fgets (3)

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.

NAME

fgets --- get a string from a stream

SYNOPSIS

#include <stdio.h>
char *fgets(char *restrict s, int n, FILE *restrict stream);

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.1-2008 defers to the ISO C standard. The fgets() function shall read bytes from stream into the array pointed to by s, until n-1 bytes are read, or a <newline> is read and transferred to s, or an end-of-file condition is encountered. The string is then terminated with a null byte. The fgets() function may mark the last data access timestamp of the file associated with stream for update. The last data access timestamp shall be marked for update by the first successful execution of fgetc(), fgets(), fread(), fscanf(), getc(), getchar(), getdelim(), getline(), gets(), or scanf() using stream that returns data not supplied by a prior call to ungetc().

RETURN VALUE

Upon successful completion, fgets() shall return s. If the stream is at end-of-file, the end-of-file indicator for the stream shall be set and fgets() shall return a null pointer. If a read error occurs, the error indicator for the stream shall be set, fgets() shall return a null pointer, and shall set errno to indicate the error.

ERRORS

Refer to fgetc().

The following sections are informative.

EXAMPLES

Reading Input

The following example uses fgets() 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 sysconf(_SC_LINE_MAX) returns -1 without setting errno. This example assumes that sysconf(_SC_LINE_MAX) will not fail.)

#include <limits.h>
#include <stdio.h>
#include <unistd.h>
#define MYLIMIT 16384
char *line;
int line_max;
if (LINE_MAX >= MYLIMIT) {
    // Use maximum line size of MYLIMIT. If LINE_MAX is
    // bigger than our limit, sysconf() can't report a
    // smaller limit.
    line_max = MYLIMIT;
} else {
    long limit = sysconf(_SC_LINE_MAX);
    line_max = (limit < 0 || limit > MYLIMIT) ? MYLIMIT : (int)limit;
}
// 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;
}
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);
...

APPLICATION USAGE

None.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

Section 2.5, Standard I/O Streams, fgetc(), fopen(), fread(), fscanf(), getc(), getchar(), getdelim(), gets(), ungetc() The Base Definitions volume of POSIX.1-2008, <stdio.h>

COPYRIGHT

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, Copyright (C) 2013 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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 www.unix.org/online.html .
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 www.kernel.org/doc/man-pages/reporting_bugs.html .