Question

I made the command cp code in c language. But when I copy a file, file permissions are not copied equally. So I want to copy the file authority as well. What should I do?

#include
#include
#include
#include
#include
#include
#include
#include
#include

int main(int argc, char *argv[])
{
struct stat st;
char ch;
int src, dst;
if(argc != 3)
{
printf("argument error \n");
printf("usage: ./a.out src dest \n");
exit(0);
}
src = open(argv[1], O_RDONLY);
if(src == -1){
perror("open source");
exit(errno);
}
dst = open(argv[2], O_WRONLY | O_CREAT|O_TRUNC, st.st_mode&(S_IRWXU | S_IRWXG | S_IRWXO));
if(dst == -1){
perror("open source");
exit(errno);
}
while(read(src, &ch,1))
write(dst,&ch,1);

close(src);
close(dst);
return 0;
}

Answer 1

file name 1.c

#include <stdio.h> 
#include <stdlib.h> 
#include <unistd.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <errno.h>
#define BUF_SIZE 1024
int main(int argc, char *argv[])
{
        struct stat st;
    int inputFd, outputFd, openFlags;
    ssize_t numRead;
    char buf[BUF_SIZE];

    if(argc != 3)
        {
                printf("argument error \n");
                printf("usage: ./a.out src dest \n");
                exit(0);
        }
/*
save the stat structure of the file which you are about to copy
it contains all the information related to file including permission assign
*/
    stat(argv[1],&st);
        
/* Open input and output files */

    if((inputFd = open(argv[1], O_RDONLY) )== EACCES)
    {
        perror("access to the file is not allowed or  the file did not exist yet ");

    } 
    else{
        printf("file open successfully\n");
     }
     
    if((outputFd = open(argv[2], O_CREAT | O_WRONLY,st.st_mode & 511)) < 0)
    {
        printf("error in opening file %d",errno);
        //get error no and search in error list 
        
    }
    else{
        printf("file open successfully\n");
    }
/*
    O_CREAT flag is use to create file on not exist.
    struct stat has member st_mode which contain the file type and file mode.
    least significant 9 bits correspond  as the file permission bits.
    so we perform and operation between st_mode and 511 to reterive permission bits.
*/
    
    /* Transfer data until we encounter end of input or an error */
        printf("%d\n",outputFd );
    while ((numRead = read(inputFd, buf, BUF_SIZE)) > 0)
    {
        if(write(outputFd,buf,numRead) <0)
        {
                printf("error write %d",errno);//get error no. and check error through list of error
        }

        }
    
    exit(0);
}

compile command

gcc -o 1 1.c

This program is successfully run in ubuntu 20.04.

for error list type below command on linux terminal

errno -l