CTRACE(1) INTERACTIVE UNIX System CTRACE(1)
NAME
ctrace - C program debugger
SYNOPSIS
ctrace [options] [file]
DESCRIPTION
The ctrace command allows you to follow the execution of a C
program, statement-by-statement. The effect is similar to
executing a shell procedure with the -x option. The ctrace
command reads the C program in file (or from standard input
if you do 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. You must put the output of
ctrace into a temporary file because the cc(1) command does
not allow the use of a pipe. You then compile and execute
this file.
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, 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 warning message is printed every 1000 times
through the loop to help you detect infinite loops. The
trace output goes to the standard output so you can put it
into a file for examination with an editor or the bfs(1) or
tail(1) commands.
The options commonly used are:
-f functions Trace only these functions.
-v functions Trace all but these functions.
You 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. You 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:
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CTRACE(1) INTERACTIVE UNIX System CTRACE(1)
-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 Run the C preprocessor on the input before tracing
it. You can also use the -D, -I, and -U cpp(1)
options.
These options are used to tailor the run-time trace package
when the traced program will run in a non-UNIX System
environment:
-b Use only basic functions in the trace code, that is,
those in ctype(3C), printf(3S), and string(3C).
These are usually available even in cross-compilers
for microprocessors. In particular, this option is
needed when the traced program runs under an operat-
ing system that does not have signal(2), fflush(3S),
longjmp(3C), or setjmp(3C).
-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 lets you change the entire print func-
tion, instead of just the name and leading arguments
(see the -p option).
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 }
and you enter these commands and test data:
cc lc.c
a.out
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CTRACE(1) INTERACTIVE UNIX System CTRACE(1)
1
(cntl-d)
the program will be compiled and executed. The output of
the program will be the number 2, which is not correct
because there is only one line in the test data. The error
in this program is common, but subtle. If you invoke 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 you enter 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 you now enter an end-of-file character (cntl-d) the final
output will be:
/* c == -1 */
10 printf("%d\n", nl);
/* nl == 2 */2
return
Note that the program output printed at the end of the trace
line for the nl variable. Also note the return comment
added by ctrace at the end of the trace output. This shows
the implicit return at the terminating brace in the func-
tion.
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
your attention is drawn to this if statement, you will
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CTRACE(1) INTERACTIVE UNIX System CTRACE(1)
probably realize that you used the assignment operator (=)
in place of the equality operator (==). You can easily miss
this error during code reading.
EXECUTION-TIME TRACE CONTROL
The default operation for ctrace is to trace the entire pro-
gram file, unless you use the -f or -v options to trace
specific functions. This does not give you statement-by-
statement control of the tracing, nor does it let you turn
the tracing off and on when executing the traced program.
You can do both of these by adding ctroff() and ctron()
function calls to your program to turn the tracing off and
on, respectively, at execution time. Thus, you can code
arbitrarily complex criteria for trace control with if
statements, and you can even conditionally include this code
because ctrace defines the CTRACE preprocessor variable.
For example:
#ifdef CTRACE
if (c == '!' && i > 1000)
ctron();
#endif
You can also call these functions from sdb(1) if you compile
with the -g option. For example, to trace all but lines 7
to 10 in the main function, enter:
sdb a.out
main:7b ctroff()
main:11b ctron()
r
You can also turn the trace off and on by setting static
variable tr_ct_ to 0 and 1, respectively. This is useful if
you are using a debugger that cannot call these functions
directly.
FILES
/usr/lib/ctrace/runtime.c run-time trace package
SEE ALSO
signal(2), ctype(3C), fclose(3S), printf(3S), setjmp(3C),
string(3C).
bfs(1), tail(1) in the INTERACTIVE UNIX System User's/System
Administrator's Reference Manual.
DIAGNOSTICS
This section contains diagnostic messages from both ctrace
and cc(1), since the traced code often gets some cc warning
messages. You can get cc error messages in some rare cases,
all of which can be avoided.
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CTRACE(1) INTERACTIVE UNIX System CTRACE(1)
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 you are using tabs to indent your 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.
'if ... else if' sequence too long
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. If you still get the error message, check the
WARNINGS section below.
Cc Diagnostics
warning: illegal combination of pointer and integer
warning: statement not reached
warning: sizeof returns 0
Ignore these messages.
compiler takes size of function
See the ctrace "possible syntax error" message above.
yacc stack overflow
See the ctrace "'if ... else if' sequence too long"
message above.
out of tree space; simplify expression
Use the -t option to reduce the number of traced vari-
ables per statement from the default of 10. Ignore the
"ctrace: too many variables to trace" warnings you will
now get.
redeclaration of signal
Either correct this declaration of signal(2), or remove
it and #include <signal.h>.
WARNINGS
You will get a ctrace syntax error if you omit the semicolon
at the end of the last element declaration in a structure or
union, just before the right brace (}). This is optional in
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CTRACE(1) INTERACTIVE UNIX System CTRACE(1)
some C compilers.
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.
The ctrace command 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.
BUGS
The ctrace command 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 assignment 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.
Pointer values are always treated as pointers to character
strings.
The loop trace output elimination is done separately for
each file of a multifile program. This can result in func-
tions called from a loop still being traced, or the elimina-
tion of trace output from one function in a file until
another in the same file is called.
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