The **`rename` function** is part of the `<stdio.h>` header file in C. It is used to change the name of an existing file to a new one. 

The function takes in two parameters: 
1. old filename 
2. new filename 

It returns an `int` which is 0 in case of success and a non-zero number in case of an error.

The illustration below shows how `rename` works:

# Declaration

The `rename` function is defined as follows:

```C
int rename (const char *oldName, const char *newName);
```
It takes in two parameters. The first is the character pointer to the old name, and the second is the character pointer to the new name.

> Pointer to `char` can be in the form of a `char` array.

# Example

The following code snippet shows how we can use `rename`:

> We will first create a file and name it "oldFile.txt". Then we will rename it to "newFile.txt".


#include<stdio.h> // Including header file

int main(){

  FILE *fptr; // creating a pointer to file
  char oldName[] = "oldFile.txt"; // setting variable for original filename
  fptr = fopen(oldName, "w"); // creating a file

  char newName[] = "newFile.txt"; // setting variable for new filename
  int checkReturn; // variable to hold return value

  checkReturn = rename(oldName, newName); // renaming files
  if(checkReturn == 0){
    printf("File name changed successfully");
  }
  else{
    printf("Error Occured");
  }
  return 0;
}

As we can see above, the filename has been changed successfully.

> Errors usually occur if a file of the specified name does not exist.

What is rename in C?

The rename function in C changes an existing filename to a new one and returns 0 on success.