ctrace(1)ctrace(1)NAMEctrace - C program debugger
SYNOPSISctrace [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 simi‐
lar 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 variables referenced or
modified in the statement; these variable names and values will be fol‐
lowed 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 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) com‐
mand.
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 appropri‐
ate. char, short, and int variables are also printed as signed inte‐
gers 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 out‐
put, instead of the default of 20. Use 0 to get all the trace
output from loops.
-s Suppress redundant trace output from simple assignment state‐
ments 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 diagnostics section explains
when to use this option.
-P Preprocess the input before tracing it. The user can also use
the -D, -I, and -Ucc(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 useful 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 output. 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 program 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 statements, and this code can even be condition‐
ally 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 variable 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 ALSObfs(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 statement.
'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.
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 differ‐
ent 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 struc‐
tures, 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.
The loop trace output elimination is done separately for each file of a
multi-file program. Separate output elimination can result in func‐
tions 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.
March 1998 0a
