Segmentation fault error - C programming - c++

Question

I am receiving a Segmentation Fault error when I run this program. To summarize, the program reads in multiple data files (129 of them). Each file contains information about a specific name. The information includes how many people were named a specific name in a specific year and the gender. For now, I am trying to store each name in a linked list. Whenever I read in more than about 4 data files, I get the Segmentation Fault error. I have written this program in Java, which was much simpler. If anybody could simply point me in the right direction, as I am almost certain this has to do with memory allocation, but I cannot seem to solve this myself. Thank you.
#include <iostream>
#include <fstream>
#include <stdio.h>
#include <string.h>
#include <cstdlib>
using namespace std;
//typedef struct Node NodeStruct;
struct Node {
char *namePtr;
Node *nextPtr;
};
int main() {
// Declare variables
Node *headPtr = NULL;
Node *tempPtr;
Node *currentPtr;
// Variables for reading in a file
FILE *filePtr;
char fileName[20];
int i;
int nameLength;
char inputLine[81];
cout << "Reading from data files, please be patient...\n";
// Loop through files
for (i = 1880; i <= 2009; i++) {
sprintf(fileName, "data/yob%d.txt", i);
filePtr = fopen(fileName, "r"); // Open the file
// Check to ensure file was opened
if(filePtr == NULL) {
cout << "Error opening input file...check location of data files\n";
exit(-1); // Exit program
} // End if statement
while (fscanf(filePtr, "%s", inputLine) != EOF) {
// Create a node
tempPtr = (Node *) malloc(sizeof(Node));
tempPtr->nextPtr = NULL;
// Set the head pointer of first node
if (headPtr == NULL) {
headPtr = tempPtr;
currentPtr = tempPtr;
} // End if statement
// Link the list
currentPtr->nextPtr = tempPtr;
currentPtr = currentPtr->nextPtr;
// Create pointer variables
char *startPtr = inputLine;
char *endPtr = NULL;
endPtr = strchr(inputLine, ','); // Point to end of name
int length = endPtr - inputLine; // Calculate length
// Create space for the name
tempPtr->namePtr = (char *) malloc(sizeof(length + 1));
strncpy(tempPtr->namePtr, startPtr, length); // Store pointer to name
// cout << tempPtr->namePtr << endl;
}
} // End of for (i = 1880...
cout << "Done reading from data files...\n";
} // End of main function

Answer 1

Surely
tempPtr->namePtr = (char *) malloc(sizeof(length + 1));
should be
tempPtr->namePtr = (char *) malloc(length + 1);
since you copy that many characters to the string. sizeof (length + 1) would evaluate to four on a 32-bit machine (eight on 64-bit). Not enough memory was being allocated, so the strncpy which followed this was overwriting memory not belonging to you.

Answer 2

Rather than find your bug, let's try to teach you some practical lessons. Here are my C++ rules for C programmers:
1) Don't use pointers to track your data. The standard containers work just fine for that.
2) Don't use pointers to manage your strings. The standard string type works just fine for that.
3) Don't use pointers for anything else, until you need to learn how polymorphism works.
4) Don't use malloc at all. Ever.
5) Don't use new, hardly at all.
The neat thing about not using pointers (or arrays), is that you will never make pointer bugs. No more buffer overflows, no more segmentation faults. Joy!
Here is my translation of your program into idiomatic C++. Because I let std::list do all of the list management, all of the silly headPtr, nextPtr, etc, goes away. Because I let std::string do all of the string management, I don't need to malloc(strlen()) (or fix my bug and malloc(strlen()+1). Because I use the RAII idiom, I don't have to worry about closing my files.
It all just works.
#include <iostream>
#include <fstream>
#include <string>
#include <list>
#include <cstdlib>
#include <sstream>
using std::string;
using std::cout;
using std::list;
using std::ifstream;
using std::stringstream;
int main() {
// Declare variables
list<string> list;
cout << "Reading from data files, please be patient...\n";
// Loop through files
for (int i = 1880; i <= 2009; i++) {
stringstream fileName;
fileName << "data/yob" << i << ".txt";
ifstream filePtr(fileName.str().c_str());
if(!filePtr.is_open()) {
cout << "Error opening input file: " << fileName.str() << " ...check location of data files\n";
exit(-1); // Exit program
}
string inputLine;
while (filePtr >> inputLine) {
list.push_back(inputLine);
}
} // End of for (i = 1880...
cout << "Done reading from data files...\n";
} // End of main function

Answer 3

I know this one is already answered, but you should also be in the habit of closing your files. For you with something like fclose(filePtr). By the way if you learn how to use std::ifstream then you don't have to close, it auto closes when the std::ifstream goes out of scope.