Small query regarding function "char * strerror(int errnum)" Post: 302762859

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Top Forums Programming Small query regarding function "char * strerror(int errnum)" Post 302762859 by JohnGraham on Tuesday 29th of January 2013 06:14:24 AM

01-29-2013

Registered User

Quote:

Originally Posted by Praveed_218

The MAN page tells under the BUG section that "For unknown error numbers, the strerror() function will return its result in a static buffer which may be overwritten by subsequent calls."

Quote:

Originally Posted by Praveen_218

The primary reason to ask this query is because of the fact that the MAN pages themselves not making me sure if the programmer needs to handle any kind of cleanup post calling strerror()?

The man page tells you that the result may be in a static buffer. It tells you this to let you know two things:

You do not need to free the result returned from this function. A static buffer (as in when you use "static" within a function) exists throughout the lifetime of the program and does not need to be released by anyone.
You should not rely on the result being available over multiple calls to strerror(), as in:
Code:
```
char *err1 = strerror(errno);
some_call();
char *err2 = strerror(errno);
printf("Got errors %s and %s\n", err1, err2);
```
This is because it's possible that the result of both calls to strerror() will be a pointer to the same (static) buffer.

Some advice: When asking questions on forums, ask what you really want to ask, as opposed to second-guessing what you want. When you ask where the result of strerror() will be and if you can edit it, my first instinct is to just say "no, don't do any of that forget about it!", but if you just ask whether you should free() the result and why the man page mentions a static buffer, you'll get a much better response.

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LEARN ABOUT REDHAT

strerror

STRERROR(3)						     Linux Programmer's Manual						       STRERROR(3)

NAME

       strerror, strerror_r - return string describing error code

SYNOPSIS

       #include <string.h>

       char *strerror(int errnum);
       int strerror_r(int errnum, char *buf, size_t n);

DESCRIPTION

       The  strerror()	function  returns a string describing the error code passed in the argument errnum, possibly using the LC_MESSAGES part of
       the current locale to select the appropriate language.  This string must not be modified by the application, but may be modified by a  sub-
       sequent call to perror() or strerror().	No library function will modify this string.

       The strerror_r() function is similar to strerror(), but is thread safe. It returns the string in the user-supplied buffer buf of length n.

RETURN VALUE

       The  strerror()	function  returns the appropriate error description string, or an unknown error message if the error code is unknown.  The
       value of errno is not changed for a successful call, and is set to a nonzero value upon error.  The strerror_r() function returns 0 on suc-
       cess and -1 on failure, setting errno.

ERRORS

       EINVAL The value of errnum is not a valid error number.

       ERANGE Insufficient storage was supplied to contain the error description string.

CONFORMING TO

       SVID 3, POSIX, BSD 4.3, ISO/IEC 9899:1990 (C89).
       strerror_r()  with prototype as given above is specified by SUSv3, and was in use under Digital Unix and HP Unix. An incompatible function,
       with prototype

       char *strerror_r(int errnum, char *buf, size_t n);

       is a GNU extension used by glibc (since 2.0), and must be regarded as obsolete in view of SUSv3.  The GNU version may, but  need  not,  use
       the  user-supplied buffer.  If it does, the result may be truncated in case the supplied buffer is too small. The result is always NUL-ter-
       minated.

SEE ALSO

       errno(3), perror(3), strsignal(3)

								    2001-10-16							       STRERROR(3)