C File I/O bug in my code - c++

Question

I attempted writing a thesaurus program which reads a thesaurus file, for example:
drink:beverage
clever:smart,witty
and a .txt document, changing up the words it finds from the thesaurus and creating a new document with the modified text. However there appears to be a bug, I have narrowed it down to the while loop in getReplacement(), by checking a print operation before and after. I would really appreciate someone finding why it won't work.
#include <stdio.h>
#include <ctype.h>
#include <string.h>
#include <iostream>
char* getReplacement(char* original, FILE* file);
int main(int argc, char* argv[])
{
using namespace std;
FILE* thes = fopen(argv[1], "r");
FILE* text = fopen(argv[2], "r+");
FILE* nText = fopen("temp.txt", "w");
if(thes == NULL || text == NULL || nText == NULL)
return 1;
char word[20] = {};
char c;
int bytesW=0;
while((c = fgetc(text)) != EOF)
{
fputc(c, nText);
bytesW++;
if(isalpha(c))
{
int len = strlen(word);
word[len] = c;
word[len + 1] = '\0';
}
else
{
if(word == "")
continue;
cout << 7<<endl;
char* replacement = getReplacement(word, thes);
if(replacement == NULL)
continue;
fseek(nText,bytesW-1-strlen(word),SEEK_SET);
for(int i=0;i<strlen(replacement);i++)
fputc(replacement[i],nText);
int diff = strlen(word) - strlen(replacement);
while(diff-- >0)
fputc(' ', nText);
bytesW = bytesW-1-strlen(word)+strlen(replacement);
fseek(nText, bytesW, SEEK_SET);
}
}
fclose(thes);
fclose(text);
fclose(nText);
return 0;
}
char* getReplacement(char* const original, FILE* file)
{
using namespace std;
char* line="";
const short len = strlen(original);
int numOfOptions=1;
int toSkip=0; // number of commas to skip over
outer: while(fgets(line,1000,file) != NULL)
{
for(int i=0;i<len;i++)
if(line[i] != original[i])
{
goto outer;
}
if(line[len] != ':')
goto outer;
for(int i=0;i<len;i++)
line++;
for(int i=0;i<strlen(line);i++)
if(line[i] == ',')
numOfOptions++;
toSkip = rand()%numOfOptions;
while(toSkip >0)
{
if(line[0] == ',')
toSkip--;
line++;
}
return line;
}
return NULL;
}

Answer 1

char* line="";
// ... snip ...
outer: while(fgets(line,1000,file) != NULL)
Here's your problem. You are trying to read into a literal string; you instead need to allocate an array, on the stack or via malloc() to read into.
A string that you write in quotes in C is known as a literal. This means that this string gets embedded in the code of your program, and later loaded into memory when your programs is loaded. Usually it gets loaded into memory that's marked read-only, but that's platform dependent. That string that you wrote has room only for the null terminator. But you are trying to read up to 1000 characters into it. This will either lead to a segmentation fault because you were writing to read-only memory, or will lead to you writing all over some other memory, producing who knows what behavior.
What you want to do instead is allocate a buffer that you can read into:
char line[1000];
or, if you have limited stack space:
char *line = malloc(1000 * sizeof(char));
Furthermore, in your main() function, you do:
char c;
while((c = fgetc(text)) != EOF)
fgetc() returns an int, not a char. This way, it can return a value corresponding to a valid character if a value is read, or a value that is outside that range if you hit the end of file.
You can't compare strings in C using ==; what that does is compare whether they are the same pointer, not whether they have the same contents. It doesn't really make sense to recalculate the length of the current word each time; why not just keep track of len yourself, incrementing it every time you add a character, and then when you want to check if the word is empty, check if len == 0? Remember to reset len to 0 after the end of the word so you'll start over on the next word. Also remember to reset if len goes over sizeof(word); you don't want to write more than word can hold, or you will start scribbling all over random stuff on your stack and lots of things will break.