Man Page ctrace.1
NAME
ctrace - C program debugger
SYNOPSIS
ctrace [options] [file]
DESCRIPTION
The ctrace command allows the user to monitor the sequential
execution of a C program as each program statement executes.
The effect is similar to executing a shell procedure with
the -x option. ctrace reads the C program in file (or from
standard input if the user does not specify file), inserts
statements to print the text of each executable statement
and the values of all variables referenced or modified, and
writes the modified program to the standard output. The
output of ctrace must be placed into a temporary file
because the cc(1) commands do not allow the use of a pipe.
This file can then be compiled and executed.
As each statement in the program executes, it will be listed
at the terminal, followed by the name and value of any vari-
ables referenced or modified in the statement; these vari-
able names and values will be followed by any output from
the statement. Loops in the trace output are detected and
tracing is stopped until the loop is exited or a different
sequence of statements within the loop is executed. A warn-
ing message is printed after each 1000 loop cycles to help
the user detect infinite loops. The trace output goes to
the standard output so the user can put it into a file for
examination with an editor or the tail(1) command.
The options commonly used are:
-f functions Trace only these functions.
-v functions Trace all but these functions.
The user may want to add to the default formats for printing
variables. Long and pointer variables are always printed as
signed integers. Pointers to character arrays are also
printed as strings if appropriate. char, short, and int
variables are also printed as signed integers and, if
appropriate, as characters. double variables are printed as
floating point numbers in scientific notation. The user can
request that variables be printed in additional formats, if
appropriate, with these options:
-o Octal
-x Hexadecimal
-u Unsigned
-e Floating point
These options are used only in special circumstances:
-l n Check n consecutively executed statements for looping
trace output, instead of the default of 20. Use 0 to
get all the trace output from loops.
-s Suppress redundant trace output from simple assign-
ment statements and string copy function calls. This
option can hide a bug caused by use of the = operator
in place of the == operator.
-t n Trace n variables per statement instead of the
default of 10 (the maximum number is 20). The diag-
nostics section explains when to use this option.
-P Preprocess the input before tracing it. The user can
also use the -D, -I, and -U cc(1) options.
-p string
Change the trace print function from the default of
printf. For example, fprintf(stderr, would send the
trace to the standard error output.
-r f Use file f in place of the runtime.c trace function
package. This replacement lets the user change the
entire print function, instead of just the name and
leading arguments (see the -p option).
-V Prints version information on the standard error.
-Qarg If arg is y, identification information about ctrace
will be added to the output files. This can be use-
ful for software administration. Giving n for arg
exlicitly asks for no such information, which is the
default behavior.
EXAMPLE
If the file lc.c contains this C program:
1 #include <stdio.h>
2 main() /* count lines in input */
3 {
4 int c, nl;
5
6 nl = 0;
7 while ((c = getchar()) != EOF)
8 if (c = '\n')
9 ++nl;
10 printf("%d\n", nl);
11 }
these commands and test data are entered:
cc lc.c
a.out
1
(cntl-d)
the program will be compiled and executed. The output of
the program will be the number 2, which is incorrect because
there is only one line in the test data. The error in this
program is common, but subtle. If the user invokes ctrace
with these commands:
ctrace lc.c >temp.c
cc temp.c
a.out
the output will be:
2 main()
6 nl = 0;
/* nl == 0 */
7 while ((c = getchar()) != EOF)
The program is now waiting for input. If the user enters
the same test data as before, the output will be:
/* c == 49 or '1' */
8 if (c = '\n')
/* c == 10 or '\n' */
9 ++nl;
/* nl == 1 */
7 while ((c = getchar()) != EOF)
/* c == 10 or '\n' */
8 if (c = '\n')
/* c == 10 or '\n' */
9 ++nl;
/* nl == 2 */
7 while ((c = getchar()) != EOF)
If an end-of-file character (cntl-d) is entered, the final
output will be:
/* c == -1 */
10 printf("%d\n", nl);
/* nl == 2 */2
return
Note the information printed out at the end of the trace
line for the nl variable following line 10. Also note the
return comment added by ctrace at the end of the trace out-
put. This shows the implicit return at the terminating
brace in the function.
The trace output shows that variable c is assigned the value
'1' in line 7, but in line 8 it has the value '\n'. Once
user attention is drawn to this if statement, he or she will
probably realize that the assignment operator (=) was used
in place of the equality operator (==). This error can
easily be missed during code reading.
EXECUTION-TIME TRACE CONTROL
The default operation for ctrace is to trace the entire pro-
gram file, unless the -f or -v options are used to trace
specific functions. The default operation does not give the
user statement-by-statement control of the tracing, nor does
it let the user turn the tracing off and on when executing
the traced program.
The user can do both of these by adding ctroff() and ctron()
function calls to the program to turn the tracing off and
on, respectively, at execution time. Thus, complex criteria
can be arbitrarily coded for trace control with if state-
ments, and this code can even be conditionally included
because ctrace defines the CTRACE preprocessor variable.
For example:
#ifdef CTRACE
if (c == '!' && i > 1000)
ctron();
#endif
These functions can also be called from dbx(1) if they are
compiled with the -g option. For example, to trace all but
lines 7 to 10 in the main function, enter:
dbx a.out
when at 7 {call ctroff();}
when at 7 {call ctron();}
run
The trace can be turned off and on by setting static vari-
able tr_ct_ to 0 and 1, respectively. This on/off option is
useful if a user is using a debugger that can not call these
functions directly.
FILES
runtime.c run-time trace package
SEE ALSO
bfs(1), dbx(1), tail(1), ctype(3C), fclose(3S), printf(3S),
string(3C)
DIAGNOSTICS
This section contains diagnostic messages from both ctrace
and cc(1), since the traced code often gets some cc warning
messages. The user can get cc error messages in some rare
cases, all of which can be avoided.
ctrace Diagnostics
warning: some variables are not traced in this statement
Only 10 variables are traced in a statement to prevent
the C compiler "out of tree space; simplify expression"
error. Use the -t option to increase this number.
warning: statement too long to trace
This statement is over 400 characters long. Make sure
that tabs are used to indent the code, not spaces.
cannot handle preprocessor code, use -P option
This is usually caused by #ifdef/#endif preprocessor
statements in the middle of a C statement, or by a
semicolon at the end of a #define preprocessor state-
ment.
Split the sequence by removing an else from the middle.
possible syntax error, try -P option
Use the -P option to preprocess the ctrace input, along
with any appropriate -D, -I, and -U preprocessor
options.
NOTES
Defining a function with the same name as a system function
may cause a syntax error if the number of arguments is
changed. Just use a different name.
ctrace assumes that BADMAG is a preprocessor macro, and that
EOF and NULL are #defined constants. Declaring any of these
to be variables, e.g., "int EOF;", will cause a syntax
error.
Pointer values are always treated as pointers to character
strings.
ctrace does not know about the components of aggregates like
structures, unions, and arrays. It cannot choose a format
to print all the components of an aggregate when an assign-
ment is made to the entire aggregate. ctrace may choose to
print the address of an aggregate or use the wrong format
(e.g., 3.149050e-311 for a structure with two integer
members) when printing the value of an aggregate.
The loop trace output elimination is done separately for
each file of a multi-file program. Separate output elimina-
tion can result in functions called from a loop still being
traced, or the elimination of trace output from one function
in a file until another in the same file is called.