Tcl_Panic, Tcl_PanicVA, Tcl_SetPanicProc − report fatal error and abort


#include <tcl.h>

(format, arg, arg, ...)

(format, argList)



const char* format (in)

A printf-style format string.

arg (in)

Arguments matching the format string.

va_list argList (in)

An argument list of arguments matching the format string. Must have been initialized using va_start, and cleared using va_end.

Tcl_PanicProc *panicProc (in)

Procedure to report fatal error message and abort.



When the Tcl library detects that its internal data structures are in an inconsistent state, or that its C procedures have been called in a manner inconsistent with their documentation, it calls Tcl_Panic to display a message describing the error and abort the process. The format argument is a format string describing how to format the remaining arguments arg into an error message, according to the same formatting rules used by the printf family of functions. The same formatting rules are also used by the built-in Tcl command format.

In a freshly loaded Tcl library, Tcl_Panic prints the formatted error message to the standard error file of the process, and then calls abort to terminate the process. Tcl_Panic does not return.

Tcl_SetPanicProc may be used to modify the behavior of Tcl_Panic. The panicProc argument should match the type Tcl_PanicProc:

typedef void Tcl_PanicProc(
const char *format,
, arg,...);

After Tcl_SetPanicProc returns, any future calls to Tcl_Panic will call panicProc, passing along the format and arg arguments. To maintain consistency with the callers of Tcl_Panic, panicProc must not return; it must call abort. panicProc should avoid making calls into the Tcl library, or into other libraries that may call the Tcl library, since the original call to Tcl_Panic indicates the Tcl library is not in a state of reliable operation.

The typical use of Tcl_SetPanicProc arranges for the error message to be displayed or reported in a manner more suitable for the application or the platform. As an example, the Windows implementation of wish calls Tcl_SetPanicProc to force all panic messages to be displayed in a system dialog box, rather than to be printed to the standard error file (usually not visible under Windows).

Although the primary callers of Tcl_Panic are the procedures of the Tcl library, Tcl_Panic is a public function and may be called by any extension or application that wishes to abort the process and have a panic message displayed the same way that panic messages from Tcl will be displayed.

Tcl_PanicVA is the same as Tcl_Panic except that instead of taking a variable number of arguments it takes an argument list.


abort(3), printf(3), exec(n), format(n)


abort, fatal, error


Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.

Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.

Free Software

Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.

Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.

Free Books

The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.

Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.


Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.

Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.