Re: beginner's questions

From: SM Ryan (wyrmwif_at_tango-sierra-oscar-foxtrot-tango.fake.org)
Date: 02/18/05 

Date: Fri, 18 Feb 2005 11:12:41 -0000

gruzdnev@gmail.com wrote:
# Hi all,
#
# I've started to program in C not long ago, and I've got some questions:
# (I work on Linux 2.4.22/Debian)
#
# 1. Why the "**var" construct is used? What are the cases when it is
# commonly needed? I'd like to read more about it, but there's nothing in
# K&R on this theme, AFAIR.

Simulate pass by name.
        int afgets(char **linevar,int size,FILE *fp) {
          char *line = malloc(size);
          if (fgets(line,size,fp)) {*linevar = line; return 1;}
          else {free(line); *linevar = 0; return 0;}
        }
        ...
        char *line;
        while (afgets(&line,20,stdin)) {
          doWahDiddy(line); free(line);
        }

Pointer chasing in linked lists. You'll need to draw some pictures to understand this
        typedef struct T{struct T *link; int data;} T;

        T *removeAll(T *head0,int data) {
          T *head = head0; T **current = &head;
          while (*current) {
            if ((*current)->data==data) {
              T *sacrafice = *current;
              *current = sacrafice->link;
              free(sacrafice);
            }else {
              current = &((*current)->link);
            }
          }
          return head;
        }
        
# 2. Suppose, I want to see the source code of the "fopen" function used
# by my system. Where do I have to look to learn it? Is it some header

Depends on your vendor. Some (like gnu) supply the source code, some don't.

# 3. Why is "___P" used in declaraion of functions like "main __P((int,
# char *[]));"? What is the sense of it? Does using it have any pluses?
# Where do I read about it (online, preferably)?

Probably somewhere it defines
        #define __P(x) x
or
        #define __P(x)

The first #define converts those declarations into ANSI-C function prototypes. The
second #define converts them into old style K+R function declarations. It lets you
use the same header file for two different styles of compilation. 

--
SM Ryan http://www.rawbw.com/~wyrmwif/
I have no idea what you just said.
I get that alot.

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