Problem with linked lists Post: 302276724

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Top Forums Programming Problem with linked lists Post 302276724 by brinch on Wednesday 14th of January 2009 12:58:31 PM

01-14-2009

Registered User

Problem with linked lists

I am working on a problem for which I need to use a linked list of a sort. For this particular application I need each node to refer to a set of other nodes.
A simplified version of the code looks as follows:

Code:

#include <stdio.h>
#include <stdlib.h>

struct record {
  int id;
  struct record **friends;
};

int main(){
    int i;
    struct record *data;

/* This uncommented piece of code works as it is supposed to */
//  data=malloc(3*sizeof(struct record));
//  data[0].friends=malloc(2*sizeof(struct record));    
//  for(i=0;i<3;i++) data[i].id=i;
//  data[0].friends[0]=&data[1];
//  printf("%d %d\n", data[0].id, data[0].friends[0]->id);  

    subroutine(&data);
}

void subroutine(struct record **data){
    int i;
    
/* This piece of code produces a wrong output */
    *data=malloc(3*sizeof(struct record));
    (*data)[0].friends=malloc(2*sizeof(struct record));
    for(i=0;i<3;i++) (*data)[i].id=i;
    (*data)[0].friends[0]=data[1];
    printf("%d %d\n", (*data)[0].id, (*data)[0].friends[0]->id);  
}

A successful run results in the printing of "0 1". The uncommented code in main() gives the correct result, but if I pass the data to the subroutine, I get something weird. Can anybody help me?

brinch

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LEARN ABOUT MOJAVE

scandir_b

SCANDIR(3)						   BSD Library Functions Manual 						SCANDIR(3)

NAME

     scandir, scandir_b, alphasort -- scan a directory

SYNOPSIS

     #include <sys/types.h>
     #include <dirent.h>

     int
     scandir(const char *dirname, struct dirent ***namelist, int (*select)(const struct dirent *),
	 int (*compar)(const struct dirent **, const struct dirent **));

     int
     alphasort(const struct dirent **d1, const struct dirent **d2);

     int
     scandir_b(const char *dirname, struct dirent ***namelist, int (^select)(const struct dirent *),
	 int (^compar)(const struct dirent **, const struct dirent **));

DESCRIPTION

     The scandir() function reads the directory dirname and builds an array of pointers to directory entries using malloc(3).  It returns the num-
     ber of entries in the array.  A pointer to the array of directory entries is stored in the location referenced by namelist (even if the num-
     ber of entries is 0).

     The select argument is a pointer to a user supplied subroutine which is called by scandir() to select which entries are to be included in the
     array.  The select routine is passed a pointer to a directory entry and should return a non-zero value if the directory entry is to be
     included in the array.  If select is null, then all the directory entries will be included.

     The compar argument is a pointer to a user supplied subroutine which is passed to qsort(3) to sort the completed array.  If this pointer is
     null, the array is not sorted.

     The alphasort() function is a routine which can be used for the compar argument to sort the array alphabetically.

     The memory allocated for the array can be deallocated with free(3), by freeing each pointer in the array and then the array itself.

     The scandir_b() function works the same way as the scandir() function, except that select and compar are blocks instead of subroutines.

DIAGNOSTICS

     Returns -1 if the directory cannot be opened for reading or if malloc(3) cannot allocate enough memory to hold all the data structures.

SEE ALSO

     directory(3), malloc(3), qsort(3), dir(5)

HISTORY

     The scandir() and alphasort() functions appeared in 4.2BSD.  The scandir_b() function appeared in Mac OS X 10.6.

BSD
								   May 20, 2008 							       BSD