The GNU C Library
2. Error Reporting
Many functions in the GNU C library detect and report error conditions,
and sometimes your programs need to check for these error conditions.
For example, when you open an input file, you should verify that the
file was actually opened correctly, and print an error message or take
other appropriate action if the call to the library function failed.
This chapter describes how the error reporting facility works. Your
program should include the header file `errno.h' to use this