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fgets.3p (4889B)

'\" et
.TH FGETS "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
fgets
\(em get a string from a stream
.SH SYNOPSIS
.LP
.nf
#include <stdio.h>
.P
char *fgets(char *restrict \fIs\fP, int \fIn\fP, FILE *restrict \fIstream\fP);
.fi
.SH DESCRIPTION
The functionality described on this reference page is aligned with the
ISO\ C standard. Any conflict between the requirements described here and the
ISO\ C standard is unintentional. This volume of POSIX.1\(hy2017 defers to the ISO\ C standard.
.P
The
\fIfgets\fR()
function shall read bytes from
.IR stream
into the array pointed to by
.IR s
until
.IR n \-1
bytes are read, or a
<newline>
is read and transferred to
.IR s ,
or an end-of-file condition is encountered. A null byte shall be
written immediately after the last byte read into the array.
If the end-of-file condition is encountered before any bytes
are read, the contents of the array pointed to by
.IR s
shall not be changed.
.P
The
\fIfgets\fR()
function may mark the last data access timestamp of the file associated
with
.IR stream
for update. The last data access timestamp shall be marked for update
by the first successful execution of
\fIfgetc\fR(),
\fIfgets\fR(),
\fIfread\fR(),
\fIfscanf\fR(),
\fIgetc\fR(),
\fIgetchar\fR(),
\fIgetdelim\fR(),
\fIgetline\fR(),
\fIgets\fR(),
or
\fIscanf\fR()
using
.IR stream
that returns data not supplied by a prior call to
\fIungetc\fR().
.SH "RETURN VALUE"
Upon successful completion,
\fIfgets\fR()
shall return
.IR s .
If the stream is at end-of-file, the end-of-file indicator for
the stream shall be set and
\fIfgets\fR()
shall return a null pointer.
If a read error occurs, the error indicator for the stream shall be set,
\fIfgets\fR()
shall return a null pointer,
and shall set
.IR errno
to indicate the error.
.SH ERRORS
Refer to
.IR "\fIfgetc\fR\^(\|)".
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Reading Input"
.P
The following example uses
\fIfgets\fR()
to read lines of input. It assumes that the file it is reading is a text
file and that lines in this text file are no longer than 16384 (or
{LINE_MAX}
if it is less than 16384 on the implementation where it is running)
bytes long. (Note that the standard utilities have no line length limit if
.IR sysconf (_SC_LINE_MAX)
returns \-1 without setting
.IR errno .
This example assumes that
.IR sysconf (_SC_LINE_MAX)
will not fail.)
.sp
.RS 4
.nf
#include <limits.h>
#include <stdio.h>
#include <unistd.h>
#define MYLIMIT 16384
.P
char *line;
int line_max;
if (LINE_MAX >= MYLIMIT) {
    // Use maximum line size of MYLIMIT. If LINE_MAX is
    // bigger than our limit, sysconf() cannot report a
    // smaller limit.
    line_max = MYLIMIT;
} else {
    long limit = sysconf(_SC_LINE_MAX);
    line_max = (limit < 0 || limit > MYLIMIT) ? MYLIMIT : (int)limit;
}
.P
// line_max + 1 leaves room for the null byte added by fgets().
line = malloc(line_max + 1);
if (line == NULL) {
    // out of space
    ...
    return error;
}
.P
while (fgets(line, line_max + 1, fp) != NULL) {
    // Verify that a full line has been read ...
    // If not, report an error or prepare to treat the
    // next time through the loop as a read of a
    // continuation of the current line.
    ...
    // Process line ...
    ...
}
free(line);
\&...
.fi
.P
.RE
.SH "APPLICATION USAGE"
None.
.SH RATIONALE
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.5" ", " "Standard I/O Streams",
.IR "\fIfgetc\fR\^(\|)",
.IR "\fIfopen\fR\^(\|)",
.IR "\fIfread\fR\^(\|)",
.IR "\fIfscanf\fR\^(\|)",
.IR "\fIgetc\fR\^(\|)",
.IR "\fIgetchar\fR\^(\|)",
.IR "\fIgetdelim\fR\^(\|)",
.IR "\fIgets\fR\^(\|)",
.IR "\fIungetc\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<stdio.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .