Not allocating memory for pointers, is this causing my segmentation fault (core dumped) in C?

Question

I am able to run it on my windows computer in eclipse, but when I try running it on a Unix computer I get "Segmentation fault (core dumped)". How can I find which line is causing the error? I read this page http://www.cs.mun.ca/~michael/c/problems.html which says it could be due to not allocating memory for pointers, but I am getting errors when I try for example char *users[1000];

#include <stdio.h>
#include <ctype.h>
#include <string.h>

#define MAX_CHANGE (10.0/86400.0)       /* 10kg/day */
    /* seconds in a day is 24 hours * 60 minutes * 60 seconds */

main() {
    char line[1024];
    char lineC[1024];

    int countToken = 0;
    int lasttime = 0;
    char *tokens;
    char *users;
    int timestamp;
    int duration;
    char userID[1000];
    char lastuser[1000];
    float weight;
    float lastweight;
    float change;
    float changePerTime;

    while (fgets(line,1024,stdin) != NULL) {
        strcpy(lineC, line);
        tokens = strtok(line, " ");
        sscanf(tokens, "%d", &timestamp);   //first token is timestamp
        while(tokens != NULL){
            countToken++;
            tokens = strtok(NULL, " ");
        }

        int countTemp = countToken;
        users = strtok(lineC, " ");
        while(countTemp > 1){
            if(countTemp == countToken){
                countTemp--;
            }
            else{
                users = strtok(NULL, " .0123456789");
                strcat(userID, users);
                countTemp--;
            }
        }
        users = strtok(NULL, " ");
        sscanf(users, "%f", &weight);

        if (countToken < 1 || timestamp == 0) {
            printf("Invalid time\n");
            continue;
            }
        else if (countToken < 2 || ! (isalpha(userID[0]) || userID[0] == '_') )
            printf("Illegal userID\n");
        else if (countToken < 3 || weight < 30.0 || weight > 300.0)
            printf("Illegal weight\n");
        else if (lasttime >= timestamp)
                printf("Nonmonotonic timestamp\n");
        else {
            duration = timestamp - lasttime;
            change = weight - lastweight;
            changePerTime = change / duration;
            int g = strcmp(lastuser, userID);
            if (lasttime > 0 && (changePerTime < -MAX_CHANGE || changePerTime > MAX_CHANGE) && (g==0))
                printf("Suspiciously large weight change\n");
            else
                printf("OK\n");

            lastweight = weight;
            lasttime = timestamp;
            }
        strcpy(lastuser, userID);
        lasttime = timestamp;
        countToken = 0;
        strcpy(userID,  "");

    }
}

Answer 1

Sometimes it can be hard to start developing for a new platform, but once you understand the basic concepts it is just as easy as any other platform. :)

I would recommend you to do the following:

1. Compile your code with debugging symbols. This will enable you to debug kind of similar to the way you would do it in VS. Using the gcc compiler (and some others) you should use the -g argument to do it.
2. Configure your terminal to generate coredumps. A coredump is an exact copy of the process state at the time where it crashed. Enabling this depends on the terminal you are using (bash, csh, etc) but you can try the following: 'ulimit', 'limits', 'limit'
3. Run gdb. If you used the -g argument to compile, and you have the coredump, you can do 'gdb -c COREDUMP_FILENAME BinaryFilename' It will take you exactly to the line that caused the crash 2.a Alternately you can run your program from gdb 'gdb BinaryFilename' and step every line until your program crashes.

Answer 2

您正在滥用NULL指针，并且在修复代码之前，请先阅读有关指针（特别是NULL指针）的更多信息，然后选中此按钮。