Variables used by Tcl

The following global variables are created and managed automatically by the Tcl library. Except where noted below, these variables should normally be treated as read-only by application-specific code and by users.

env

This variable is maintained by Tcl as an array whose elements are the environment variables for the process. Reading an element will return the value of the corresponding environment variable. Setting an element of the array will modify the corresponding environment variable or create a new one if it doesn't already exist. Unsetting an element of env will remove the corresponding environment variable. Changes to the env array will affect the environment passed to children by commands like exec. If the entire env array is unset then Tcl will stop monitoring env accesses and will not update environment variables.

errorCode

After an error has occurred, this variable will be set to hold additional information about the error in a form that is easy to process with programs. errorCode consists of a Tcl list with one or more elements. The first element of the list identifies a general class of errors, and determines the format of the rest of the list. The following formats for errorCode are used by the Tcl core; individual applications may define additional formats.

ARITH code msg

This format is used when an arithmetic error occurs (e.g. an attempt to divide by zero in the expr command). Code identifies the precise error and msg provides a human-readable description of the error. Code will be either DIVZERO (for an attempt to divide by zero), DOMAIN (if an argument is outside the domain of a function, such as acos(-3)), IOVERFLOW (for integer overflow), OVERLFLOW (for a floating-point overflow), or UNKNOWN (if the cause of the error cannot be determined).

CHILDKILLED pid sigName msg

This format is used when a child process has been killed because of a signal. The second element of errorCode will be the process's identifier (in decimal). The third element will be the symbolic name of the signal that caused the process to terminate; it will be one of the names from the include file signal.h, such as SIGPIPE. The fourth element will be a short human-readable message describing the signal, such as ``write on pipe with no readers'' for SIGPIPE.

CHILDSTATUS pid code

This format is used when a child process has exited with a non-zero exit status. The second element of errorCode will be the process's identifier (in decimal) and the third element will be the exit code returned by the process (also in decimal).

CHILDSUSP pid sigName msg

This format is used when a child process has been suspended because of a signal. The second element of errorCode will be the process's identifier, in decimal. The third element will be the symbolic name of the signal that caused the process to suspend; this will be one of the names from the include file signal.h, such as SIGTTIN. The fourth element will be a short human-readable message describing the signal, such as ``background tty read'' for SIGTTIN.

NONE

This format is used for errors where no additional information is available for an error besides the message returned with the error. In these cases errorCode will consist of a list containing a single element whose contents are NONE.

POSIX errName msg

If the first element of errorCode is POSIX, then the error occurred during a POSIX kernel call. The second element of the list will contain the symbolic name of the error that occurred, such as ENOENT; this will be one of the values defined in the include file errno.h. The third element of the list will be a human-readable message corresponding to errName, such as ``no such file or directory'' for the ENOENT case.

To set errorCode, applications should use library procedures such as Tcl_SetErrorCode and Tcl_PosixError or they may invoke the error command. If one of these methods hasn't been used, then the Tcl interpreter will reset the variable to NONE after the next error.

errorInfo

After an error has occurred, this string will contain one or more lines identifying the Tcl commands and procedures that were being executed when the most recent error occurred. Its contents take the form of a stack trace showing the various nested Tcl commands that had been invoked at the time of the error.

tcl_precision

If this variable is set, it must contain a decimal number giving the number of significant digits to include when converting floating-point values to strings. If this variable is not set then 6 digits are included. 17 digits is used for IEEE floating-point because it allows double-precision values to be converted to strings and back to binary with no loss of precision.