I'm running into buffer overflows when reading a file in C and copying character arrays. There are three potentially offending pieces of code and I can't figure out where I'm going wrong.
The first reads a file and populates it into a hashmap:
bool load_file(const char* in_file, hmap hashtable[]) {
for(int x = 0; x < HASH_SIZE; x++) {
hashtable[x] = NULL;
}
FILE *fptr = fopen(in_file, "r");
char c[LENGTH] = "";
c[0] = '\0';
while (fgets(c, sizeof(c)-1, fptr) != NULL) {
node *n = malloc(sizeof(node));
hmap new_node = n;
new_node->next = NULL;
strncpy(new_node->content, c, LENGTH-1);
// do stuff to put it into the hashtable
}
fclose(fptr);
return true;
}
The second checks whether given content is in the hashmap:
bool check_content(const char* content, hmap hashtable[]) {
char c_content[LENGTH] = "";
strncpy(c_content, content, LENGTH-1);
// do stuff to check if it's in the hashmap
return false;
}
and the third parses a given file and checks whether its content is in the hashmap:
int check_file(FILE* fp, hmap hashtable[], char * not_found[]) {
int num_not_found = 0;
char c[1000] = "";
while (fgets(c, sizeof(c)-1, fp) != NULL) {
char * pch;
char curToken[LENGTH] = "";
pch = strtok (c," ");
strncpy(curToken, pch, LENGTH-1);
curToken[LENGTH]=0;
if(!check_content(curToken, hashtable)) {
not_found[num_not_found] = malloc(LENGTH*sizeof(not_found[num_not_found]));
strncpy(not_found[num_not_found], curToken, LENGTH-1);
num_not_found++;
}
}
fclose(fp);
return num_not_found;
}
Finally, main calls these and frees mallocs:
int main (int argc, char *argv[])
{
hmap hashtable[HASH_SIZE];
load_file(argv[2], hashtable);
FILE *fptr = fopen(argv[1], "r");
char * not_found[MAX_ENTRIES];
int num_not_found = check_file(fptr, hashtable, not_found);
for(int x=0; x<num_not_found; x++) {
free(not_found[x]);
}
for(int y=0; hashtable[y] != NULL; y++) {
free(hashtable[y]);
}
return 0;
}
My question is this: for each of the three code snippets, what have I done that causes buffer overflows? Many thanks in advance!
I finally got rid of the buffer overflow problems mostly by following David's advice in the comments, plus figuring out that I had one more malloc than I needed. The fixes were:
new_node->next
needed a malloc new_node->next
should happen only if it's actually going to be used. not_found[num_not_found] = malloc(LENGTH*sizeof(not_found[num_not_found]));
was wrong and should have been notfound[num_not_found] = malloc(sizeof(char) * (strlen(pch)+1))
(assuming pch wasn't null terminated). (Side note, for whatever reason, on my computer, malloc(sizeof(char) * strlen(pch)+1)
is not the same as malloc(strlen(pch)+1)
)
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