function-declaration

Recently I've learnt about implicit function declarations in C . The main idea is clear but I have some troubles with understanding of the linkage process in this case. Consider the following code ( file a.c ): #include <stdio.h> int main() { double someValue = f(); printf("%f\n", someValue); return 0; } If I try to compile it: gcc -c a.c -std=c99 I see a warning about implicit declaration of function f() . If I try to compile and link: gcc a.c -std=c99 I have an undefined reference error. So everything is fine. Then I add another file (file b.c ): double f(double x) { return x; } And invoke

I was going through some code when I encountered this in one of the source files . int st_insert(table, key, value) register st_table *table; register st_data_t key; st_data_t value; { unsigned int hash_val, bin_pos; register st_table_entry *ptr; hash_val = do_hash(key, table); FIND_ENTRY(table, ptr, hash_val, bin_pos); if (ptr == 0) { ADD_DIRECT(table, key, value, hash_val, bin_pos); return 0; } else { ptr->record = value; return 1; } } What is this style? Is it some obscure way to declare functions? Is there some reason one might use this over normal function declarations? Vlad from Moscow

I was just browsing through gcc source files. In gcc.c , I found something like extern int main (int, char **); int main (int argc, char **argv) { Now my doubt is extern is to tell the compiler that the particular function is not in this file but will be found somewhere else in the project. But here, definition of main is immediately after the extern declaration. What purpose is the extern declaration serving then? It seems like, in this specific example, extern seems to be behaving like export that we use in assembly, wherin we export a particular symbol outside of the module Any ideas? You

Nowadays , i was reading the APUE.and i found the function defined as below: void (*signal(int signo, void (*func)(int)))(int); i was confused, i know signal is pointer to a function and the last (int) is his parameter. i did not know what is (int signo,void (*func)(int)). John Bode The general procedure: find the leftmost identifier and work your way out. Absent an explicit grouping with parentheses, postfix operators such as () and [] bind before unary operators like * ; thus, the following are all true: T *x[N] -- x is an N-element array of pointer to T T (*x)[N] -- x is a pointer to an N

This question already has an answer here: In C++, what does & mean after a function's return type? 11 answers In this piece of code, why f() is declared as "double & f(..."? What does it mean and how does it work? I don't even know what to google to find the answer to my question. Please help. double a = 1, b = 2; double & f (double & d) { d = 4; return b; } I know ampersand sign means the address of a variable or a function, but I don't see why it would make sense to write it when you are declaring a function. When the & operator is used in a declaration form, preceded by a type it doesn't