Name

Library

Synopsis

Return values

Detailed description

The new descriptor returned by the call is the lowest numbered descriptor currently not in use by the process.

The object referenced by the descriptor does not distinguish between oldd and newd in any way. Thus if newd and oldd are duplicate references to an open file, read, write and lseek calls all move a single pointer into the file, and append mode, non-blocking I/O and asynchronous I/O options are shared between the references. If a separate pointer into the file is desired, a different object reference to the file must be obtained by issuing an additional open system call.

In dup2, the value of the new descriptor newd is specified. If this descriptor is already in use and oldd != newd, the descriptor is first deallocated as if the close system call had been used. If oldd is not a valid descriptor, then newd is not closed. If oldd == newd and oldd is a valid descriptor, then dup2 is successful, and does nothing.

Limitation: dup2(int oldfd, int newfd); The return value of dup2 can be different from the one user expected to be (newfd). So, user of dup2 have to use return value of dup2 as the new allocated fd rather than the one which is been passed to dup2 (newfd). As per the standard, newfd and return values are the same, if dup2 is successful.

Examples

/**
  *Detailed description : Sample usage of dup system call
**/
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
int main()
{
  int fd;
  FILE *Fil;
  int Newfd;
  fd = open("Example.txt" , O_CREAT | O_RDWR , 0666);
  if(fd < 0 )  {
     printf("Failed to open file Example.txt \n");
     return -1;
  }
  Newfd  = dup(fd );
  if(Newfd < 0 ) 
  {
    printf("Failed to duplicate file descriptor \n");
    return -1 ;
  }
  close(fd);
  close(Newfd);
  printf("New Duped fd is %d \n" , Newfd);
  return 0;
}

         

Output

New Duped fd is 4

         

/**
  *Detailed description : Sample usage of dup system call
**/
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <stdio.h>
int main()
{
  int fd;
  FILE *Fil;
  int Newfd;
  fd = open("Example.txt" , O_CREAT | O_RDWR , 0666);
  if(fd < 0 )  {
     printf("Failed to open file Example.txt \n");
     return -1;
  }
  Newfd  = dup2(fd  , 4);
  if(Newfd < 0 )  {
    printf("Failed to duplicate file descriptor \n");
    return -1;
  }
  close(fd);
  close(Newfd);
  printf("New Duped fd is %d \n" , Newfd);
  return 0;
}

         

Output

New Duped fd is 4

         

Errors

See also

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