Segmentation Fault when initializing struct

Question

Having trouble figuring out why I run into a segmentation fault with the longer piece of code.

I take this and run it fine.

struct Earthquake
    {
        char *prevString;
        char *magString;
        char *postString;
    };  
int main(void)
{
    struct Earthquake eq;
    eq.prevString="PREVIOUS STRING";
    eq.magString = "50";
    eq.postString = "POST STRING";

    printf("\n%s",eq.prevString);
    printf("\n%s",eq.magString);
    printf("\n%s\n",eq.postString);
}

When I try to run this I get a segmentation fault, which I stop getting if I comment out the struct initialization. You might be able to tell with what I have commented out, but once I get this working I'm trying to make a struct array at which point I know the eq needs to be an Earthquake pointer but I can't even get this working.

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
//#include <stddef.h>
//#include <paths.h>

struct Earthquake
{
    char *prevString;
    char *magString;
    char *postString;
};


int main()
{
    int numLines = 0;
    FILE *fileA = fopen("./all_month.csv", "r");
    char c[1];

    do
    {
        *c = fgetc(fileA);
        //printf("%c", c);

    }while(*c != '\n');

    do
    {
        *c = fgetc(fileA);
        //printf("%c", c);
        if(*c == '\n')
        {
            numLines++;
        }

    }while(*c != EOF);

    //printf("%d",numLines);
    fclose(fileA);
    FILE *fileB = fopen("./all_month.csv", "r");
    char prevStringTemp[60];
    char postStringTemp[150];
    char magStringTemp[10];

    struct Earthquake eq; 
    //if(NULL == (entries = (Earthquake *)malloc(sizeof(Earthquake) * numLines)));
    //printf("\nmalloc failed\n");

    do
    {
        *c = fgetc(fileB);
        //printf("%c", c);

    }while(*c != '\n');
    char line[200];
    int commaCount = 0;
    do{
        *c = fgetc(fileB);
        if(*c==',')
        {
            commaCount++;
            //printf("\n%d", commaCount);
        }
        strcat(prevStringTemp, c);
    }while(commaCount<4);

    do
    {
        *c = fgetc(fileB);
        strcat(magStringTemp, c);
    }while(*c!=',');

    do
    {
        *c = fgetc(fileB);
        strcat(postStringTemp, c);
    }while(*c!='\n');

    //strcpy(entries[0].prevString, prevString);
    //printf(entries[0].prevString);

    //fscanf(fileB,"%s",line);

    //printf("\n%s\n", line);
    fclose(fileB);
    //free(entries);
    return 0;
}

Answer 1

This is the problem

strcat(prevStringTemp, c);

beacause strcat() expects nul terminated strings which niether the parameters is in your case.

Your c array should be

char c[2] = {0};

and the first element of these

char prevStringTemp[60];
char postStringTemp[150];
char magStringTemp[10];

should be initialized to '\\0' , like this

prevStringTemp[0] = '\0';
postStringTemp[0] = '\0';
magStringTemp[0]  = '\0';

after fixing this, strcat() will behave as you expect.