Question

This code is an expression of cp command in c language. But I don't understand this code very well. Please explain in notes one by one.

#define SIZE 1024

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

int main(int argc, char *argv[]){

if(argc != 3){
perror("argument 부족\n");
exit(0);
}

struct stat frstatbuf;
FILE* fr = fopen(argv[1], "r");

if(fr == NULL){
perror("read file 읽기 오류\n");
exit(0);
}

int frfd = fileno(fr);

fstat(frfd, &frstatbuf);

FILE* fw=fopen(argv[2], "w+");

int fwfd=fileno(fw);

fchmod(fwfd,frstatbuf.st_mode&(S_IRWXU|S_IRWXG|S_IRWXO));

char buf[1024];

while(1){
int n=fread(buf,sizeof(char),SIZE,fr);

if(n<SIZE){
fwrite(buf,sizeof(char),n,fw);

printf("파일을 다 읽었음\n");
exit(0);
}
fwrite(buf,sizeof(char),n,fw);
}

fclose(fr);
fclose(fw);
return 0;
}

Answer 1

#define SIZE 1024

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


int main(int argc, char *argv[]) {
    
    // validate command line arguments.
    if(argc != 3){
        perror("argument 부족\n");
        exit(0);
    }

    struct stat frstatbuf;
    
    // Open the file and get a file pointer
    FILE* fr = fopen(argv[1], "r");
    if(fr == NULL){
        perror("read file 읽기 오류\n");
        exit(0);
    }
    
    // Get file descriptor from file pointer
    int frfd = fileno(fr);
    
    // Read file related information into file status buffer: frstatbuf
    fstat(frfd, &frstatbuf);
    
    // Open a new file for writing
    FILE* fw=fopen(argv[2], "w+");
    
    // get file descriptor for file to write
    int fwfd=fileno(fw);
    
    // Change the file permissio of writing file
    // Give read write execute permission to group, owner and others.
    fchmod(fwfd, frstatbuf.st_mode & (S_IRWXU|S_IRWXG|S_IRWXO));
    
    // buffer to read file contents.
    char buf[1024];

    while(1){
        
        // try reading SIZE bytes maximum from file with descriptor fr
        // (from input file)
        // The return value is the number of bytes read successfully
        int n=fread(buf,sizeof(char),SIZE,fr);
        
        // If we are unable to read all the required number of bytes
        // then it is a errror
        if(n < SIZE){
            fwrite(buf,sizeof(char),n,fw);

            printf("파일을 다 읽었음\n");
            
            // end the program here after printing the error.
            exit(0);
        }
        
        // Else, everything was fine, we were able to read the data from file
        // Now, write the same number of bytes to output file with descriptor fw.
        fwrite(buf,sizeof(char),n,fw);
    }
    
    // at last close all the resources
    fclose(fr);
    fclose(fw);
    return 0;
}

**************************************************

please see inline comments.

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