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4870614
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2011-02-02T03:52:55.320
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2011-02-02T03:52:55.320
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4869739
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You can either use <code>perror()</code> if you don't mind working within the constraints of its formatting, or you can do the equivalent of: <pre><code>fprintf(stderr, "Failed to open %s (%d: %s)\n", filename, errno, strerror(errno)); </code></pre> You should only ever look at <code>errno</code> after a function has reported failure. It may also be set after a function that succeeds. For example, on Solaris, if the output is not a 'tty', you will often find ENOTTY in <code>errno</code> after a successful print operation - because it checked whether the output file descriptor was a tty and actually it wasn't. But the function succeeded. If there are any more function calls in your code than in the line I showed, it is best to preserve <code>errno</code> early on - because it gets tampered with by other functions (one of the many demerits of a global variable). So, I often write: <pre><code>{ int errnum = errno; ...any other processing... fprintf(stderr, "%s: failed to open %s for reading (%d: %s)\n", prog_name, file_name, errnum, strerror(errnum)); } </code></pre> Well, I write the logical equivalent of that; I have a library that reports errors, so I'd actually write: <pre><code>err_sysrem("failed to open %s for reading", file_name); </code></pre> All the functions in the package start <code>err_</code>. The function <code>err_sysrem()</code> is a remark (does not exit) and includes the system error information - <code>errno</code> and <code>strerror(errno)</code>. If I called <code>err_syserr()</code> instead, it would terminate the program, and <code>err_remark()</code> would not include the information from <code>errno</code>, and neither would <code>err_error()</code>. I find the brevity provided by the functions beneficial. The functions all automatically include the program name at the start of the message; you can configure it to include time stamps or PIDs or other information too. You use <code>err_setarg0(argv[0]);</code> in <code>main()</code> to set the program name; you can, of course, change it at any time.
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1. POHow could I go about finding the error from opening the file?
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1. USJonathan Leffler
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