Store strtok in an array ~ C-CodePudding

I was given the parseInput function from a teacher. I am not able to get it to run without a segmentation fault and I hope you guys can help me.

I think it has to do with how I pass total_cmd_words, but I'm not sure.

// Libraries
#include <stdio.h>
#include <string.h>

int parseInput(char *input, char* splitWords[]){
      int wordInd = 0;
      splitWords[0] = strtok(input, " "); // VSCODE says this line is causing the fault
      while(splitWords[wordInd] != NULL){
              splitWords[  wordInd] = strtok(NULL, " ");
      }

      return wordInd;
}

int main(){

  char* total_cmd = "file1 > file2";
  char* total_cmd_words[] = {};

  parseInput(total_cmd, total_cmd_words);

}

gdb gives this output:

__GI___strtok_r (s=0x555555556006 "file1 > file2", delim=0x555555556004 " ", 
    save_ptr=0x7ffff7fb0ca8 <olds>) at strtok_r.c:72
72  strtok_r.c: No such file or directory.

Changing: char* total_cmd_words[] = {}; to this: char* total_cmd_words[100] = {}; Still results in a segmentation fault.

CodePudding user response：

There are two errors in your main function.

First, your declaration of total_cmd_words is wrong: as it stands, you're declaring an array of zero length – so there is no space to actually store the pointrs returned by the strtok function in your parseInput function. You need to specify a dimension for the array inside the [] – one that is large enough to hold the maximum number of values that your are likely to encounter.

Second, a call to strtok modifies the string given as its first argument. However, your total_cmd is a pointer to a non-mutable string literal. Instead, declare that as an array of char and initialize it with a copy of the string literal.

Here's a possible working version of your main:

int main()
{
    char total_cmd[] = "file1 > file2";
    char* total_cmd_words[10] = {0,};
    int p = parseInput(total_cmd, total_cmd_words);
    printf("%d\n", p);
}

CodePudding user response：

When you declared

char* total_cmd_words[] = {};

you didn't specify an array length. Therefore, it's length was determined by your initializer which was empty.

So, when you do

splitWords[0] = strtok(input, " ");

you're assigning to the first element of a 0-length array. That's undefined behavior and the most likely cause of your segmentation fault.

CodePudding user response：

Added sizes to total_cmd_words and total_cmd

// Libraries
#include <stdio.h>
#include <string.h>


int parseInput(char *input, char* splitWords[]){
      int wordInd = 0;
      splitWords[0] = strtok(input, " ");
      while(splitWords[wordInd] != NULL){
              splitWords[  wordInd] = strtok(NULL, " ");
      }

      return wordInd;
}


int main(){

  char total_cmd[100] = "file1 > file2";
  char* total_cmd_words[10] = {0,};

  int p = parseInput(total_cmd, total_cmd_words);
  
  printf("%d \n", p);

}

Thank you everyone for you help!