c - open() using O_APPEND does not update EOF -

i writing copy of dup() function (i studying book linux api).

i have file named temp.txt contains 1 line following string: hello, world.

here code:

#include <sys/types.h> #include <sys/stat.h> #include <fcntl.h> #include <errno.h> #include <unistd.h> #include <stdlib.h> #include <stdio.h>  //naive implementation int dup_t(int old) {     if(old == -1) {         errno = ebadf;         return -1;     }      int flags;     flags = fcntl(old, f_getfl);     if(flags == - 1) {         errno = ebadf;         return -1;     }      int new;     if((new = fcntl(old, f_dupfd)) == - 1) {         errno = ebadf;         return - 1;     }     return new; }  int main(int argc, char *argv[]) {      if(argc == 1) {         printf("error, no arguments given\n");         exit(-1);     }      int fd, cpfd;      if((fd = open(&argv[1][0], o_rdwr | o_append)) == -1) {         printf("error opening file\n");         exit(-1);     }      cpfd = dup_t(fd);     if(cpfd == -1) {         printf("error dup_t()\n");         exit(-1);     }      if(close(fd) == -1) {         printf("error closing fd\n");         exit(-1);     }      if(write(cpfd, "kostas", 6) == - 1) {         printf("error writting\n");         exit(-1);     }      if(close(fd) == -1) {         printf("error closing fd\n");         exit(-1);     }      if(write(cpfd, "kostas", 6) == - 1) {         printf("error writting\n");         exit(-1);     }      if(close(cpfd) == -1) {         printf("error closing cpfd\n");         exit(-1);     } } 

so, running ./prog temp.txt successfully, file temp.txt must contain following string. hello, worldkostas

by running command cat temp.txt output hello, world, but, if open file on text editor nano hello, world (followed new line contains) kostas.

why cat command produce incorrect output? why there new line added @ end of string hello, world ?

i not looking workaround, interested in finding out reason of error/problem.

1. the file has been updated to:

   hello, world[eol] kostas[no end-of-line here] 

2. cat output file content exactly, wont output final eol, terminal show after cat:

   bash> cat xxx hello, world kostasbash> 

note: bash> prompt, prompt may contains carrier return, put cursor @ beginning of line, consider situation

before output prompt:

    bash> cat xxx     hello, world     kostas           ^cursor here 

after carrier return:

    bash> cat xxx     hello, world     kostas     ^cursor here 

finally output prompt:

    bash> cat xxx     hello, world     bash>           ^cursor here 

so, prompt may overwrite last line of output of cat if no eol @ end of file.

btw:

1. when using vim opening file, [noeol] shown @ bottom left indicate file without eol @ end of file
2. when using zsh, % shown if last command doesn't output eol @ end, , zsh output eol.