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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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FGETS(3P) POSIX Programmer's Manual FGETS(3P)
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.
fgets — get a string from a stream
#include <stdio.h>
char *fgets(char *restrict s, int n, FILE *restrict stream);
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.1‐2017 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.
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 s shall not be changed.
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().
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.
Refer to fgetc(3p).
The following sections are informative.
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() cannot 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);
...
None.
None.
None.
Section 2.5, Standard I/O Streams, fgetc(3p), fopen(3p),
fread(3p), fscanf(3p), getc(3p), getchar(3p), getdelim(3p),
gets(3p), ungetc(3p)
The Base Definitions volume of POSIX.1‐2017, stdio.h(0p)
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 .
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 .
IEEE/The Open Group 2017 FGETS(3P)
Pages that refer to this page: stdio.h(0p), fgetc(3p), getdelim(3p), gets(3p)
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HTML rendering created 2023-12-22 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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