How to XOR a file buffer in C and output to a new file

Why does my output contain extra characters? Why is only the first line of every file via notepad++ being encrypted and not the entire file? Happy coding! P.S I have the Second Edition of C programming language by Kernighan and Ritchie

EDIT: This code is my code after I fixed it, the question's has been answered. Thank you guys!

Here is my source NEW code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define getchar() getc(stdin)
#define putchar() putc((c),stdout)
#define XOR_BYTE 0x9E

char * xorBuffer(char *buffer, long bufferSize){

    int i;
    for(i = 0;i <= bufferSize;i++){
        buffer[i] ^= XOR_BYTE;
    }
    return buffer;
}

int xorFile(char *fileIn, char * fileOut){

    FILE *fpi, *fpo;
    char *fileBuffer = NULL;

    fpi = fopen(fileIn,"rb");
    fpo = fopen(fileOut,"wb");

    if(NULL == fpi){
        printf("Error opening input file %s: %s\n", fileIn, strerror(errno));
        return 1;
    }
    if(NULL == fpo){
        printf("Error opening output file %s: %s\n", fileOut, strerror(errno));
        return 2;
    }

    fseek(fpi,0L,SEEK_END);
    long fileSize = ftell(fpi); 
    fileBuffer = malloc(sizeof(char)* (fileSize + 1));  
    fseek(fpi,0L,SEEK_SET);     
    size_t length = fread(fileBuffer, sizeof(char), fileSize,fpi);      
    fileBuffer[length];
    fileBuffer = (char *)xorBuffer(fileBuffer,fileSize);    
    int c;  
    for(c = 0;c < fileSize;c++){ 
        putc(((fileBuffer[c])),fpo);
    }

    fclose(fpi);
    fclose(fpo);
    free(fileBuffer);
    return 0;
}

int main(int argc, char*argv[]){
    if(argc == 3){
        if(xorFile(argv[1],argv[2]) == 0)
            printf("File encryption was successful.");
        else
            printf("An error occured.");
    }else{
        printf("usage --- xor [input file][output file]");
    }
}

Solution

Your prototype for XOR_FILE is incorrect: you should take 2 strings.

There are more issues in your code:

you must learn to indent your code and use spaces wisely. Use the Kernighan and Ritchie style shown in the book.
you cannot reliably get the file size with fseek and ftell, it is not needed in general and you can implement a bufferized version with a fixed sized buffer anyway.
avoid overwriting the input file in your program. If you make a mistake, or if the program fails or is interrupted, the file contents may be corrupted or lost.
you do not need to null terminate the array into which you read the file, just iterate over all bytes, but stop at the size read: use for (i = 0; i < newLen; i++) otherwise you would output an extra byte when encrypting and one more when deciphering...
do not iterate until '\0' in XOR_BUFFER(char *FILE_BUFFER), pass the size and use it. Otherwise you will fail to encrypt binary files that contain null bytes.
you forget to close fpo with fclose(fpo);
do not redefine standard functions such as getchar() and putchar().
do not use uppercase letters for function names and/or variable names, but it is indeed common practice to use uppercase letters for macros.

Here is a simplified version:

#include <errno.h>
#include <stdio.h>
#include <string.h>

#define XOR_BYTE   0x9E
                    
int xor_file(const char *infile, const char *outfile) {
    FILE *fpi, *fpo;
    int c;

    if ((fpi = fopen(infile, "rb")) == NULL) {
        fprintf(stderr, "cannot open input file %s: %s\n", infile, strerror(errno));
        return 1;
    }
    if ((fpo = fopen(outfile, "wb")) == NULL) 
        fprintf(stderr, "cannot open output file %s: %s\n", outfile, strerror(errno));
        fclose(fpi);
        return 2;
    }

    while ((c = getc(fpi)) != EOF) {
        putc(c ^ XOR_BYTE, fpo);
    }
    fclose(fpi);
    fclose(fpo);
    return 0;
}

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

    if (argc == 3) {
        status = xor_file(argv[1], argv[2]);
    } else {
        fprintf(stderr, "usage: xor_file input_file output_file\n");
        status = 3;
    }
    //getch();  // avoid the need for this by running your program in the terminal
    return status;
}