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Top Forums Programming Small query regarding function "char * strerror(int errnum)" Post 302762669 by Praveen_218 on Tuesday 29th of January 2013 01:04:50 AM
Old 01-29-2013
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 function 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 those few liner examples (sometimes they might just overlook error handling part just to give us the feel of the function).

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 to forward this query to the forum.

Last edited by Praveen_218; 01-29-2013 at 02:28 AM..
 

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explain_lstat(3)					     Library Functions Manual						  explain_lstat(3)

NAME
explain_lstat - explain lstat(3) errors SYNOPSIS
#include <libexplain/lstat.h> const char *explain_lstat(const char *pathname, const struct stat *buf); void explain_message_lstat(char *message, int message_size, const char *pathname, const struct stat *buf); const char *explain_errno_lstat(int errnum, const char *pathname, const struct stat *buf); void explain_message_errno_lstat(char *message, int message_size, int errnum, const char *pathname, const struct stat *buf); DESCRIPTION
These functions may be used to obtains explanations for lstat(2) errors. explain_lstat const char *explain_lstat(const char *pathname, const struct stat *buf); The explain_lstat function is used to obtain an explanation of an error returned by the lstat(2) function. The least the message will con- tain is the value of strerror(errno), but usually it will do much better, and indicate the underlying cause in more detail. The errno global variable will be used to obtain the error value to be decoded. This function is intended to be used in a fashion similar to the following example: if (lstat(pathname, &buf) < 0) { fprintf(stderr, '%s0, explain_lstat(pathname, &buf)); exit(EXIT_FAILURE); } pathname The original pathname, exactly as passed to the lstat(2) system call. buf The original buf, exactly as passed to the lstat(2) system call. Returns: The message explaining the error. This message buffer is shared by all libexplain functions which do not supply a buffer in their argument list. This will be overwritten by the next call to any libexplain function which shares this buffer, including other threads. Note: This function is not thread safe, because it shares a return buffer across all threads, and many other functions in this library. explain_errno_lstat "const char *explain_errno_lstat(int errnum, const char *pathname, const struct stat *buf); The explain_errno_lstat function is used to obtain an explanation of an error returned by the lstat(2) function. The least the message will contain is the value of strerror(errnum), but usually it will do much better, and indicate the underlying cause in more detail. This function is intended to be used in a fashion similar to the following example: if (lstat(pathname, &buf) < 0) { int err = errno; fprintf(stderr, '%s0, explain_errno_lstat(err, pathname, &buf)); exit(EXIT_FAILURE); } errnum The error value to be decoded, usually obtained from the errno global variable just before this function is called. This is neces- sary if you need to call any code between the system call to be explained and this function, because many libc functions will alter the value of errno. pathname The original pathname, exactly as passed to the lstat(2) system call. buf The original buf, exactly as passed to the lstat(2) system call. Returns: The message explaining the error. This message buffer is shared by all libexplain functions which do not supply a buffer in their argument list. This will be overwritten by the next call to any libexplain function which shares this buffer, including other threads. Note: This function is not thread safe, because it shares a return buffer across all threads, and many other functions in this library. explain_message_lstat void explain_message_lstat(char *message, int message_size, const char *pathname, const struct stat *buf); The explain_message_lstat function is used to obtain an explanation of an error returned by the lstat(2) function. The least the message will contain is the value of strerror(errno), but usually it will do much better, and indicate the underlying cause in more detail. The errno global variable will be used to obtain the error value to be decoded. This function is intended to be used in a fashion similar to the following example: if (lstat(pathname, &buf) < 0) { char message[3000]; explain_message_lstat(message, sizeof(message), pathname, &buf); fprintf(stderr, '%s0, message); exit(EXIT_FAILURE); } message The location in which to store the returned message. Because a message return buffer has been supplied, this function is thread safe. message_size The size in bytes of the location in which to store the returned message. pathname The original pathname, exactly as passed to the lstat(2) system call. buf The original buf, exactly as passed to the lstat(2) system call. explain_message_errno_lstat void explain_message_errno_lstat(char *message, int message_size, int errnum, const char *pathname, const struct stat *buf); The explain_message_errno_lstat function is used to obtain an explanation of an error returned by the lstat(2) function. The least the message will contain is the value of strerror(errnum), but usually it will do much better, and indicate the underlying cause in more detail. This function is intended to be used in a fashion similar to the following example: if (lstat(pathname, &buf) < 0) { int err = errno; char message[3000]; explain_message_errno_lstat(message, sizeof(message), err, pathname, &buf); fprintf(stderr, '%s0, message); exit(EXIT_FAILURE); } message The location in which to store the returned message. Because a message return buffer has been supplied, this function is thread safe. message_size The size in bytes of the location in which to store the returned message. errnum The error value to be decoded, usually obtained from the errno global variable just before this function is called. This is neces- sary if you need to call any code between the system call to be explained and this function, because many libc functions will alter the value of errno. pathname The original pathname, exactly as passed to the lstat(2) system call. buf The original buf, exactly as passed to the lstat(2) system call. COPYRIGHT
libexplain version 0.52 Copyright (C) 2008 Peter Miller AUTHOR
Written by Peter Miller <pmiller@opensource.org.au> explain_lstat(3)
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