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Error Handling
Netpbm Programming Library Errors
As part of Netpbm’s
mission to make writing graphics programs quick and
easy, Netpbm recognizes that no programmer likes to deal
with error
conditions. Therefore, very few Netpbm programming library
functions return
error information. There are no return codes to check. If
for some reason a
function can’t do what was asked of it, it
doesn’t return at all.
Netpbm’s response to encountering an error is called "throwing an error."
The typical way a Netpbm
function throws an error (for example, when you
attempt to open a non-existent file with pm_openr()) is that
the function
writes an error message to the Standard Error file and then
causes the
program to terminate with an exit() system call. The
function doesn’t do any
explicit cleanup, because everything a library function sets
up gets cleaned
up by normal process termination.
In many cases, that simply
isn’t acceptable. If you’re calling Netpbm
functions from inside a server program, you’d want the
program to recognize
that the immediate task failed, but keep running to do other
work.
So as an alternative, you can
replace that program exit with a longjmp
instead. A longjmp is a classic Unix exception handling
concept. See the
documentation of the standard C library setjmp() and
longjmp() functions.
In short, you identify a point
in your programs for execution to hyperjump
to from whatever depths of whatever functions it may be in
at the time it
detects an exception. That hyperjump is called a longjmp.
The longjmp
unwinds the stack and puts the program in the same state as
if the
subroutines had returned all the way up to the function that
contains the
jump point. A longjmp does not in itself undo things like
memory
allocations. But when you have a Netpbm function do a
longjmp, it also
cleans up everything it started.
To select this form of throwing
an error, use the pm_setjmpbuf() function.
This alternative is not available before Netpbm 10.27 (March
2005).
Issuing of the error message is
a separate thing. Regardless of whether a
library routine exits the program or executes a longjmp, it
issues an error
message first.
You can customize the error
message behavior too. By default, a Netpbm
function issues an error message by writing it to the
Standard Error file,
formatted into a single line with the program name prefixed.
But you can
register your own error message function to run instead with
pm_setErrorMsgFn().
pm_setjmpbuf()
pm_setjmpbuf() sets up the
process so that when future calls to the Netpbm
programming library throw an error, they execute a longjmp
instead of
causing the process to exit as they would by default.
This is not analogous to
setjmp(). You do a setjmp() first, then tell the
Netpbm programming library with pm_setjmpbuf() to use the
result.
Example:
#include <setjmp.h>
#include <pam.h>
jmp_buf jmpbuf;
int rc;
rc = setjmp(jmpbuf);
if (rc == 0) {
struct pam pam;
pm_setjmpbuf(&jmpbuf);
pnm_readpam(stdin, &pam, PAM_STRUCT_SIZE(tuple_type));
printf("pnm_readpam() succeeded!0);
} else {
printf("pnm_readpam() failed. You should have seen
"
"messages to Standard Error telling you why.0);
}
This example should look really
strange to you if you haven’t read the
documentation of setjmp(). Remember that there is a
hyperjump such that the
program is executing the pnm_readpam() and then suddenly is
returning a
second time from the setjmp()!
Even pm_error() works this way
-- if you set up a longjmp with
pm_setjmpbuf() and then call pm_error(), pm_error() will,
after issuing your
error message, execute the longjmp.
pm_setjmpbuf() was new in Netpbm
10.27 (March 2005). Before that, Netpbm
programming library functions always throw an error by
exiting the program.
User Detected Errors
The Netpbm programming library
provides a function for you to throw an error
explicitly: pm_error(). pm_error() does nothing but throw an
error, and does
so the same way any Netpbm library function you call would.
pm_error() is
more convenient than most standard C facilities for handling
errors.
If you don’t want to throw
an error, but just want to issue an error
message, use pm_errormsg(). It issues the message in the
same way as
pm_error() but returns normally instead of longjmping or
exiting the
program.
Note that libnetpbm
distinguishes between an error message and an
informational message (use pm_errormsg() for the former;
pm_message() for
the latter). The only practical difference is which user
message function it
calls. So if you don’t register any user message
function, you won’t see any
difference, but a program is still more maintainable and
easier to read when
you use the appropriate one of these.
pm_error()
Overview
void pm_error( char * fmt, ... );
Example
if (argc-1 < 3)
pm_error("You must specify at least 3 arguments. "
"You specified" only %d", argc-1);
pm_error() is a printf() style
routine that simply throws an error. It
issues an error message exactly like pm_errormsg() would in
the process.
pm_errormsg()
Overview
void pm_errormsg( char * fmt, ... );
Example
if (rc = -1)
pm_errormsg("Could not open file. errno=%d",
errno);
return -1;
pm_errormsg() is a printf()
style routine that issues an error message. By
default, it writes the message to Standard Error, but you
can register a
user error message routine to be called instead, and that
might do something
such as write the message into a log file. See
pm_setusererrormsgfn.
There is very little advantage
to using this over traditional C services,
but it issues a message in the same way as libnetpbm library
functions do,
so the common handling might be valuable.
Note that the arguments specify
the message text, not any formatting of it.
Formatting is handled by pm_errormsg(). So don’t put
any newlines or tabs in
it.
pm_setusererrormsgfn()
Overview
void pm_setusererrormsgfn(pm_usererrormsgfn * function);
Example
static pm_usererrormsgfn logfilewrite;
static void
logfilewrite(const char * const msg) {
fprintf(myerrorlog, "Netpbm error: %s", msg);
}
pm_setusererrormsgfn(&logfilewrite);
pm_errormsg("Message for the error log");
pm_setusererrormsg() registers a
handler for error messages, called a user
error message routine. Any library function that wants to
issue an error
message in the future will call that function with the
message as an
argument.
The argument the user error
message routine gets is English text designed
for human reading. It is just the text of the message; there
is no attempt
at formatting in it (so you won’t see any newline or
tab characters).
You can remove the user error
message routine, so that the library issues
future error messages in its default way (write to Standard
Error) by
specifying a null pointer for function.
The user error message routine
does not handle informational messages. It
handles only error messages. See pm_setusermessagefn().
Error Handling In Netpbm Programs
Most Netpbm programs respond to
encountering an error by issuing a message
describing the error to the Standard Error file and then
exiting with exit
status 1.
Netpbm programs generally do not
follow the Unix convention of very terse
error messages. Conventional Unix programs produce error
messages as if they
had to pay by the word. Netpbm programs tend to give a
complete description
of the problem in human-parseable English. These messages
are often many
terminal lines long.
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