Why am I getting a seg fault while calling strtok_r?

Question

So, I am getting a seg fault whenever I try to call strtok_r, however I am not completely sure why. I am getting this error within this block of code.

I know my error is coming from this section of code, because if I were to take out this while loop, and replace with simply just a printf(str); I get no errors and it prints correctly.

Below is my method containing strtok_r:

void parseCommand(char *str)
{
  char *curr;
  char *temp = str;
  char **args=NULL;

  int i = 0;
  curr=strtok_r(temp, " ", &temp);
  while(curr!=NULL){
    args[i]=curr;
    curr=strtok_r(NULL," ",&temp);
    i++;
  }

  for(int j = 0;j<i-1;j++){
    printf("%s", args[j]);
  }
}

If my understanding of seg faults are correct, this is happening because I am trying to access some sort of memory that this process doesn't have permission to access, but I am unsure why. Anything helps!

EDIT: I have tried to experiment with this a little bit, and had assumed that there wasn't a null terminator at the end of my string, however when I added the line

strncat(cur,"\0",50);

to the inside of my while loop, just before I assign to args[i] the same result is yielded.

EDIT: updated strtok_r usage

char **args=NULL ... args[i]=curr;. You never allocated any space for args. — Christian Gibbons
How would I know how much space to allocate, if my input can be of variable length? — backward forward
If the input string has length N, there can be at most N/2 tokens. — user3386109
@backwardforward Any way that you want. There are lots of options. You can guess and re-allocate more if needed. You can allocate more than you will ever need and "right size" afterwards. There are lots of other ways too. — David Schwartz
Also notice that you call strtok_r incorrectly. Only the first call shall pass temp. All following calls shall pass NULL — 4386427

4386427 4386427 · Accepted Answer · 2020-09-14T20:46:05

I see two problems:

No memory assigned to args
Wrong use of strtok_r

To solve the first problem you can use realloc when new tokens are found.

To solve the second problem, you need to call strtok_r once using str and then NULL the rest of the time.

Something like:

void parseCommand(char *str)
{
  char *curr;
  char **args=NULL;
  char *saveptr;  

  int i = 0;

  // Look for first token, i.e. pass str
  curr = strtok_r(str, " ", &saveptr);
  
  while(curr != NULL){
    // Allocate memory for the pointer
    char **p = realloc(args, (i + 1) * sizeof *p);
    if (p == NULL) exit(1);
    args = p;

    // Save the pointer
    args[i]=curr;
    i++;

    // Look for next token, i.e. pass NULL
    curr = strtok_r(NULL, " ", &saveptr);
  }

  for(int j = 0; j < i; j++){
    printf("%s\n", args[j]);
  }
}

When you call the function do:

char some_text[] = "Hello how are you";
parseCommand(some_text);

Do not call like

parseCommand("Hello how are you");  // Illegal

strtok_r will modify the passed string so you must pass it a string that can be modified.

Why am I getting a seg fault while calling strtok_r?

1 Answers