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.\" References:
.\"   glibc manual and source
.TH error 3 2024-05-02 "Linux man-pages 6.9.1"
.SH NAME
error, error_at_line, error_message_count, error_one_per_line,
error_print_progname \- glibc error reporting functions
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <error.h>
.P
.BI "void error(int " status ", int " errnum ", const char *" format ", ...);"
.BI "void error_at_line(int " status ", int " errnum ", const char *" filename ,
.BI "                   unsigned int " linenum ", const char *" format ", ...);"
.P
.BI "extern unsigned int " error_message_count ;
.BI "extern int " error_one_per_line ;
.P
.BI "extern void (*" error_print_progname ")(void);"
.fi
.SH DESCRIPTION
.BR error ()
is a general error-reporting function.
It flushes
.IR stdout ,
and then outputs to
.I stderr
the program name, a colon and a space, the message specified by the
.BR printf (3)-style
format string \fIformat\fP, and, if \fIerrnum\fP is
nonzero, a second colon and a space followed by the string given by
.IR strerror(errnum) .
Any arguments required for
.I format
should follow
.I format
in the argument list.
The output is terminated by a newline character.
.P
The program name printed by
.BR error ()
is the value of the global variable
.BR program_invocation_name (3).
.I program_invocation_name
initially has the same value as
.IR main ()'s
.IR argv[0] .
The value of this variable can be modified to change the output of
.BR error ().
.P
If \fIstatus\fP has a nonzero value, then
.BR error ()
calls
.BR exit (3)
to terminate the program using the given value as the exit status;
otherwise it returns after printing the error message.
.P
The
.BR error_at_line ()
function is exactly the same as
.BR error (),
except for the addition of the arguments
.I filename
and
.IR linenum .
The output produced is as for
.BR error (),
except that after the program name are written: a colon, the value of
.IR filename ,
a colon, and the value of
.IR linenum .
The preprocessor values \fB__LINE__\fP and
\fB__FILE__\fP may be useful when calling
.BR error_at_line (),
but other values can also be used.
For example, these arguments could refer to a location in an input file.
.P
If the global variable \fIerror_one_per_line\fP is set nonzero,
a sequence of
.BR error_at_line ()
calls with the
same value of \fIfilename\fP and \fIlinenum\fP will result in only
one message (the first) being output.
.P
The global variable \fIerror_message_count\fP counts the number of
messages that have been output by
.BR error ()
and
.BR error_at_line ().
.P
If the global variable \fIerror_print_progname\fP
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
.IR stderr .
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lb lb lbx
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR error ()
T}	Thread safety	MT-Safe locale
T{
.na
.nh
.BR error_at_line ()
T}	Thread safety	T{
.na
.nh
MT-Unsafe\ race: error_at_line/\:error_one_per_line locale
T}
.TE
.P
The internal
.I error_one_per_line
variable is accessed (without any form of synchronization, but since it's an
.I int
used once, it should be safe enough) and, if
.I 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 cancelation, so it can be interrupted only after
one of the two variables is modified.
After that,
.BR error_at_line ()
is very much like
.BR error ().
.SH STANDARDS
GNU.
.SH SEE ALSO
.BR err (3),
.BR errno (3),
.BR exit (3),
.BR perror (3),
.BR program_invocation_name (3),
.BR strerror (3)