fgets(3p) — Linux manual page


FGETS(3P)               POSIX Programmer's Manual              FGETS(3P)

PROLOG         top

       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         top

       fgets — get a string from a stream

SYNOPSIS         top

       #include <stdio.h>

       char *fgets(char *restrict s, int n, FILE *restrict stream);

DESCRIPTION         top

       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().

RETURN VALUE         top

       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         top

       Refer to fgetc(3p).

       The following sections are informative.

EXAMPLES         top

   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 ...



RATIONALE         top




SEE ALSO         top

       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)

COPYRIGHT         top

       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)