Reading words from file error - c

Question

I've got a problem with reading words from file and passing it to binary tree. When I debug it, it says:
Unhandled exception at 0x76E7773B(ntdll.dll) in Projekt.exe: 0.C00000005:
Access violation reading location 0x0037902A.
Here is the source code:
#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
typedef struct Tree {
int val;
char *word;
struct Tree *left;
struct Tree *right;
} Tree;
void show(Tree *hd) {
if (hd != NULL) {
show(hd->left);
show(hd->right);
printf("%s -- %d\n", hd->word, hd->val);
}
}
void zero(Tree *aTree) {
if (aTree == NULL)
return;
zero(aTree->left);
free(aTree);
zero(aTree->right);
}
int alpha(char *word1, char *word2) {
if (word1[0] == 0 && word2[0] == 0)
return 2;
else
if (word1[0] == word2[0])
return alpha(&word1[1], &word2[1]);
else
if (word1[0] < word2[0])
return 1;
else
return 0;
}
Tree *create(char *word) {
Tree *temp;
temp = (Tree*)malloc(sizeof(Tree));
temp->left = temp->right = NULL;
temp->val = 1;
temp->word = (char*)malloc(sizeof(char));
strcpy(temp->word, word);
return temp;
}
Tree *insert(Tree *aTree, char *word) {
if (aTree == NULL) {
aTree = create(word);
} else
if (alpha(aTree->word, word) == 0) {
aTree->left = insert(aTree->left,word);
} else
if (alpha(aTree->word, word) == 1) {
aTree->right = insert(aTree->right, word);
} else
if (alpha(aTree->word, word) == 2) {
aTree->val++;
}
return aTree;
}
int main(int argc, char *argv[]) {
Tree *myTree = NULL;
char buffer[256] = { 0 };
char temp = 0;
int i = 0;
FILE *fp = fopen(argv[1], "r");
if (fp) {
while (temp != EOF) {
temp = getc(fp);
temp = toupper(temp);
if (temp >= 65 && temp <= 90) {
buffer[i] = temp;
i++;
} else {
if (buffer[0] != 0) {
puts(buffer);
myTree = insert(myTree, buffer);
memset(buffer, 0, sizeof(buffer));
i = 0;
}
}
}
}
fclose(fp);
show(myTree);
return 0;
}

Answer 1

Your program has several problems:
in function zero, you free the pointer too soon, you should move the free(aTree); as the last statement, otherwise you invoke undefined behavior, possibly a crash (but not the one you have, since you never call this function):
void zero(Tree *aTree) {
if (aTree != NULL) {
zero(aTree->left);
zero(aTree->right);
free(aTree);
}
In function alpha, you use recursion where a simple loop would suffice. The compiler may convert this to a loop, but it does have to. This is not a bug but why not use a more idiomatic approach such as:
int alpha(const char *word1, const char *word2) {
for (size_t i = 0;; i++) {
if (word1[i] == '\0' && word2[i] == '\0')
return 2;
if (word1[i] == word2[i])
continue;
if (word1[i] < word2[i])
return 1;
else
return 0;
}
}
In function create, you allocate a single byte for the string, this is definitely a cause for the crash. You should allocate strlen(word) + 1 or use strdup(word). You should not cast the return value of malloc() either:
Tree *create(const char *word) {
Tree *temp;
temp = malloc(sizeof(Tree));
temp->left = temp->right = NULL;
temp->val = 1;
temp->word = strdup(word);
return temp;
}
In function insert you call alpha multiple times, this is inefficient: you could use a switch statement:
Tree *insert(Tree *aTree, const char *word) {
if (aTree == NULL) {
return create(word);
switch (alpha(aTree->word, word)) {
case 0:
aTree->left = insert(aTree->left, word);
break;
case 1:
aTree->right = insert(aTree->right, word);
break;
case 2:
aTree->val++;
break;
}
}
return aTree;
}
function main has multiple issues:
You do not check if argv[1] is provided to the program. It would be NULL if the program is run without a command line argument.
Your test for end of file is incorrect: temp should be defined as int and you should test its value after reading the byte from the file with getc(), it is idiomatic to name c a variable used for this.
You should use character literals instead of hard coded ASCII values.
the test if (c >= 'A' && c <= 'Z') would work for ASCII, which is almost universal today, but it is more reliable to use isupper(c) instead.
You do not need to clear the buffer, setting a '\0' at the end before inserting the word is enough.
You should also check for buffer overflow and refuse to handle words longer than 255 characters.
You should not call fclose(fp) when fp is NULL, this is undefined behavior.
Here is a corrected version:
int main(int argc, char *argv[]) {
Tree *myTree = NULL;
char buffer[256];
int c;
size_t i;
FILE *fp;
if (argc < 2) {
printf("missing argument\n");
return 2;
}
fp = fopen(argv[1], "r");
if (fp == NULL) {
printf("cannot open %s\n", argv[1]);
return 1;
}
i = 0;
while ((c = getc(fp)) != EOF) {
c = toupper(c);
if (isupper(c)) {
if (i < sizeof(buffer))
buffer[i] = c;
i++;
} else {
if (i > 0 && i < sizeof(buffer)) {
buffer[i] = '\0';
puts(buffer);
myTree = insert(myTree, buffer);
i = 0;
}
}
}
fclose(fp);
show(myTree);
return 0;
}