c++ - How Can I use the system call write() to output to a file (write to a file)? -

so learning operating systems course , right learning how system calls. ,i want copy input file openclose.in file called openclose.out. far here code

#include <cstdio> #include <unistd.h> #include <cstdlib> #include <sys/types.h> //needed open #include <sys/stat.h>  //needed open #include <fcntl.h>     //needed open #include <errno.h>      //must use perror #include <grp.h> #include <pwd.h>   using namespace std;  int main (int argc, char *argv[]) {       int infile; int outfile;     int n_char=0; char buffer[32]; int fstat(int fildes, struct stat *buf); struct stat statbuf; struct group grp; struct passwd pwd;  int err; int err2;       if(argc != 3)      {      printf("usage: openclose <infile> <outfile> \n");     //return 1;     exit(1); }       infile=open(argv[1],o_rdonly); outfile=open(argv[2],o_rdwr);     if (infile==-1)     {        printf("");        perror(argv[1]);        exit(1);      }      //use read system call obtain 32 characters infile     while( (n_char=read(infile, buffer, 32))!=0)     {             //display characters read             n_char=write(argv[2],buffer,n_char);     }  err = fstat(infile, &statbuf); printf("the status of %d\n", argv[1]); printf("file size \t owner \t group id \t last modified \n %d \t \t %d \t %d \t \t %d \n",statbuf.st_size,statbuf.st_uid,stat,statbuf.st_gid,statbuf.st_mtime);       return 0;  } 

but couldn't seem working, reason, don't know why not able see output file.

i running program terminal 2 arguments following

open(executable) inputfile1 inputfile2

your use of write() not correct:

        n_char=write(argv[2],buffer,n_char); 

the first argument should file descriptor, use outfile instead of file name in argv[2].

by way, should take habit of verifying if opening succeded, before trying file i/o. did infile, outfile