A different take on strtok_r.

tokenize.c

#include <stdlib.h>
#include <stdio.h>

typedef struct tokenize_state_t {
    const char * position;
} tokenize_state_t;

int check_delim(char c, const char * delim) {
    for (const char * cur = delim; *cur != '\0'; ++cur) {
        if (c == *cur) {
            return 1;
        }
    }
    return 0;
}

int tokenize(const char * str, const char * delim, char ** token, tokenize_state_t ** state) {
    if (delim == NULL || token == NULL || state == NULL) {
        goto error;
    }

    const char * start = str;

    if (*state == NULL) {
        *state = calloc(1, sizeof(tokenize_state_t));
        if (*state == NULL) {
            goto error;
        }
    }
    else {
        start = (*state)->position;
    }

    if (start == NULL) {
        // No string given, and no state struct.
        goto error;
    }

    // Skip leading delim characters.
    while (*start && check_delim(*start, delim)) start++;

    const char * cur = start;
    *token = (char *)start;

    // Find end of token.
    while (*cur && !check_delim(*cur, delim)) cur++;

    (*state)->position = cur;

    return cur - start;

error:
    return 0;
}


int main() {
    const unsigned int BUFFER_SIZE = 1024;
    char buffer[BUFFER_SIZE];

    if (fgets(buffer, BUFFER_SIZE, stdin)) {
        tokenize_state_t * state = NULL;
        char * tok_start = buffer;
        int tok_len = 0;

        do {
            tok_len = tokenize(tok_start, " ", &tok_start, &state);
            fwrite(tok_start, sizeof(char), tok_len, stdout);
        } while(tok_len > 0);
    }

    return 0;
}