Error Handling

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, defines an array of sys_nerr 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.
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