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Top Forums Programming Small query regarding function "char * strerror(int errnum)" Post 302762681 by Don Cragun on Tuesday 29th of January 2013 01:32:31 AM
Old 01-29-2013
Quote:
Originally Posted by Praveen_218
John,
I was confused especially after reading the MAN page itself that mentioned of memory location for “unknown error numbers” (which I referred above) but didn't talk of the memory location of the address returned by this system call under normal circumstances.

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()? Just like the call to stat(), lstat(), fstat() which create objects of type struct stat on the heap itself (whose address is returned through the pointer struct stat *sb) and puts the onus of the cleanup on the programmer only.

Available examples too didn't give the clarity as they all called the function strerror() and used the address returned into their processing and were of just few lines only. Hence I was not getting sure of the error handling aspect of the few liner examples.

The code I was working was critical enough not to have scope for silly errors. Hence couldn't have overlooked the cleanup part if the allocation happens to be on the heap and couldn't have just called to free the address either if it belonged to a different memory region.

Hope you got the dilemma making me forward this query to the forum.
If you call strerror(errno), after getting an error indication from a function that sets errno, strerror() will probably return a pointer to a statically allocated string. If you call strerror(random) with a value that is not a valid error number on your system, strerror() may return a pointer to a string like "Unknown error number xxx" where xxx is random converted to a decimal digit string. The application calling strerror() doesn't need to free the memory to which the return value from strerror() points in either case. But, if you intend to call strerror() multiple times and print the results later, you need to allocate space for a buffer to hold the returned string, copy it to that buffer, and free the buffer when you are done with it.
 

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strerror(3)						     Library Functions Manual						       strerror(3)

NAME
strerror, strerror_r - Access message explaining function error LIBRARY
Standard C Library (libc.so, libc.a) SYNOPSIS
#include <string.h> char *strerror( int errnum); The following function does not conform to current standards and is supported only to maintain backward compatibility. int strerror_r( int errnum, char *strerrbuf, int buflen); STANDARDS
Interfaces documented on this reference page conform to industry standards as follows: strerror(): XSH4.2 Refer to the standards(5) reference page for more information about industry standards and associated tags. PARAMETERS
Specifies an error-number value. [Tru64 UNIX] Specifies a buffer that will hold the error message. [Tru64 UNIX] Specifies the length of the buffer. DESCRIPTION
The strerror() function maps the error number specified by the errnum parameter to a error message string and returns a pointer to the string. The string pointed to by the return value is not modified by the program, but may be overwritten by a subsequent call to this function. The implementation behaves as though no other function calls the strerror() function. If a library message catalog is available for the current locale, the strerror() function stores the message from that catalog. Otherwise, it uses the default messages. The LC_MESSAGES category in the setlocale() call specifies the message catalog language, and the NLSPATH environment variable specifies the directory search path for message catalogs. The strerror_r() function is the reentrant version of the strerror() function. It is supported to maintain backward compatibility with operating system versions prior to Tru64 UNIX Version 4.0. RETURN VALUES
On successful completion, the strerror() function returns a pointer to the generated message string. If the error number is not valid, errno is set to indicate the error. On successful completion, strerror_r() provides the error message in strerrbuf, and returns a value of 0 (zero). Otherwise it returns a value of -1. ERRORS
The strerror() and strerror_r() functions set errno to the specified values for the following conditions: The errnum parameter is an invalid error number. [Tru64 UNIX] The strerrorbuf is inaccessible to the strerror_r() function. Note that the strerror_r() function truncates the error message if strerrbuf is too small. RELATED INFORMATION
Functions: intro(2), catgets(3), catopen(3), perror(3), setlocale(3) Standards: standards(5) delim off strerror(3)
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