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Posts by alx
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 (1979). Maybe even further...
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...
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 ...
And with the same logic, shouldn't the function prototypes in gpio.h also be extern for my source files to see them? Function declarations default to being extern. If you don't specify any st...
strncpy(3) strncpy(3) was originally invented in Seventh Edition Unix (a.k.a., V7). It was added with one use case in mind: copying a source string into a destination character sequence in a fixe...
Let's say we have a function void f(int argc, ...); where argc is the number of variadic arguments. Can we write a macro of the form #define F(...) f(CNT(__VA_ARGS__), __VA_ARGS__) which...
Can I programmatically get the name of the remote branch which my local branch is tracking? I'm interested in scripting around git-range-diff(1), so that I could do something like this: [alias] ...
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...
I need to wrap grep(1) in a script, and I'd like the script to have an exit code compatible with that of grep(1) --that is, it should return 0 if it found something, 1 if it didn't, and 2 on error-...
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) ...
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...
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...
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...
I'd like to implement a macro strnul(): #define strnul(s) (s + strlen(s)) But that expands and evaluates s twice. I'd like to avoid that. I could do (using GNU extensions) #define strnul(s)...
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...
I developed this macro (using GNU C) similar to C++'s const_cast(). #define const_cast(t, p) \ ({ ...
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...
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...
