Difference between dup() and dup2().

For more info visitdup() and dup2() are system calls which are used to duplicate file descriptors. Each process has a file descriptor table which contains the pointers to the file tables, process is using.

File descriptors are generally indexes of that table. The real information, i.e. the File pointer, is stored corresponding to these indexes. File pointer points to the file table which contains various attributes of files, such as current offset of file, file status flag, v-node information etc. Whenever a process forks, i.e creates a new child process, all the information in this table is copied to the child process's memory space.

While the information is copied from parent to child process, file pointer information from file descriptor table is also copied, so both the file pointers in both the memory spaces(child as well as Parent) point to the same File table. All the attributes in the File table( for the copied file pointers) are common for both, child as well as Parent.

This was the brief description about how the files descriptors work in the process. For detail information about file descriptors visit.

Now, as told earlier, dup() and dup2() are system calls which are used to duplicate file descriptors. By duplicating file descriptors we mean, the pointer information is copied from one location of the descriptor table to other location, no new file table is made for this process, only the file table is pointed by one more location in the descriptor table.

dup() : dup(int filedes) takes a file descriptor as its argument and duplicates the next lowest available file descriptor with the same file table pointer. It returns the new file descriptor value.

Suppose in a process, we want to duplicate the output file descriptor (1). Call

dup(1)

which will return the next lowest available file descriptor. If the process has

0 as the input descriptor

1 as output descriptor

2 as error output descriptor.

then calling the dup() will return 3 as the next available file descriptor, which will point to the terminal(same as 1).

dup2() : Using dup(), we are never sure about the value which will be returned. It will automatically return the next lowest available file descriptor. In some cases, we may need to duplicate a file descriptor as another file descriptor, we use dup2().

dup2(int filedes1, int filedes2) takes two file descriptors as its arguments. Let's suppose, we want to duplicate file descriptor 5 with file descriptor 1, i.e we want the file descriptor 5 to point at the same file table as file descriptor 1.

We will call dup2(1,5). It first checks, if file descriptor 5 is already open. If it's open, it closes the file descriptor 5 and opens it again, with the pointer pointing to the same file table, in the same way as 1.

If file descriptor 5 was already pointing to that table, it returns -1.

One more thing, we could have implemented the same functionality as below.

close(filedes2)

dup(filedes1)

But we don't do it like this. Why?

If we see, the above two operations are not atomic, i.e. we have a slight open time window between call to close and call to dup, so there is a possibility of some signal handler function(in case we receive a signal between these two calls) to alter the pattern of file descriptors. 

Generally, we use these duplicate system calls in case of Inter Process Communication, where we make pipes for the communication between two processes.