Password generator

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

FILE *file;

void writeFile(const char* buffer, size_t buffer_len) {
    assert(file);

    fwrite(buffer, sizeof(char), buffer_len, file);
    fputc('\n', file);
}

void gain(char* buffer, size_t buffer_idx, int remain_length);

inline void put_symbol(char* buffer, size_t buffer_idx, int remain_length, char symbol)
{
    buffer[buffer_idx] = symbol;
    buffer_idx++;
    if (remain_length > 0)
    {
        gain(buffer, buffer_idx, remain_length - 1);
    } else {
        writeFile(buffer, buffer_idx);
    }
}

void gain(char* buffer, size_t buffer_size, int remain_length)
{
    char i;
    for (i = 'a'; i <= 'z'; i++)
        put_symbol(buffer, buffer_size, remain_length, i);

    for (i = '0'; i <= '9'; i++)
        put_symbol(buffer, buffer_size, remain_length, i);

    for (i = 'A'; i <= 'Z'; i++)
        put_symbol(buffer, buffer_size, remain_length, i);

    char* symbols = "[]*.:;' \"|0";

    for (i = 0; symbols[i] != 0; i++)
        put_symbol(buffer, buffer_size, remain_length, symbols[i]);
}

int main(int argc, char** argv)
{
    if (argc <= 1)
    {
        fprintf(stderr, "Error: you must set password length\n");
    } else {
        file = fopen("passlist.txt", "w");
        if (argc == 2) {
            int password_length = atoi(argv[1]);
            char* buffer = malloc(sizeof(char) * (password_length));

            gain(buffer, 0, password_length - 1);
        } else if (argc == 3) {
            int min_password_length = atoi(argv[1]);
            int max_password_length = atoi(argv[2]);

            char *buffer = malloc(sizeof(char) * max_password_length);

            for (int i = min_password_length - 1; i < max_password_length; i++)
            {
                gain(buffer, 0, i);
                printf("Generated passwords with length: %i", i + 1);
            }
        }
    }

    if (file)
        fclose(file);

    return 0;
}