c++ - I am trying to use fread and fwrite to make a copy function but I keep getting strange output depending on the buffer size -

i trying write copy function takes in 3 arguments when run in command line. program run using name of program followed user specified buffer size , name of input file , name of output file.

e.g. >>copyf 1024 input.txt output.txt stored in argv[0] argv[3] respectively.

when run code buffer size of 1024 this:

this beginning of test file , want see how far makes in file before cutting stuff off.ÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌ

when run code buffer size of 32 this:

this beginning of test file , want see how far makes in file before cutting stuff off.tting stuff

here code:

#include <stdio.h> #include <stdlib.h> #include <time.h> #include <string>  int main(int argc, char*argv[]) {     int end = 0;     char buffer[65536];     //error checking     if (argc != 4)     {         end = 1;         printf("error-insufficient arguments (%d/4)\n", argc);       }     file*fp, *outfile;     fp = fopen(argv[2], "r");     outfile = fopen(argv[3], "a");     if (fp == null)     {         end = 1;         printf("error-input file fake\n");     }      //no errors. proceed copy code     int size = atoi(argv[1]);     printf("buffer size is: %d bytes\n\n",size);      if (end == 0)     {         printf("no errors detected\n\n");         while (!feof(fp))         {              fread(&buffer, size, 1, fp);              fwrite(&buffer, size, 1, outfile);              (int = 0; < 65536; i++)             {                 buffer[i] = null;             }         }      }     fclose(fp);     fclose(outfile);      clock_t endtime = clock();     float endtimesec = endtime;     printf("runtime = %f\n\n", endtimesec / 1000);     return 0; } 

any suggestions on doing wrong appreciated. have feeling source of problem understanding of how fread , fwrite work in context.

there several problems code.

1. you doing little error handling (and not matters)

2. you opening files reading/writing text data instead of binary data, line breaks (if any) in file data possibly converted different format. true copy, don't want data change @ all.

3. you asking fread() read 1 element size bytes in size. if input file size not multiple of size, last element fail , ignored. account that, need ask fread() read size elements 1 byte in size.

4. you ignoring return value of fread(), tells how bytes read. don't write more number of bytes output file.

try more instead:

#include <stdio.h> #include <stdlib.h> #include <time.h> #include <string>  int main(int argc, char*argv[]) {     file *fp, *outfile;     char buffer[65536];     int bufsize, numread;      if (argc != 4)     {         printf("error-insufficient arguments (%d/4)\n", argc);         return 0;     }      bufsize = atoi(argv[1]);     if ((bufsize < 1) || (bufsize > sizeof(buffer))     {         printf("error-invalid buffer size (%d)\n", bufsize);         return 0;     }      fp = fopen(argv[2], "rb");     if (fp == null)     {         printf("error-input file not opened\n");         return 0;     }      outfile = fopen(argv[3], "wb");     if (outfile == null)     {         printf("error-output file not created\n");         fclose(fp);         return 0;     }      //no errors. proceed copy code     printf("buffer size is: %d bytes\n\n", bufsize);          {         numread = fread(buffer, 1, bufsize, fp);         if (numread < 1)         {             if ((ferror(fp) != 0) && (feof(fp) == 0))                 printf("error-reading input file\n");             break;         }          if (fwrite(buffer, numread, 1, outfile) != 1)         {             printf("error-writing output file\n");             break;         }     }     while (1);      fclose(fp);     fclose(outfile);      clock_t endtime = clock();     float endtimesec = endtime;     printf("runtime = %f\n\n", endtimesec / 1000);      return 0; }