using strtok to get input from file - c

Question

I am trying to create a program that takes in a number of processes (name, start time, remaining time) from a file, then uses a round robin algorithm to handle the queue.
The issue is, when I try to tokenize each line of the file by using strtok() and fgets(), the name of the process is always wrong.
For example, if the first line is P1 0 3 the output is like this:
void RoundRobin(char *filename) {
Queue *q = initQueue();
char string[MAX_SIZE];
FILE *file;
Process process[20];
char *token;
file = fopen(filename, "r");
if (!file) {
printf("File Cannot Be Opened");
}
fgets(string, 150, file);
token = strtok(string, "=");
token = strtok(NULL, "+");
int time_quantum = atoi(token);
int process_count = 0;
while (fgets(string, 150, file)) {
char *token1;
token1 = strtok(string, " ");
process[process_count].name = token1;
token1 = strtok(NULL, " ");
process[process_count].starting_time = atoi(token1);
token1 = strtok(NULL, " ");
process[process_count++].remaining_time = atoi(token1);
token1 = strtok(NULL, " ");
}
for (int i = 0; i < process_count; i++) {
printf("%s %d %d\n", process[i].name, process[i].starting_time, process[i].remaining_time);
}
fclose(file);
}

Answer 1

You are reusing a single char[] for all of your token parsing. fgets() will overwrite the contents of that char[] each time, and strtok() will return pointers to memory inside of that char[]. Thus, each time you read a new line from the file, the previous pointers you already stored in the process[] array are still pointing at the same memory, but the contents of that memory have been altered.
You need to instead allocate a separate char[] string for each name that you want to save in the process[] array. You can use strdup() for that, eg:
while (fgets(string, 150, file)){
char* token1 token1 = strtok(string, " ");
process[process_count].name = strdup(token1); // <-- HERE
...
}
// use process[] as needed...
for(int i = 0; i < process_count; i++){
free(process[i].name);
}

Answer 2

The problem is strtok() returns a pointer into the line that it parses. Hence all entries in the process array point to the same string array that is modified by the call to fgets().
You must duplicate the string you store in the process description structure:
void RoundRobin(const char *filename) {
char string[MAX_SIZE];
Process process[20];
Queue *q = initQueue();
char *token;
FILE *file;
file = fopen(filename, "r");
if (!file) {
printf("Cannot open file %s\n", filename);
return;
}
int time_quantum = 0;
int process_count = 0;
if (fgets(string, sizeof string, file)
&& (token = strtok(string, "=")) != NULL
&& (token = strtok(NULL, "+")) != NULL) {
time_quantum = atoi(token);
}
while (fgets(string, sizeof string, file)) {
char *token1;
if ((token1 = strtok(string, " ")) == NULL)
contine;
process[process_count].name = strdup(token1);
if ((token1 = strtok(string, " ")) == NULL)
contine;
process[process_count].starting_time = atoi(token1);
if ((token1 = strtok(string, " ")) == NULL)
contine;
process[process_count].remaining_time = atoi(token1);
process_count++;
}
for (int i = 0; i < process_count; i++) {
printf("%s %d %d\n", process[i].name, process[i].starting_time, process[i].remaining_time);
}
for (int i = 0; i < process_count; i++) {
free(process[i].name);
}
fclose(file);
}