How do I replicate C++'s .push_back() function in C?

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

int main()
{
    int *ints = NULL;  /* a dynamic array of ints */
    size_t len = 0;    /* size in BYTES of ints */
    FILE *mem; /* in memory data structure */

    if( (mem = open_memstream( (char **)&ints, &len)) == NULL ) {
        /* handle error */
        return 1;
    }


    for(int i = 0; i < 10; ++i) {
        int n = i * 2;
        ssize_t written = fwrite(&n, sizeof n, 1, mem); /* "push_back" an int  */
        if(written != 1) {
            /* handle error */
        }
    }

    fflush(mem); /* needed to update ints buffer */

    size_t nints = len / sizeof(int);;
    for(int i = 0; i < nints; ++i) {
        printf("%d\n", ints[i]);
    }

    /* add some more to ints */

    for(int i = 0; i < 10; ++i) {
        int n = (100+i);
        size_t written = fwrite(&n, sizeof n, 1, mem); /* "push_back" an int  */
        if(written != 1) {
            /* handle error */
        }
    }


    fclose(mem); /* flushes memory to ints, closes file handle */
    /* "ints" is still valid here */
    nints = len/sizeof(int);
    for(int i = 0; i < nints; ++i) {
        printf("%d\n", ints[i]);
    }

    free(ints);  /* clean up ints array */

    return 0;
}

struct MEMDATA {
     char *data;       /* pointer to memory buffer */
     size_t data_size; /* buffer size in bytes */
     size_t mem_size;  /* size of one data element */
     FILE *fp;    
};