Node:Handling Errors, Previous:Dynamic Wind, Up:Control Mechanisms

26.10 How to Handle Errors in C Code

Error handling is based on catch and throw. Errors are always thrown with a key and four arguments:

• key: a symbol which indicates the type of error. The symbols used by libguile are listed below.
• subr: the name of the procedure from which the error is thrown, or #f.
• message: a string (possibly language and system dependent) describing the error. The tokens ~A and ~S can be embedded within the message: they will be replaced with members of the args list when the message is printed. ~A indicates an argument printed using display, while ~S indicates an argument printed using write. message can also be #f, to allow it to be derived from the key by the error handler (may be useful if the key is to be thrown from both C and Scheme).
• args: a list of arguments to be used to expand ~A and ~S tokens in message. Can also be #f if no arguments are required.
• rest: a list of any additional objects required. e.g., when the key is 'system-error, this contains the C errno value. Can also be #f if no additional objects are required.

In addition to catch and throw, the following Scheme facilities are available:

scm-error key subr message args rest

Scheme Procedure

Throw an error, with arguments as described above.

error msg arg ...

Scheme Procedure

Throw an error using the key 'misc-error. The error message is created by displaying msg and writing the args.

display-error stack port subr message args rest	Scheme Procedure
scm_display_error (stack, port, subr, message, args, rest)	C Function

Display an error message to the output port port. stack is the saved stack for the error, subr is the name of the procedure in which the error occurred and message is the actual error message, which may contain formatting instructions. These will format the arguments in the list args accordingly. rest is currently ignored.

The following are the error keys defined by libguile and the situations in which they are used:

• error-signal: thrown after receiving an unhandled fatal signal such as SIGSEGV, SIGBUS, SIGFPE etc. The rest argument in the throw contains the coded signal number (at present this is not the same as the usual Unix signal number).
• system-error: thrown after the operating system indicates an error condition. The rest argument in the throw contains the errno value.
• numerical-overflow: numerical overflow.
• out-of-range: the arguments to a procedure do not fall within the accepted domain.
• wrong-type-arg: an argument to a procedure has the wrong type.
• wrong-number-of-args: a procedure was called with the wrong number of arguments.
• memory-allocation-error: memory allocation error.
• stack-overflow: stack overflow error.
• regex-error: errors generated by the regular expression library.
• misc-error: other errors.

26.10.1 C Support

SCM scm_error (SCM key, char *subr, char *message, SCM args, SCM rest)

Throws an error, after converting the char * arguments to Scheme strings. subr is the Scheme name of the procedure, NULL is converted to #f. Likewise a NULL message is converted to #f.

The following procedures invoke scm_error with various error keys and arguments. The first three call scm_error with the system-error key and automatically supply errno in the "rest" argument: scm_syserror generates messages using strerror, scm_sysmissing is used when facilities are not available. Care should be taken that the errno value is not reset (e.g. due to an interrupt).

• void scm_syserror (char *subr);
• void scm_syserror_msg (char *subr, char *message, SCM args);
• void scm_sysmissing (char *subr);
• void scm_num_overflow (char *subr);
• void scm_out_of_range (char *subr, SCM bad_value);
• void scm_wrong_num_args (SCM proc);
• void scm_wrong_type_arg (char *subr, int pos, SCM bad_value);
• void scm_memory_error (char *subr);
• static void scm_regex_error (char *subr, int code); (only used in rgx.c).

Exception handlers can also be installed from C, using scm_internal_catch, scm_lazy_catch, or scm_stack_catch from libguile/throw.c. These have not yet been documented, however the source contains some useful comments.