1STRERROR(3P) POSIX Programmer's Manual STRERROR(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 strerror, strerror_l, strerror_r — get error message string
13
15 #include <string.h>
16 char *strerror(int errnum);
18 char *strerror_l(int errnum, locale_t locale);
19 int strerror_r(int errnum, char *strerrbuf, size_t buflen);
20
22 For strerror(): The functionality described on this reference page is
23 aligned with the ISO C standard. Any conflict between the requirements
24 described here and the ISO C standard is unintentional. This volume of
25 POSIX.1‐2017 defers to the ISO C standard.
26 The strerror() function shall map the error number in errnum to a
28 locale-dependent error message string and shall return a pointer to it.
29 Typically, the values for errnum come from errno, but strerror() shall
30 map any value of type int to a message.
31 The application shall not modify the string returned. The returned
33 string pointer might be invalidated or the string content might be
34 overwritten by a subsequent call to strerror(), or by a subsequent call
35 to strerror_l() in the same thread. The returned pointer and the string
36 content might also be invalidated if the calling thread is terminated.
37 The string may be overwritten by a subsequent call to strerror_l() in
39 the same thread.
40 The contents of the error message strings returned by strerror() should
42 be determined by the setting of the LC_MESSAGES category in the current
43 locale.
44 The implementation shall behave as if no function defined in this vol‐
46 ume of POSIX.1‐2017 calls strerror().
47 The strerror() and strerror_l() functions shall not change the setting
49 of errno if successful.
50 Since no return value is reserved to indicate an error of strerror(),
52 an application wishing to check for error situations should set errno
53 to 0, then call strerror(), then check errno. Similarly, since str‐
54 error_l() is required to return a string for some errors, an applica‐
55 tion wishing to check for all error situations should set errno to 0,
56 then call strerror_l(), then check errno.
57 The strerror() function need not be thread-safe.
59 The strerror_l() function shall map the error number in errnum to a
61 locale-dependent error message string in the locale represented by
62 locale and shall return a pointer to it.
63 The strerror_r() function shall map the error number in errnum to a
65 locale-dependent error message string and shall return the string in
66 the buffer pointed to by strerrbuf, with length buflen.
67 If the value of errnum is a valid error number, the message string
69 shall indicate what error occurred; if the value of errnum is zero, the
70 message string shall either be an empty string or indicate that no
71 error occurred; otherwise, if these functions complete successfully,
72 the message string shall indicate that an unknown error occurred.
73 The behavior is undefined if the locale argument to strerror_l() is the
75 special locale object LC_GLOBAL_LOCALE or is not a valid locale object
76 handle.
77
79 Upon completion, whether successful or not, strerror() shall return a
80 pointer to the generated message string. On error errno may be set,
81 but no return value is reserved to indicate an error.
82 Upon successful completion, strerror_l() shall return a pointer to the
84 generated message string. If errnum is not a valid error number, errno
85 may be set to [EINVAL], but a pointer to a message string shall still
86 be returned. If any other error occurs, errno shall be set to indicate
87 the error and a null pointer shall be returned.
88 Upon successful completion, strerror_r() shall return 0. Otherwise, an
90 error number shall be returned to indicate the error.
91
93 These functions may fail if:
94 EINVAL The value of errnum is neither a valid error number nor zero.
96 The strerror_r() function may fail if:
98 ERANGE Insufficient storage was supplied via strerrbuf and buflen to
100 contain the generated message string.
101 The following sections are informative.
103
105 None.
106
108 Historically in some implementations, calls to perror() would overwrite
109 the string that the pointer returned by strerror() points to. Such
110 implementations did not conform to the ISO C standard; however, appli‐
111 cation developers should be aware of this behavior if they wish their
112 applications to be portable to such implementations.
113
115 The strerror_l() function is required to be thread-safe, thereby elimi‐
116 nating the need for an equivalent to the strerror_r() function.
117 Earlier versions of this standard did not explicitly require that the
119 error message strings returned by strerror() and strerror_r() provide
120 any information about the error. This version of the standard requires
121 a meaningful message for any successful completion.
122 Since no return value is reserved to indicate a strerror() error, but
124 all calls (whether successful or not) must return a pointer to a mes‐
125 sage string, on error strerror() can return a pointer to an empty
126 string or a pointer to a meaningful string that can be printed.
127 Note that the [EINVAL] error condition is a may fail error. If an
129 invalid error number is supplied as the value of errnum, applications
130 should be prepared to handle any of the following:
131 1. Error (with no meaningful message): errno is set to [EINVAL], the
133 return value is a pointer to an empty string.
134 2. Successful completion: errno is unchanged and the return value
136 points to a string like "unknownerror" or "errornumberxxx" (where
137 xxx is the value of errnum).
138 3. Combination of #1 and #2: errno is set to [EINVAL] and the return
140 value points to a string like "unknownerror" or "errornumberxxx"
141 (where xxx is the value of errnum). Since applications frequently
142 use the return value of strerror() as an argument to functions like
143 fprintf() (without checking the return value) and since applica‐
144 tions have no way to parse an error message string to determine
145 whether errnum represents a valid error number, implementations are
146 encouraged to implement #3. Similarly, implementations are encour‐
147 aged to have strerror_r() return [EINVAL] and put a string like
148 "unknownerror" or "errornumberxxx" in the buffer pointed to by str‐
149 errbuf when the value of errnum is not a valid error number.
150 Some applications rely on being able to set errno to 0 before calling a
152 function with no reserved value to indicate an error, then call str‐
153 error(errno) afterwards to detect whether an error occurred (because
154 errno changed) or to indicate success (because errno remained zero).
155 This usage pattern requires that strerror(0) succeed with useful
156 results. Previous versions of the standard did not specify the behavior
157 when errnum is zero.
158
160 None.
161
163 perror()
164 The Base Definitions volume of POSIX.1‐2017, <string.h>
166
168 Portions of this text are reprinted and reproduced in electronic form
169 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
170 table Operating System Interface (POSIX), The Open Group Base Specifi‐
171 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
172 Electrical and Electronics Engineers, Inc and The Open Group. In the
173 event of any discrepancy between this version and the original IEEE and
174 The Open Group Standard, the original IEEE and The Open Group Standard
175 is the referee document. The original Standard can be obtained online
176 at http://www.opengroup.org/unix/online.html .
177 Any typographical or formatting errors that appear in this page are
179 most likely to have been introduced during the conversion of the source
180 files to man page format. To report such errors, see https://www.ker‐
181 nel.org/doc/man-pages/reporting_bugs.html .
182IEEE/The Open Group 2017 STRERROR(3P)