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Posts by alx
What does it mean that we can use memcmp(3) on invalid values? ISO C allows comparing an invalid value through memcmp(3), because it doesn't read the value, but rather its representation, and "rea...
Is it valid to partly allocate an object, as long as you only use the allocated part of it? #include <stdio.h> #include <stdlib.h> struct s { int i[100]; }; int main(void) {...
How can we use git-send-email(1) to sign patches (emails) with the gpg(1) keyring? I've heard it can be done, but couldn't find anything in the git-send-email(1) documentation nor in a web search.
Let's consider the following code: const char a[][4] = {"aa", "aaa"}; const char *b[] = {"bb", "bbb"}; const char *const c[] = {"cc", "ccc"}; For shared libraries, both b and c arrays require...
It's a long story. Once upon a time, there were three ways to express a null pointer constant: NULL 0 ((char *) 0) This was true at least as far back as Unix v7. Maybe even further back i...
I've seen macros use parentheses to enclose its arguments. Most of these make sense, as in #define sum(a, b) ((a) + (b)) The outer prevents the following: #define sum_bad(a, b) (a) + (b) ...
Let's say we have an object, we store it in a union (into some other narrower type, but with memcpy(3), so it's allowed --I guess--), and then read it from the union via it's original type (so no a...
char *p, *q; p = malloc(1); free(p); q = p; // lvalue conversion Is the last lvalue conversion (= p;) Undefined Behavior or not? We didn't take the address of the local p. C11::6.3.2.1...
Is it legal ISO C to declare a function as noreturn with a non-void return type (but of course not actually returning)? As far as I can read from the standard, it seems legal. Example: noreturn ...
strcpy(3) can be safe. Some compilers, such as GCC and Clang, use a feature test macro, _FORTIFY_SOURCE, (see feature_test_macros(7) https://man7.org/linux/man-pages/man7/feature_test_macros.7.htm...
Let's analyze this code, assuming an architecture where the alignment of int64_t is the same as that of double: void bar(double *f, int64_t *j) { *(int64_t *)f = *j; } void foo(void) ...
I'd like to assert that some code can be optimized out, and is not present in the final binary object. #define CONSTANT 0 #if (!CONSTANT) [[landmine_A]] #endif static int foo(void); void...
Is -isystem/path/to/sys/includes a standard compiler option, or is it a compiler extension implemented by gcc, clang, and maybe other compilers? Can I rely on its availability? I couldn't find th...
How to write a macro that discards the const qualifier, for any type? I hope some combination of typeof and a cast will do, but haven't found the combination. I tried this, without luck: #define...
I was directed a few days ago to a post about a string copy function, which IMO improves the commonly known string copy functions, including strlcpy(3BSD), strlcat(3BSD), and strscpy(9). It define...
It can't be done with git-send-email(1), but there's a tool that integrates with it, and is very simple to use: patatt(1). Install the tool: $ sudo apt-get install patatt And then for each rep...
I developed this macro (using GNU C) similar to C++'s const_cast(). #define const_cast(t, p) \ ({ ...
Original problem: https://stackoverflow.com/questions/37538/how-do-i-determine-the-size-of-my-array-in-c/57537491#57537491 Arrays in C are problematic. They are transformed into pointers too easi...
Calling an undefined function will have that behavior at link time: landmine.c: #ifndef CONSTANT #define CONSTANT 0 #endif #define assert_not_in_binary_if(e) do \ { \ if (e) \ undefi...
I heard git pull is dangerous. Is it really dangerous? If so, why or how is it dangerous? Are there any flags that mitigate or remove the danger? What are the alternatives if it...
What is the portability of malloc(0);? Which platforms return NULL without setting errno? Which platforms return a non-null pointer? Do any platforms have some other behavior?
git pull is dangerous, but not a security problem. The only danger is that it's difficult or inconvenient (but not impossible) to recover to the state prior to the pull, if it would be necessary. ...
After addressing @Lundin 's suggestions: Implemented in terms of libc functions for performance. const correctness style improvements I have a few more that I realized after trying to repla...
git-send-email(1) uses sendmail(8) as the MTA by default. However, this can be changed by passing the --sendmail-cmd option. $ man git-send-email | sed -n '/--sendmail-cmd=/,/^$/p' --sendma...
There are several things to consider when calling the malloc(3) family of functions: nelem * sizeof(elem) or sizeof(elem) * nelem? Use the type or the pointer name in sizeof()? To cast or not ...
