Com_err displays an error message on the standard error stream stderr (see stdio(3S)) composed of the whoami string, which should specify the program name or some subportion of a program, followed by an error message generated from the code value (derived from compile_et(1)), and a string produced using the format string and any following arguments, in the same style as fprintf(3).
The behavior of com_err can be modified using set_com_err_hook; this defines a procedure which is called with the arguments passed to com_err, instead of the default internal procedure which sends the formatted text to error output. Thus the error messages from a program can all easily be diverted to another form of diagnostic logging, such as syslog(3). Reset_com_err_hook may be used to restore the behavior of com_err to its default form. Both procedures return the previous hook value. These hook procedures must have the declaration given for proc above in the synopsis.
The initialize_XXXX_error_table routine is generated mechanically by compile_et(1) from a source file containing names and associated strings. Each table has a name of up to four characters, which is used in place of the XXXX in the name of the routine. These routines should be called before any of the corresponding error codes are used, so that the com_err library will recognize error codes from these tables when they are used.
The com_err.h header file should be included in any source file that uses routines from the com_err library; executable files must be linked using -lcom_err in order to cause the com_err library to be included.