Documentation/technical/api-error-handling.txt

   1 Error reporting in git
   2 ======================
   3
   4 `die`, `usage`, `error`, and `warning` report errors of various
   5 kinds.
   6
   7 - `die` is for fatal application errors.  It prints a message to
   8   the user and exits with status 128.
   9
  10 - `usage` is for errors in command line usage.  After printing its
  11   message, it exits with status 129.  (See also `usage_with_options`
  12   in the link:api-parse-options.html[parse-options API].)
  13
  14 - `error` is for non-fatal library errors.  It prints a message
  15   to the user and returns -1 for convenience in signaling the error
  16   to the caller.
  17
  18 - `warning` is for reporting situations that probably should not
  19   occur but which the user (and Git) can continue to work around
  20   without running into too many problems.  Like `error`, it
  21   returns -1 after reporting the situation to the caller.
  22
  23 Customizable error handlers
  24 ---------------------------
  25
  26 The default behavior of `die` and `error` is to write a message to
  27 stderr and then exit or return as appropriate.  This behavior can be
  28 overridden using `set_die_routine` and `set_error_routine`.  For
  29 example, "git daemon" uses set_die_routine to write the reason `die`
  30 was called to syslog before exiting.
  31
  32 Library errors
  33 --------------
  34
  35 Functions return a negative integer on error.  Details beyond that
  36 vary from function to function:
  37
  38 - Some functions return -1 for all errors.  Others return a more
  39   specific value depending on how the caller might want to react
  40   to the error.
  41
  42 - Some functions report the error to stderr with `error`,
  43   while others leave that for the caller to do.
  44
  45 - errno is not meaningful on return from most functions (except
  46   for thin wrappers for system calls).
  47
  48 Check the function's API documentation to be sure.
  49
  50 Caller-handled errors
  51 ---------------------
  52
  53 An increasing number of functions take a parameter 'struct strbuf *err'.
  54 On error, such functions append a message about what went wrong to the
  55 'err' strbuf.  The message is meant to be complete enough to be passed
  56 to `die` or `error` as-is.  For example:
  57
  58         if (ref_transaction_commit(transaction, &err))
  59                 die("%s", err.buf);
  60
  61 The 'err' parameter will be untouched if no error occurred, so multiple
  62 function calls can be chained:
  63
  64         t = ref_transaction_begin(&err);
  65         if (!t ||
  66             ref_transaction_update(t, "HEAD", ..., &err) ||
  67             ret_transaction_commit(t, &err))
  68                 die("%s", err.buf);
  69
  70 The 'err' parameter must be a pointer to a valid strbuf.  To silence
  71 a message, pass a strbuf that is explicitly ignored:
  72
  73         if (thing_that_can_fail_in_an_ignorable_way(..., &err))
  74                 /* This failure is okay. */
  75                 strbuf_reset(&err);