Base change program in C complies but does not show any output

Question

I'm recently new to programming but this assignment has proven to be my most difficult. The program is suppose to read in a .txt file with the following format

input_base number output_base

and outputs the results. the bases can only range from 2-36 For example:

input: 2 10 4
output: 2

My program reads in each part of the line and stores them respectively. I then go through the steps to convert the number into the output base and then print it backwards. My problem is that the program runs fine and prints all the stored values and calculated values, but once I add in my "base_conversion" function the program no longer works and my friend even said it gave him a segmentation fault. I don't really know what could be causing it. Any help would be appreciated. Thank you.

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

void read_file_to_buffer(FILE *file);
char *buffer = NULL;
void string_check(char *buffer);
int char_to_int(char c);
int int_conversion(int x, int y);
void base_conversion(int end_number, int out_base);

int main(int argc, char *argv[]){

FILE *file = NULL;


if ( argc != 2 ){
    fprintf(stderr, "Error: To many/few arguments\n");
    exit(EXIT_FAILURE);
}

else {
    file = fopen(argv[1], "rb");

    if (file == NULL) {
        fprintf(stderr, "Error: could not open file '%s'\n", argv[1]);
        exit(EXIT_FAILURE);
    }
    else{
        printf("File %s opened!\n", argv[1]);
    }
}

read_file_to_buffer(file);
string_check(buffer);

fclose(file);
free(buffer);
buffer = NULL;

exit(EXIT_SUCCESS);
}



void read_file_to_buffer(FILE *file) {
long file_size = 0;

if(buffer != NULL){
    fprintf(stderr, "buffer in use\n");
    exit(EXIT_FAILURE);
}
rewind(file);
if (fseek(file, 0, SEEK_END) != 0){
    perror("Could not seek to end of file");
    exit(EXIT_FAILURE);
}

file_size = ftell(file);
if (file_size < 0){
    perror("could not tell size of file");
    exit(EXIT_FAILURE);
}
rewind(file);

buffer = (char *)malloc(sizeof(char) * (file_size + 1));
if (buffer == NULL){
    fprintf(stderr, "Could not allocate memory");
    exit(EXIT_FAILURE);
}

if(fread(buffer, sizeof(char), (size_t)file_size, file) != file_size){
    fprintf(stderr, "Could not read file\n");
    exit(EXIT_FAILURE);
}
buffer[file_size] = '\0';

return;
}


void string_check(char *buffer){

int i = 0;
int j = 0;
char base_in[2];
char number[50];
char base_out[2];
int actual_number[100];
int end_number = 0;
int x = 0;
int y = 0;
int in_base;
int out_base;

while( buffer[i] != '\0'){
    if (buffer[i] != '#' && buffer[i] != ' ' && buffer[i] != '\n' &&     buffer[i] != '\r'){
        while (buffer[i] != ' '){
            base_in[j] = buffer[i];
            i++;
            j++;
        }
        j = 0;
        i++;
        if (base_in[1] != '\0'){
            x = char_to_int(base_in[0]);
            y = char_to_int(base_in[1]);
            in_base = int_conversion(x, y);
            x = 0;
            y = 0;
        }
        else{

            in_base = char_to_int(base_in[0]);
        }
        while (buffer[i] != ' '){
            number[j] = buffer[i];
            i++;
            j++;

        }
        int  q;
        q=j;
        j=0;
        while (number[j] != '\0'){
            actual_number[j] = char_to_int(number[j]);
            end_number = end_number + pow(in_base, q-1) * actual_number[j];
            j++;
            q--;
        }
        j = 0;
        i++;

        while (buffer[i] != '\n' && buffer[i] != '\0'){
            base_out[j] = buffer[i];
            i++;
            j++;
        }
        if (base_out[1] != '\0'){
            x = char_to_int(base_out[0]);
            y = char_to_int(base_out[1]);
            out_base = int_conversion(x, y);
            x = 0;
            y = 0;
        }
        else{
            out_base = char_to_int(base_out[0]);
        }
        j = 0;
        i++;

        base_conversion(end_number, out_base);
    }
    else{
        while (buffer[i] != '\n' && buffer[i] != '\0'){
            i++;
        }
    }
    i++;
}

return;
}

int char_to_int(char c){

char map[] = "0123456789abcdefghijklmnopqrstuvwxyz";
int result = -1;
char *next = map;

while(*next != '\0'){
    if(*next == c){
        result = next - map;
        break;
    }
    next++;
}
return result;
}

int int_conversion(int x, int y){
int value;

value = (x * 10) + y;

return value;
}

void base_conversion(int end_number, int out_base){
int remainder[100];
char map[] = "0123456789abcdefghijklmnopqrstuvwxyz";
int index = 0;
int i;

while (end_number != 0){
    remainder[index] = end_number % out_base;
    end_number = end_number / out_base;
    index++;
}
for (i=0; i<index; i++){
    printf("%c", map[remainder[index-1]]);
}
printf("\n");
return;
}

Answer 1

OP's base_conversion() is messed.

Loop prints same character repeatedly.

for (i=0; i<index; i++){
    printf("%c", map[remainder[index-1]]);  // Why same character?
}

Code is using signed math and % and can create negative remainders which may be used as array index.

remainder[index] = end_number % out_base;  // result may be negative.
...
printf("%c", map[remainder[index-1]]);     // accessing out of bounds with negative

---

Suggested simplification.

void base_conversion_helper(unsigned end_number, int out_base){
  if (end_number >= out_base) base_conversion_helper(end_number/out_base, out_base);
  putchar("0123456789abcdefghijklmnopqrstuvwxyz"[end_number % outbase]);
}

void base_conversion(unsigned end_number, int out_base){
  assert(out_base >= 2 && out_base <= 36);
  base_conversion_helper(end_number, out_base);
  putchar('\n');
}