Final version

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <stdint.h>

#define IMAX 262144
#define PAGE_SIZE 4096

static size_t bump_size(char ** buffer, size_t existing, size_t additional) {
    size_t desired;
    size_t ideal;

    if (existing + additional > (existing << 1)) {
        desired = existing + additional;
    } else {
        desired = existing << 1;
    }

    ideal = ((desired + PAGE_SIZE) >> 12) << 12;

    if (!((*buffer) = realloc(*buffer, ideal))) {
        perror("Error increasing memory allocation. Aborting.");
        exit(EXIT_FAILURE);
    }
    return ideal;
}


int main() {
    setbuf(stdout, NULL); //disable output buffering

    /* this is known at compile time, why not:
     * const char str[] = "abcdefghefghefgh";
     * ? Still, won't "cheat"... */

    /* stupid unneeded mess */
    size_t str_mem = 9;
    char * str = malloc(str_mem);
    if(str == NULL) {
        perror("Failed to allocate initial memory for str.");
        return EXIT_FAILURE;
    }

    (void) strcpy(str, "abcdefgh");

    /* not worth optimizing, since it's not in the loop, but could
     * also, wouldn't really use bump_size here, just raw realloc
     * but I am to demonstrate this is a viable general technique */
    str_mem = bump_size(&str, str_mem, 8);
    strcat(str, "efghefgh");

    /* end stupid unneeded mess that should have been statically declared */

    printf("%s", "exec.tm.sec\tstr.length\n");

    time_t starttime = time(NULL);
    char * gstr = malloc(1);
    if (!gstr) {
        perror("Error allocating memory for gstr. Exiting.");
        return EXIT_FAILURE;
    }
    gstr[0] = '\0';

    size_t gstr_len = 0;
    size_t gstr_size = 1;

    char * pos_c = gstr;
    size_t str_len = strlen(str);
    size_t str_size = str_len + 1;

    size_t efgh_len = strlen("efgh"); // 4. To keep it general and avoid magic constants

    size_t i = 0;
    while (i++ < IMAX+1000) {
        if ((gstr_len + str_size) > gstr_size) {
            gstr_size = bump_size(&gstr, gstr_size, str_len);
        }
        pos_c = gstr + gstr_len;
        memcpy(pos_c, str, str_size); //strcat(gstr, str);
        gstr_len += str_len;

        while((pos_c = strnstr(pos_c, "efgh", str_len))) {
            memcpy(pos_c, "____", efgh_len);
            pos_c += efgh_len;
        }

        if (gstr_len % IMAX == 0) {
            printf("%ldsec\t\t%lukb\n", time(NULL) - starttime, gstr_len / 1024);
        }
    }

    //printf("%s\n",gstr);
    free(gstr);

    return EXIT_SUCCESS;
}