March 09, 2001, 12:00 AM — This week, I will discuss the C errno facility and its interfaces.
Each system call returns a status code. A zero value indicates success
whereas small negative numbers represent error codes. The
implementation automatically stores a system call's return value in a
global variable called errno, and each invocation of the system call
overwrites the previous errno value (note that in multithreaded
environments, each thread has a distinct errno). errno and its
associated functions are declared in the standard header file
The perror() function has the following prototype:
void perror(const char * msg);
perror() takes an error description string as an argument and prints it
to stderr (typically, the console). In addition to the user-supplied
string, perror() adds a colon and a verbal description of the current
errno value. The common approach passes the program's filename as an
argument. For example:
if (open("db_file.dat", O_RDONLY)<0)
perror ("database.c"); /*pass current filename*/
The output may look as follows:
database.c: No such file or directory
Remember, do not append a newline character to the argument; perror()
will add one after the error description.
Translating an error code to its verbal description may be useful if,
for example, you wish to log the error messages or format them before
printing. Use the function strerror() for this purpose. strerror() has
the following prototype:
char * strerror(int errcode);
The function takes an error code and returns a statically allocated
string describing that code. Remember that subsequent calls to strerror
() overwrite the description string. Unless you intend to use it
immediately, you should copy it to a local string. Using strerror(),
the following example translates the current errno code to a string
that is printed to the user's screen:
printf("error: %s", strerror(errno));
In addition to these functions,
pointers to error messages called sys_errlist. The elements of
sys_errlist are const strings, so you cannot modify them. The following
example retrieves the error description associated with the current
errno value by using sys_errlist and errno as the index:
printf("error: %s", sys_errlist[errno]);
sys_errlist use is deprecated and non-portable; however, it may exist
in legacy code. The strerror() function remains the preferred
alternative for retrieving the error description associated with an
errno code.
