C - Why does char array only return last value that's put into it?

Question

I'm writing in C. I'm trying to read lines from a text file, parse the line, and put some info into an array of strings. When I test my code, every value in the array seems to be the last value inserted. What causes this?

int r;
char *users[51]; //given no more than 50 users
for (r = 0; r < 51; r++) {
    int n = 15; //arbitrary guess at length of unknown usernames
    users[r] = malloc((n + 1) * sizeof(char));
}
FILE *fp;
fp = fopen(argv[1], "r");

char *username;

int counter = 0;
char line[100];
while (fgets(line, 100, fp) != NULL) {
    username = strtok(line, ":");

    users[counter] = username;
    printf("%s\n", username);
    printf("%s\n", users[counter]);

    //counter increase for later
    counter += 1;

Answer 1

You put a sensible value into each entry in the array:

users[r] = malloc((n+1) * sizeof(char));

But then you overwrite it with a nonsensical value (a pointer into line ):

users[counter] = username;

What you presumably wanted to do was copy the string pointed to by username into the space allocated at users[counter] . The strcpy function can do this.

Answer 2

strtok is a very confusing function:

• it modifies the array it receives a pointer to
• it returns a pointer to an element of this array.
• it keeps an internal state which makes it non-reentrant and non thread-safe.

Hence username points inside line . You store this pointer into users[counter] . At the end of the loop, all entries in users point to the same array that has been overwritten by every call to fgets() .

You should duplicate the contents of the array with strdup() :

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

int main(int argc, char *argv[) {
    char *users[51]; //given no more than 50 users
    int r;
    FILE *fp;

    if (argc < 2) {
        fprintf(stderr, "missing filename argument\n");
        return 1;
    }
    fp = fopen(argv[1], "r");
    if (fp == NULL) {
        fprintf(stderr, "cannot open file %s\n", argv[1]);
        return 1;
    }
    char line[100];
    int counter = 0;
    while (counter < 50 && fgets(line, 100, fp) != NULL) {
        char *username = strtok(line, ":");
        if (username != NULL) {
            users[counter] = strdup(username);
            //counter increase for later
            counter += 1;
        }
    }
    users[counter] = NULL;

    ...
}

Answer 3

I would just like to add to David's answer that you should also check the username string is null terminated before using strcpy() . I can't remember if strtok() null terminates but you shouldn't rely on it anyway.