)
{
printf("%d\n", a + b);
}
int
main(void)
{
print_sum(1.5, 8.5);
return 0;
}
...
- 17.4k
1
vote
0
answers
147
views
How to compile and deploy OpenSSL using osxcross on Linux
I need help compiling OpenSSL using osxcross and running that on a MacOS system.
I managed to download the latest OpenSSL version from Github (3.1.4) and I compiled it using the following command:
CC=...
- 145
0
votes
0
answers
111
views
Mapping between line:column numbers in C or C++ code before and after preprocessing
When there's a syntax error in a C or C++ source file, both GCC and Clang report the exact line and column of where the error is. For example, if we try to compile this code fragment:
#include <...
- 5,268
2
votes
1
answer
428
views
Is there any sanitizer setting in GCC or Clang that can detect the use of uninitialized heap-allocated memory?
These C and C++ programs use uninitialized memory:
C:
#include <stdio.h>
#include <stdlib.h>
int main(void) {
double* p = malloc(100*sizeof(double));
printf("%f\n", p[...
- 12.4k
1
vote
1
answer
396
views
Clang uses a linked shared lib to decide which symbols to export?
Take this toy code for an executable and a shared library:
// main.c
void foo() {}
int main() { return 0; }
// bar.c
void foo();
void bar() { foo(); }
Let's build without optimizations, first with ...
- 16.1k
8
votes
1
answer
302
views
gcc and clang emit seemingly redundant function call
typedef struct foo
{
void (*const t)(struct foo *f);
} foo;
void t(struct foo *f)
{
}
void (*const myt)(struct foo *f) = t;
foo f = {.t = t};
int main(void)
{
f.t(&f);
myt(&...
- 902
7
votes
1
answer
810
views
malloc attribute takes no arguments
I've created a pair of functions:
void destroy_foo(void *ptr);
void *create_foo(void);
As the names suggest, these function akin to malloc and free. I'd like to use the malloc gcc function attribute ...
- 6,668
0
votes
1
answer
80
views
Forcing linking through the PLT
I just did a quick benchmark of the PLT overhead, and I found I'm unable to force my functions to go through the PLT in the final executable unless I put the code in a shared library and link that.
...
- 60k
0
votes
1
answer
466
views
Is __attribute__ portable across compilers?
I want to make sure my program works on either. Currently, I compile as such:
CFLAGS := -std=c99 -Wall -Wextra -Wshadow -Wpedantic
and want to make sure it works on either Linux/macOS.
I use ...
- 1,229