added low-level error handling module

[imager.git] / error.c

/*
=head1 NAME

error.c - error reporting code for Imager

=head1 SYNOPSIS

  // user code:
  int new_fatal; // non-zero if errors are fatal
  int old_fatal = i_set_failure_fatal(new_fatal);
  i_set_argv0("name of your program");
  extern void error_cb(char const *);
  i_error_cb old_ecb;
  old_ecb = i_set_error_cb(error_cb);
  i_failed_cb old_fcb;
  extern void failed_cb(char **errors);
  old_fcb = i_set_failed_cb(failed_cb);
  if (!i_something(...)) {
    char **errors = i_errors();
  }

  // imager code:
  undef_int i_something(...) {
    i_clear_error();
    if (!some_lower_func(...)) {
      return i_failed("could not something");
    }
    return 1;
  }
  undef_int some_lower_func(...) {
    if (somethingelse_failed()) {
      i_push_error("could not somethingelse");
      return 0;
    }
    return 1;
  }

=head1 DESCRIPTION

This module provides the C level error handling functionality for
Imager.

A few functions return or pass in a char **, this is a NULL terminated
list of pointers to error messages (which are NUL terminated strings,
just as it normal in C :).  Even though these aren't passed as char
const * const * pointers, don't modify the strings or the pointers.

The interface as currently defined isn't thread safe, unfortunately.

This code uses Imager's mymalloc() for memory allocation, so out of
memory errors are I<always> fatal.

=head1 INTERFACE

These functions form the interface that a user of Imager sees (from
C).  The Perl level won't use all of this.

=over

=cut
*/

#include "image.h"
#include <stdio.h>
#include <stdlib.h>

/* we never actually use the last item - it's the NULL terminator */
#define ERRSTK 20
char *error_stack[ERRSTK];
int error_sp = ERRSTK - 1;
/* we track the amount of space used each string, so we don't reallocate 
   space unless we need to.
   This also means that a memory tracking library may see the memory 
   allocated for this as a leak. */
int error_space[ERRSTK];

static i_error_cb error_cb;
static i_failed_cb failed_cb;
static int failures_fatal;
static char *argv0;

/*
=item i_set_argv0(char const *program)

Sets the name of the program to be displayed in fatal error messages.

The simplest way to use this is just:

  i_set_argv0(argv[0]);

when your program starts.
*/
void i_set_argv0(char const *name) {
  if (!name)
    return;
  char *dupl = mymalloc(strlen(name)+1);
  strcpy(dupl, name);
  if (argv0)
    myfree(argv0);
  argv0 = dupl;
}

/*
=item i_set_failure_fatal(int failure_fatal)

If failure_fatal is non-zero then any future failures will result in
Imager exiting your program with a message describing the failure.

Returns the previous setting.

=cut
*/
int i_set_failures_fatal(int fatal) {
  int old = failures_fatal;
  failures_fatal = fatal;

  return old;
}

/*
=item i_set_error_cb(i_error_cb)

Sets a callback function that is called each time an error is pushed
onto the error stack.

Returns the previous callback.

i_set_failed_cb() is probably more useful.

=cut
*/
i_error_cb i_set_error_cb(i_error_cb cb) {
  i_error_cb old = error_cb;
  error_cb = cb;

  return old;
}

/*
=item i_set_failed_cb(i_failed_cb cb)

Sets a callback function that is called each time an Imager function
fails.

Returns the previous callback.

=cut
*/
i_failed_cb i_set_failed_cb(i_failed_cb cb) {
  i_failed_cb old = failed_cb;
  failed_cb = cb;

  return old;
}

/*
=item i_errors()

Returns a pointer to the first element of an array of error messages,
terminated by a NULL pointer.  The highest level message is first.

=cut
*/
char **i_errors() {
  return error_stack + error_sp;
}

/*
=back

=head1 INTERNAL FUNCTIONS

These functions are called by Imager to report errors through the
above interface.

It may be desirable to have functions to mark the stack and reset to
the mark.

=over

=item i_clear_error()

Called by any imager function before doing any other processing.

=cut */
void i_clear_error() {
  error_sp = ERRSTK-1;
}

/*
=item i_push_error(char const *msg)

Called by an imager function to push an error message onto the stack.

No message is pushed if the stack is full (since this means someone
forgot to call i_clear_error(), or that a function that doesn't do
error handling is calling function that does.).

=cut
*/
void i_push_error(char const *msg) {
  int size = strlen(msg)+1;

  if (error_sp <= 0)
    /* bad, bad programmer */
    return;

  --error_sp;
  if (error_space[error_sp] < size) {
    if (error_stack[error_sp])
      myfree(error_stack[error_sp]);
    /* memory allocated on the following line is only ever release when 
       we need a bigger string */
    error_stack[error_sp] = mymalloc(size);
    error_space[error_sp] = size;
  }
  strcpy(error_stack[error_sp], msg);

  if (error_cb)
    error_cb(msg);
}

/*
=item i_failed(char const *msg)

Called by Imager code to indicate that a top-level has failed.

msg can be NULL.

Calls the current failed callback, if any.

Aborts the program with an error, if failures have been set to be fatal.

Returns zero if it does not abort.

=cut
*/
int i_failed(char const *msg) {
  if (msg)
    i_push_error(msg);
  if (failed_cb)
    failed_cb(error_stack + error_sp);
  if (failures_fatal) {
    if (argv0)
      fprintf(stderr, "%s: ", argv0);
    fputs("error:\n", stderr);
    while (error_stack[error_sp]) {
      fprintf(stderr, " %s\n", error_stack[error_sp]);
      ++error_sp;
    }
    exit(EXIT_FAILURE);
  }

  return 0;
}

/*
=back

=head1 BUGS

This interface isn't thread safe.

=head1 AUTHOR

Tony Cook <tony@develop-help.com>

Stack concept by Arnar Mar Hrafnkelsson <addi@umich.edu>

=cut
*/