String "Contains" char function

I wrote this function which does the following from scratch w/o looking for ways that it is typically done:

1. Accepts a char c to search for.
2. Accepts a char* str to search within.
3. Accepts a buffer pointer, and stores all indices of c that are in str inside the provided buffer.
4. Returns the # of times that c was found in str.

Please critique and also - Are there better alternatives to export the indices other than using a buffer ptr?

#include <stdlib.h>
#include <string.h>
#include <stdio.h>
int str_contains(const char c, const char* str, int* output_buffer){
    int i, str_len;
    str_len = strlen(str);
    int *holder = malloc(str_len*sizeof(int));
    int holder_iteration = 0;
    for(i = 0; i < str_len; i++){
        if(str[i] == c){
            holder[holder_iteration++] = i;
        }
    }
    memcpy(output_buffer,holder,holder_iteration*sizeof(int));
    free(holder); 
    return holder_iteration;
}

int main(){
    char* myString = "Welcome to the jungle, baby. Do you like to juggle or jump around?";
    int* output = malloc(500*sizeof(int));
    int number = str_contains('a',myString,output);

    printf("# of times: %d\nIndices: \n",number);
    int i;
    for(i = 0; i < number; i++){
        printf("%d\n",output[i]);
}

free(output);
    return(EXIT_SUCCESS);
}