I know another seg fault

[xem2189]

Okay so this is my first post and I hope someone can help. I've been writing this program for a class and it is essentially done except for this one part which I've attached. Basically I want to open a file, in this case it is a list of words in the file linux.words, and then allocate memory so I can put the words into an array. Well I figured first that I need to know how many words were in the file and since I don't know any real shortcut on how to do that I wrote the code below to count how many times it scans in a word and use that number to allocate memory.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(int argc, char *argv[])
{
// Initialize variables
FILE *dictionaryFile; // Dictionary
char **dictionary; // Array to hold dictionary
char dictionaryword; // Word in the dictionary
int num_word = 0;

// Check if correct number of command line arguments
if(argc != 2)
{
printf("\nError, insufficient command line arguments!");
printf(" Exiting.\n\n");
return 0;
}

// Open file
dictionaryFile = fopen(argv[1], "r");

// Verify file opened successfully
if(dictionaryFile == NULL)
{
printf("\nError, dictionary file %s could not be opened\n\n", argv[1]);
return 0;
}

fscanf(dictionaryFile, "%s", &dictionaryword);

// Calculate the number of words in the dictionary
while(!feof(dictionaryFile))
{
fscanf(dictionaryFile, "%s", &dictionaryword);
printf("%d\n", num_words);
num_words ++;
}

// Close the dictionary file
fclose(dictionaryFile);

return 0;

}

Problem: I keep getting a seg fault at the end of my while(!feof....) and I have absolutely no idea why that is happening. If someone could explain this to me that would be great.

[MadCow108]

char dictionaryword; // Word in the dictionary
this has only room for a single char
so fscanf overwrites this

increase the size (e.g. char dictionaryword[100]) and replace fscanf with this:
fscanf(dictionaryFile, "%100s", dictionaryword);

[napsy]

fscanf(dictionaryFile, "%s", &dictionaryword);

You are storing a string in 1-byte allocated memory.

[xem2189]

Okay thanks guys. Sometimes it is just the simplest stuff. One more question though, and I think I know the answer but what does the 100 in %100s do?

[napsy]

it says only a string of 100 characters should be read by scanf()

[xem2189]

Okay that makes sense. Thanks. :)

[MadCow108]

this is just to make sure no buffer overflow happens because of malformed input.
it will truncate your data but that (when correctly handled) does not form a security risk

you could also use the "%as" flag so that scanf allocates its own memory of appropriate size
in that case you will have to explicitly free it again
see man fscanf
this is also a non standard extension of gcc so you will lose portability

[dwhitney67]

A safer bet when reading C-string is to use fgets(). This function will stop reading after a newline character is found or one less than the number of characters specified is read. If a newline is found, it is included in the result.

If a line has multiple words in it, then use strtok() or other "tool" to parse the line.

In the case the OP was attempting to solve, fgets() would be used as:


char dictionaryWord[100];
...
fgets(dictionaryWord, sizeof(dictionaryWord), dictionaryFile);

/* to remove a newline, if it exists */
char* nl = strchr(dictionaryWord, '\n');
if (nl) *nl = '\0';

...