How to write a file with C in Linux

Question

I want to rewrite the  cp  command of Linux  So this program will work like    a out originalfile copiedfile  I can open the file  create new file but can t write the new file  Nothing is written  What could be the reason   The current C code is    include  lt stdio h gt   include  lt sys types h gt   include  lt sys stat h gt   include  lt fcntl h gt   include  lt unistd h gt   int main int argc char  aa         int fd fd1      char buffer 100        if argc  3           printf  Usage     a out  lt original gt   lt copy gt   n            return -1             fd open aa 1  O RDONLY S IRUSR       if fd  -1           printf  file not found  n            return -1            fd1 open aa 2  O CREAT   O WRONLY S IRUSR       if fd1  -1           printf  file is created  n              ssize t n      while n read fd buffer 50            write fd1 buffer n           printf    writing   n              close fd       close fd1

User · Answer

You have to allocate the buffer with mallock, and give the read write the pointer to it.

#include <unistd.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int main(){
    ssize_t nrd;
    int fd; 
    int fd1;

    char* buffer = malloc(100*sizeof(char));
    fd = open("bli.txt", O_RDONLY);
    fd1 = open("bla.txt", O_CREAT | O_WRONLY, S_IRUSR | S_IWUSR);
    while (nrd = read(fd,buffer,sizeof(buffer))) {
        write(fd1,buffer,nrd);
    }   

    close(fd);
    close(fd1);
    free(buffer);
    return 0;
}

Make sure that the rad file exists and contains something. It's not perfect but it works.

User · Answer

You need to write   the read   data into the new file   ssize t nrd  int fd  int fd1   fd   open aa 1   O RDONLY   fd1   open aa 2   O CREAT   O WRONLY  S IRUSR   S IWUSR   while  nrd   read fd buffer 50         write fd1 buffer nrd      close fd   close fd1     Update  added the proper opens     Btw  the O CREAT can be OR d  O CREAT   O WRONLY    You are actually opening too many file handles   Just do the open once

User · Answer

You have to do write in the same loop as read

User · Answer

First of all  the code you wrote isn t portable  even if you get it to work  Why use OS-specific functions when there is a perfectly platform-independent way of doing it  Here s a version that uses just a single header file and is portable to any platform that implements the C standard library    include  lt stdio h gt   int main int argc  char   argv        FILE  sourceFile      FILE  destFile      char buf 50       int numBytes       if argc  3                printf  Usage  fcopy source destination n            return 1             sourceFile   fopen argv 1    rb        destFile   fopen argv 2    wb         if sourceFile  NULL                printf  Could not open source file n            return 2            if destFile  NULL                printf  Could not open destination file n            return 3             while numBytes fread buf  1  50  sourceFile                 fwrite buf  1  numBytes  destFile              fclose sourceFile       fclose destFile        return 0      EDIT  The glibc reference has this to say      In general  you should stick with   using streams rather than file   descriptors  unless there is some   specific operation you want to do that   can only be done on a file descriptor    If you are a beginning programmer and   aren t sure what functions to use  we   suggest that you concentrate on the   formatted input functions  see   Formatted Input  and formatted output   functions  see Formatted Output        If you are concerned about portability   of your programs to systems other than   GNU  you should also be aware that   file descriptors are not as portable   as streams  You can expect any system   running ISO C to support streams  but   non-GNU systems may not support file   descriptors at all  or may only   implement a subset of the GNU   functions that operate on file   descriptors  Most of the file   descriptor functions in the GNU   library are included in the POSIX 1   standard  however

[c] How to write a file with C in Linux?

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