Segmentation faulting in assignment - c

Question

I'm currently learning C and I came from java. Our assignment asked us to count strings from either a file that could be added or it asks for the user to input a string. We just started using pointers and i looked up the different reasons why segmentation faults happened but I have no idea how to check for which issue it is. I initialized all my pointers to NULL but it still didn't work and from what i read that was the most common reason why a segmentation fault happens.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int debug = 0;
int
main(int argc, char **argv)
{
extern char *optarg;
extern int optind;
FILE* infile = NULL;
int c, err = 0;
int lflag = 0, sflag = 0, count = 0; //flags and count
char *shortWord = NULL, *longWord = NULL; //variable for shortest and longest word
int shortest = 100, longest = 0; //longest char 100, shortest 0
char *string = NULL;
char *pch = NULL;
static char usage[] = "usage: %s [-l] [-s] [filename]\n";
while ((c = getopt(argc, argv, "ls")) != -1)
switch (c)
{
case 'l':
lflag = 1;
break;
case 's':
sflag = 1;
break;
case '?':
err = 1;
break;
}
if (err)
{
fprintf(stderr, usage, argv[0]);
exit(1);
}
//checks for file and then runs loop for word count
infile = fopen("myfile.txt","r");
if (infile != NULL)
{
fgets(string, 100, infile);
pch = strtok (string, " ,.-");
while(pch != NULL)
{
count++;
if (strlen(pch) > longest)
longWord = pch;
if (strlen(pch) < shortest)
shortWord = pch;
pch = strtok (NULL, " ,.");
}
}
//else, asks for string
else
{
printf("Enter your string: \n");
fgets(string, 100, stdin);
int len = strlen(string);
count = len;
pch = strtok ( string, " ,.-");
while(pch != NULL)
{
count++;
if (strlen(pch) > longest)
longWord = pch;
if (strlen(pch) < shortest)
shortWord = pch;
pch = strtok (NULL, " ,.");
}
}
//following lines compute value based on arguments
if(lflag == 1)
{
printf("Longest word is %s", longWord);
}
if(sflag == 1)
{
printf("Shortest word is %s", shortWord);
}
printf("Word count = %.2d\n", count);
exit(0);
}

Answer 1

Their are some issues in your code:
You initialized string to NULL, then used it as an input buffer for fgets(). fgets() reqiures a pointer to an array of chars, either declared on the stack or dynamically allocated with malloc(3). You can set an input buffer such as char string[100].
fgets() must be checked, as it returns NULL when unable to read a line.
Your delimiter for strtok() is not accounting for the \n character appended by fgets(). You can either remove this newline, or include it in the delimter. If you want to include it in the delimeter, make sure your delimiter is " ,.-\n".
You could create function which parses your input with strtok(), as this would allow your main() to be shorter and reduce the repetitiveness in the code. An example function prototype could be void longest_shortest_words(char line[], char **longest, char **shortest, size_t *word_count);, whereby you pass the longest, shortest words along with the number of words back to main() via pointers. You could also just store the longest and shortest words in a 2D array or array of pointers.
You should also explicitly check that your file was opened correctly. Something like this should be included:
infile = fopen("myfile.txt", "r");
if (infile == NULL) {
fprintf(stderr, "Failed to open file\n");
exit(EXIT_FAILURE);
}
When checking opt, checking ? as a character in your switch statement is not right. Instead of:
case '?':
err = 1;
break;
Use default, which covers any other invalid option entered. Here is how you can use it:
default:
fprintf(stderr, "usage: %s [-l] [-s] [filename]\n", argv[0]);
exit(EXIT_FAILURE);
Checking sflag and lflag at the end is not enough. You should check if longWord and shortWord are not NULL.
Here is some example code which demonstrates these points:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#define LINESIZE 100
void longest_shortest_words(char line[], char **longest, char **shortest, size_t *wordcount);
void print_output(int lflag, int sflag, char *longword, char *shortword, size_t wordcount);
void remove_newline(char line[]);
int main(int argc, char * const argv[]) {
FILE *infile;
char line[LINESIZE] = {'\0'};
int opt, sflag = 0, lflag = 0;
size_t wordcount = 0;
const char *optstr = "ls";
char *longword = NULL, *shortword = NULL;
while ((opt = getopt(argc, argv, optstr)) != -1) {
switch(opt) {
case 'l':
lflag = 1;
break;
case 's':
sflag = 1;
break;
default:
fprintf(stderr, "usage: %s [-l] [-s] [filename]\n", argv[0]);
exit(EXIT_FAILURE);
}
}
/* Checking if file is in directory */
infile = fopen("myfile.txt", "r");
if (infile == NULL) {
fprintf(stderr, "Failed to open file\n");
exit(EXIT_FAILURE);
}
/* checking if line exists in file */
if (fgets(line, LINESIZE, infile) == NULL) {
fprintf(stderr, "No line found in file.\n");
printf("\nEnter string instead:\n");
if (fgets(line, LINESIZE, stdin) != NULL) {
remove_newline(line);
longest_shortest_words(line, &longword, &shortword, &wordcount);
/* checking that longWord, shortWord and word_count are valid */
if (longword != NULL && shortword != NULL && wordcount > 0) {
print_output(lflag, sflag, longword, shortword, wordcount);
}
}
/* file has line, do stuff with it */
} else {
remove_newline(line);
longest_shortest_words(line, &longword, &shortword, &wordcount);
print_output(lflag, sflag, longword, shortword, wordcount);
}
exit(EXIT_SUCCESS);
}
/* function for printing output, can be improved */
void print_output(int lflag, int sflag, char *longword, char *shortword, size_t wordcount) {
if (lflag) {
printf("Longest word: %s\n", longword);
}
if (sflag) {
printf("Shortest word: %s\n", shortword);
}
if (wordcount > 0) {
printf("Word count = %zu\n", wordcount);
}
}
/* function for removing newline, and checking that input hasnt exceeded limit */
void remove_newline(char line[]) {
size_t slen;
slen = strlen(line);
if (slen > 0 && line[slen-1] == '\n') {
line[slen-1] = '\0';
} else {
fprintf(stderr, "\nToo many characters in input.\n");
exit(EXIT_FAILURE);
}
}
/* function which parses line, and saves longWord and shortWord in pointers */
void longest_shortest_words(char line[], char **longword, char **shortword, size_t *wordcount) {
char *word = NULL;
const char *delim = " ,.";
word = strtok(line, delim);
if (word != NULL) {
*longword = word;
*shortword = word;
*wordcount = 1;
}
while ((word = strtok(NULL, delim)) != NULL) {
(*wordcount)++;
if (strlen(word) > strlen(*longword)) {
*longword = word;
} else if (strlen(word) < strlen(*shortword)) {
*shortword = word;
}
}
}
Note: The code shown above can be improved, it is just to show you another approach to your problem.