cp.c

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <bsd/string.h>
#include <libgen.h>

#define BUFFER_SIZ  (256)

static char buffer[256];

int main(int argc, char *argv[]){
    int ret;
    char *src, *dest;
    int src_fd, dest_fd;
    struct stat src_buf, dest_buf;
    if(argc < 3){
        fprintf(stderr, "cp [OPTION]... SOURCE DEST\n");
        return 1;
    }
    argv++;
    src = *argv++;
    dest = *argv;
    src_fd = open(src, O_RDONLY);
    if(src_fd < 0){
        perror(src);
        return 1;
    }

    ret = fstat(src_fd, &src_buf);
    if(ret)
        return ret;

    if(!S_ISREG(src_buf.st_mode) ){
        fprintf(stderr, "not regular file %s\n", src);
        return 1;
    }
    
    ret = stat(dest, &dest_buf);
    if(ret == 0 && S_ISDIR(dest_buf.st_mode)){
        strlcpy(buffer, dest, BUFFER_SIZ);
        strlcat(buffer, "/", BUFFER_SIZ);
        strlcat(buffer, basename(src), BUFFER_SIZ);

        dest_fd = open(buffer, O_WRONLY | O_CREAT, src_buf.st_mode);
        if(dest_fd < 0){
            perror(buffer);
            return 1;
        }
    }else{
        dest_fd = open(dest, O_WRONLY | O_CREAT, src_buf.st_mode);
        if(dest_fd < 0){
            perror(dest);
            return 1;
        }
    }
    
    while((ret = read(src_fd, buffer, BUFFER_SIZ * sizeof(char))) > 0){
        ret = write(dest_fd, buffer, ret);
        if(ret <= 0){
            perror("write");
            break;
        }
    }
    close(src_fd);
    close(dest_fd);
    return 0;
}