1FGETS(3P) POSIX Programmer's Manual FGETS(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 fgets — get a string from a stream
13
15 #include <stdio.h>
16 char *fgets(char *restrict s, int n, FILE *restrict stream);
18
20 The functionality described on this reference page is aligned with the
21 ISO C standard. Any conflict between the requirements described here
22 and the ISO C standard is unintentional. This volume of POSIX.1‐2017
23 defers to the ISO C standard.
24 The fgets() function shall read bytes from stream into the array
26 pointed to by s until n-1 bytes are read, or a <newline> is read and
27 transferred to s, or an end-of-file condition is encountered. A null
28 byte shall be written immediately after the last byte read into the
29 array. If the end-of-file condition is encountered before any bytes
30 are read, the contents of the array pointed to by s shall not be
31 changed.
32 The fgets() function may mark the last data access timestamp of the
34 file associated with stream for update. The last data access timestamp
35 shall be marked for update by the first successful execution of
36 fgetc(), fgets(), fread(), fscanf(), getc(), getchar(), getdelim(),
37 getline(), gets(), or scanf() using stream that returns data not sup‐
38 plied by a prior call to ungetc().
39
41 Upon successful completion, fgets() shall return s. If the stream is
42 at end-of-file, the end-of-file indicator for the stream shall be set
43 and fgets() shall return a null pointer. If a read error occurs, the
44 error indicator for the stream shall be set, fgets() shall return a
45 null pointer, and shall set errno to indicate the error.
46
48 Refer to fgetc().
49 The following sections are informative.
51
53 Reading Input
54 The following example uses fgets() to read lines of input. It assumes
55 that the file it is reading is a text file and that lines in this text
56 file are no longer than 16384 (or {LINE_MAX} if it is less than 16384
57 on the implementation where it is running) bytes long. (Note that the
58 standard utilities have no line length limit if sysconf(_SC_LINE_MAX)
59 returns -1 without setting errno. This example assumes that
60 sysconf(_SC_LINE_MAX) will not fail.)
61 #include <limits.h>
64 #include <stdio.h>
65 #include <unistd.h>
66 #define MYLIMIT 16384
67 char *line;
69 int line_max;
70 if (LINE_MAX >= MYLIMIT) {
71 // Use maximum line size of MYLIMIT. If LINE_MAX is
72 // bigger than our limit, sysconf() cannot report a
73 // smaller limit.
74 line_max = MYLIMIT;
75 } else {
76 long limit = sysconf(_SC_LINE_MAX);
77 line_max = (limit < 0 || limit > MYLIMIT) ? MYLIMIT : (int)limit;
78 }
79 // line_max + 1 leaves room for the null byte added by fgets().
81 line = malloc(line_max + 1);
82 if (line == NULL) {
83 // out of space
84 ...
85 return error;
86 }
87 while (fgets(line, line_max + 1, fp) != NULL) {
89 // Verify that a full line has been read ...
90 // If not, report an error or prepare to treat the
91 // next time through the loop as a read of a
92 // continuation of the current line.
93 ...
94 // Process line ...
95 ...
96 }
97 free(line);
98 ...
99
101 None.
102
104 None.
105
107 None.
108
110 Section 2.5, Standard I/O Streams, fgetc(), fopen(), fread(), fscanf(),
111 getc(), getchar(), getdelim(), gets(), ungetc()
112 The Base Definitions volume of POSIX.1‐2017, <stdio.h>
114
116 Portions of this text are reprinted and reproduced in electronic form
117 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
118 table Operating System Interface (POSIX), The Open Group Base Specifi‐
119 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
120 Electrical and Electronics Engineers, Inc and The Open Group. In the
121 event of any discrepancy between this version and the original IEEE and
122 The Open Group Standard, the original IEEE and The Open Group Standard
123 is the referee document. The original Standard can be obtained online
124 at http://www.opengroup.org/unix/online.html .
125 Any typographical or formatting errors that appear in this page are
127 most likely to have been introduced during the conversion of the source
128 files to man page format. To report such errors, see https://www.ker‐
129 nel.org/doc/man-pages/reporting_bugs.html .
130IEEE/The Open Group 2017 FGETS(3P)