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.\" References:
.\"   glibc manual and source
.TH error 3 2022-12-15 "Linux man-pages 6.03"
.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>
.PP
.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 ", ...);"
.PP
.BI "extern unsigned int " error_message_count ;
.BI "extern int " error_one_per_line ;
.PP
.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.
.PP
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 ().
.PP
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.
.PP
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.
.PP
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.
.PP
The global variable \fIerror_message_count\fP counts the number of
messages that have been output by
.BR error ()
and
.BR error_at_line ().
.PP
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).
.ad l
.nh
.TS
allbox;
lb lb lbx
l l l.
Interface	Attribute	Value
T{
.BR error ()
T}	Thread safety	MT-Safe locale
T{
.BR error_at_line ()
T}	Thread safety	T{
MT-Unsafe\ race: error_at_line/\:error_one_per_line locale
T}
.TE
.hy
.ad
.sp 1
.PP
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
These functions and variables are GNU extensions, and should not be
used in programs intended to be portable.
.SH SEE ALSO
.BR err (3),
.BR errno (3),
.BR exit (3),
.BR perror (3),
.BR program_invocation_name (3),
.BR strerror (3)