Can sscanf in C write on char* instead of char[]? - c

Question

I'm looking for the simplest way in standard C to parse a string. The number of words inside the string is fixed, but the length of each single word is not. The code will be running on a microprocessor with limited memory so I can't just allocate an overkill buffer, I'd like to allocate just the memory that I need.
The following code works, but I'd like the single words to be char* . Is there some way around this?
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main(void) {
char * my_words = "foo bar 1 2";
char word1[20];
char word2[20];
char word3[20];
char word4[20];
int match = sscanf(my_words,"%s %s %s %s",word1,word2,word3,word4);
printf("Matches: %d\r\n",match);
printf("%s\r\n",word1);
printf("%s\r\n",word2);
printf("%s\r\n",word3);
printf("%s\r\n",word4);
return 0;
}
Thank you

Answer 1

For parsing you can use strtok() function. A simple approach can be like that also you can modify it
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main(void) {
char const *my_words = "foo bar 1 2";
char *str = malloc(1 + strlen(my_words));
strcpy(str, my_words);
int countWord = 0;
char * pch;
printf ("Splitting string \"%s\" into tokens:\n",str);
pch = strtok (str," ");
while (pch != NULL)
{
printf ("%s\n",pch);
pch = strtok (NULL, " ");
++countWord;
}
printf("Total words = %d\n", countWord);
return 0;
}

Answer 2

The answer depends on how simple and standard your code should be.
If your target supports POSIX 2008 (recent GNU libc does), then you could use m modifier as docs suggest to allocate just enough space to read data.
But if you have to stay with ANSI C, then probably you're stuck to functions like strtok/strtok_r or alike.

Answer 3

And in the event you must roll your own, the algorithm goes like this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void) {
/* your input, I'll presume its constant */
const char *input = " foo bar 1 2 ";
/* here is your array of char*.
* you indicate the number of words is fixed */
char *words[4];
/* the algo */
size_t b = 0;
size_t e = 0;
size_t l = strlen(input);
int w = 0;
while (b < l) {
b += strspn(input + b, " ");
e = b + strcspn(input + b, " ");
words[w] = malloc(e - b + 1);
strncpy(words[w], input + b, e - b);
w++;
b = e+1;
}
/* debugging, outputs in reverse order */
while (w--) {
printf("%s\n", words[w]);
free(words[w]);
}
exit(EXIT_SUCCESS);
}
Obviously, you'd want to add error checking.