error, error_at_line, error_message_count, error_one_per_line, error_print_progname - glibc error reporting functions
void error(int status, int errnum, const char *format, ...);
error_at_line(int status, int
errnum, const char *filename,
unsigned int linenum, const char *format, ...);
extern unsigned int error_message_count;
extern int error_one_per_line;
extern void (*error_print_progname) (void);
error() is a general error-reporting function. It flushes stdout, and then outputs to stderr the program name, a colon and a space, the message specified by the printf(3)-style format string format, and, if errnum is nonzero, a second colon and a space followed by the string given by strerror(errnum). Any arguments required for format should follow format in the argument list. The output is terminated by a newline character.
The program name printed by error() is the value of the global variable program_invocation_name(3). program_invocation_name initially has the same value as main()’s argv. The value of this variable can be modified to change the output of error().
If status has a nonzero value, then error() calls exit(3) to terminate the program using the given value as the exit status.
The error_at_line() function is exactly the same as error(), except for the addition of the arguments filename and linenum. The output produced is as for error(), except that after the program name are written: a colon, the value of filename, a colon, and the value of linenum. The preprocessor values __LINE__ and __FILE__ may be useful when calling error_at_line(), but other values can also be used. For example, these arguments could refer to a location in an input file.
If the global variable error_one_per_line is set nonzero, a sequence of error_at_line() calls with the same value of filename and linenum will result in only one message (the first) being output.
The global variable error_message_count counts the number of messages that have been output by error() and error_at_line().
If the global variable error_print_progname is assigned the address of a function (i.e., is not NULL), then that function is called instead of prefixing the message with the program name and colon. The function should print a suitable string to stderr.
For an explanation of the terms used in this section, see attributes(7).
The internal error_one_per_line variable is accessed (without any form of synchronization, but since it’s an int used once, it should be safe enough) and, if error_one_per_line is set nonzero, the internal static variables (not exposed to users) used to hold the last printed filename and line number are accessed and modified without synchronization; the update is not atomic and it occurs before disabling cancellation, so it can be interrupted only after one of the two variables is modified. After that, error_at_line() is very much like error().
These functions and variables are GNU extensions, and should not be used in programs intended to be portable.
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