C使用open（）read（）write（）访问文本修改它并存储在新文件中

Question

My program is supposed to take text from a file given in the command line change it to uppercase and store it in another file. It works except the output file has a whole bunch of garbage after the converted text. Thank you Edit: I changed my read to check for 0 bytes and used ret_in to write per Pyjamas it still pulls two or three garbage values. It's definitely read getting the garbage because when I output the buffer before converting it's there.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <ctype.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>

#define BUF_SIZE 500

int main(int argc, char* argv[]){
  char buffer[BUF_SIZE];
  int ret_in;
  char inputf[100],outputf[100],txt[4],up[3];
  // Takes input and adjusts it to the correct file type. 
  strcpy(inputf,argv[1]);
  strcpy(outputf,argv[1]);
  strcat(outputf,".up");
  printf("%s\n",outputf);
  strcat(inputf,".txt");
  printf("%s\n",inputf);
  int output, input,wrt;
  int total;


    //opens input file
input=open(inputf, O_RDONLY);
if (input == -1) {
    printf("Failed to open file\n");
    exit(1);
}
ret_in = read(input,buffer,BUF_SIZE);
total = ret_in;
// output to console
while (ret_in!= 0) {
  //  printf("%s\n", buffer);
    ret_in = read(input,buffer,BUF_SIZE);
    total += ret_in;
}
//ret_in= read(input,&buffer,BUF_SIZE);
puts(buffer);   
close(input); 
int i = 0; 
while(buffer[i]) {
      buffer[i] = toupper(buffer[i]);
      i++;
   }
// output buffer in console   
puts(buffer);   
//output filename in console
printf("%s\n",outputf);
  // Opens or creates output file with permissions.
 output = open(outputf, O_CREAT| S_IRUSR | O_RDWR);
  if (output == -1) {
      printf("Failed to open or create the file\n");
    exit(1);
}
// write to output file
wrt = write(output, buffer,total);  

close(output);


  return 0;
}

Answer 1

Because you read ret_in bytes from file, but you write BUF_SIZE to the file, you should write ret_in bytes to the file. You are not supposed to read BUF_SIZE bytes from file every time, it depends, right?

write(output, buffer,BUF_SIZE);//wrong
write(output, buffer,ret_in);  //right