Thread: Can anyone show me what is wrong here? Segmentation fault

  1. #16
    Registered User
    Join Date
    Nov 2007
    Anyone have any ideas on how to alphabetize by last name the list of names in the file?

  2. #17
    Jack of many languages Dino's Avatar
    Join Date
    Nov 2007
    Katy, Texas
    I got your code working. Tough assignment for me. (still learning C). Been working on it since noon, about 9-10 hours of work I guess. Thought I'd share.

    One of the issues you had was the fscanf() reading the count. It was leaving the CRLF in the input stream, and the first fgets() to read a record read only the CRLF.

    Also, your pointers are a level deeper than the need to be.

    #include <stdio.h>
    #include <string.h>
    #include <stdlib.h>
    int read_file(char **first_name[], char **last_name[], char **age[]) {
    	int count, i;
    	char line[80];
    	FILE *fp = fopen("names.txt", "r");
    	if (fp==NULL) return 0 ; 
    	//printf("fp = %08X, *fp = %08X\n", &fp, fp );
    	fgets(line, 80, fp); //count = how many names in the file on the first line
    	count = atoi(line) ; 
    	*last_name =  malloc(count * sizeof(char *));
    	//printf(" 3 word area for last name pointers starts @ %p\n", *last_name);
    	*first_name = malloc(count * sizeof(char *));
    	//printf(" 3 word area for first name pointers starts @ %p\n", *first_name);
    	*age = malloc(count * sizeof(char *));
    	//printf(" 3 word area for age pointers starts @ %p\n", *age);
    	for (i = 0; i < count; i++) {
    		fgets(line, 80, fp);
    		(*last_name)[i]  = malloc( 21 * sizeof(last_name) ); // area for last name 
    		(*first_name)[i] = malloc( 21 * sizeof(first_name) ); // area for first name 
    		(*age)[i]        = malloc(  4 * sizeof(age) ); // area for age 
    		strcpy( (*last_name)[i] , strtok(line,", \n") ) ; 
    		strcpy( (*first_name)[i], strtok(NULL,", \n") ) ; 
    		strcpy( (*age)[i]       , strtok(NULL,", \n") ) ; 
    	return count ; 
    int main() { 
    	int i, count ; 
    	char **ages, **first_names, **last_names ;
    	char **ln, **fn, **age ; 
    	count = read_file(&first_names, &last_names, &ages);
    	ln  = (char **) last_names  ; 
    	fn  = (char **) first_names ; 
    	age = (char **) ages ; 
    	for ( i = 0 ; i < count ; i++ ) {
    		printf("Lastname=%s, Firstname=%s, Age=%s\n", 
    			*fn , 
    			*age ) ; 
    			ln  += 1 ; 
    			fn  += 1 ; 
    			age += 1 ; 
    	return 0;
    I added code at the end to print out the records too. It's really not that much code when it comes down to it.


  3. #18
    Algorithm Dissector iMalc's Avatar
    Join Date
    Dec 2005
    New Zealand
    Using triple-pointers can be okay, at a push.
    The reason you're using them in this case though is simply to pass double-pointers as output parameters. That being the case, inside your function you should begin by declaring some double-pointers, and then work with those throughout the function. Then when it comes time to return them you can simply do an assignment to *first_name *last_name and *age from the double-pointers.
    And viola, you're down to double-pointer for most of the code.
    Then, you can take that a step furthur doing the same thing within your loops, and all of a sudden you're down to the familiar plain old single-pointer and the whole thing seems so much more comprehensable!

    Not to mention it will probably be faster then as well, since the compiler will have less trouble working out what pointer aliasing could be going on, and can hence do a better job of optimisation.
    Last edited by iMalc; 11-22-2007 at 01:25 AM.
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