C - split string into an array of strings

Question

I'm not completely sure how to do this in C:

char* curToken = strtok(string, ";");
//curToken = "ls -l" we will say
//I need a array of strings containing "ls", "-l", and NULL for execvp()

How would I go about doing this?

If you want to split based on spaces, why have you specified ; as the delimiter? — Oliver Charlesworth
– Oliver Charlesworth, Commented Jun 25, 2012 at 23:03

Filip Roséen · Accepted Answer · 2024-12-17 03:26:40Z

69

Since you've already looked into strtok just continue down the same path and split your string using space (' ') as a delimiter, then use something as realloc to increase the size of the array containing the elements to be passed to execvp.

See the below example, but keep in mind that strtok will modify the string passed to it. If you don't want this to happen you are required to make a copy of the original string, using strcpy or similar function.

char    str[]= "ls -l";
char ** res  = NULL;
char *  p    = strtok(str, " ");
int n_spaces = 0, i;


/* split string and append tokens to 'res' */

while(p) {
  res = realloc(res, sizeof (char*) * ++n_spaces);

  if (res == NULL)
    exit(-1); /* memory allocation failed */

  res[n_spaces-1] = p;

  p = strtok(NULL, " ");
}

/* realloc one extra element for the last NULL */

res = realloc(res, sizeof (char*) * (n_spaces+1));
res[n_spaces] = 0;

/* print the result */

for (i = 0; i < (n_spaces+1); ++i)
  printf("res[%d] = %s\n", i, res[i]);

/* free the memory allocated */

free (res);

res[0] = ls
res[1] = -l
res[2] = (null)

edited Dec 17, 2024 at 3:26

answered Jun 25, 2012 at 23:03

Filip Roséen

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3 Comments

Honinbo Shusaku Over a year ago

@FilipRoséen-refp Can you explain the last code block before the printing and freeing the memory, the: /* realloc one extra element for the last NULL */? I'm having difficulty understanding it

Charles Over a year ago

@Abdul I believe usually there is a null character at the end of each array so that the computer can differentiate between two different arrays.

MattSom Over a year ago

if I wrap this into a function and return the res pointer, I get a warning after freeing the pointer I assigned res to, saying: warning: attempt to free a non-heap object [-Wfree-nonheap-object]. Do you have any insight of the cause?

Chris O · Accepted Answer · 2012-06-25 23:05:17Z

6

Here is an example of how to use strtok borrowed from MSDN.

And the relevant bits, you need to call it multiple times. The token char* is the part you would stuff into an array (you can figure that part out).

char string[] = "A string\tof ,,tokens\nand some  more tokens";
char seps[]   = " ,\t\n";
char *token;

int main( void )
{
    printf( "Tokens:\n" );
    /* Establish string and get the first token: */
    token = strtok( string, seps );
    while( token != NULL )
    {
        /* While there are tokens in "string" */
        printf( " %s\n", token );
        /* Get next token: */
        token = strtok( NULL, seps );
    }
}

answered Jun 25, 2012 at 23:05

Chris O

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4 Comments

Jordan Over a year ago

I understand this much, but this does not give me an array of strings from the tokens. I guess I don't understand that specific part of it.

Charles Over a year ago

Why token = strtok(NULL, seps);? Why the NULL?

Chris O Over a year ago

@c650 See the linked page from MSDN, subsequent calls need the NULL.

Charles Over a year ago

Ah yes I did much Googling and have found out that strtok() uses a static variable to save its place.

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