how to rename all file with program c

hello I have some trouble for my project. I need to rename all file extension .png to namefile_grey.png.

example :
1.png
2.png
3.png

output :
1_grey.png
2_grey.png
3_grey.png

can anyone solve this problem ??

To clarify: are you just trying to modify strings representing filenames? Or are you trying to rename the files in a particular directory that have a ".png" extension? If the latter, is this really meant to be a C program, or is this a practical problem for your actual C project that you're hoping to solve quickly to get it over and done with?

it will be the second option for me ( " trying to rename the files in a particular directory that have a ".png" extension " ) best regeards

Do you know the names of the files in advance? If not, you will need to use OS specific functionality for that, so you should then state the OS this is for, or what cross platform API you might wish to use.