function of “errnoâ€
What function does “errno†do?
Sometimes it is nice to know what specific error occurred; for this purpose all system calls, when appropriate, leave an error number in an external integer called calls, when appropriate, leave an error number in an external integer called errno. By using errno, your program can, for example, determine whether an attempt to open a file failed because it did not exist or because you lacked permission to read it.