ctrace -- C program debugger


ctrace [ options ] [ file ]


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. ctrace 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 standard output. You must put the output of ctrace into a temporary file because cc(CP) 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 variables 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(C) or more(C) 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.

String arguments to the string(S) functions and return values from fgets(S), gets(S), and sprintf(S) are printed as strings. You can request that variables be printed in additional formats, if appropriate, with these options:




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.

Suppress redundant trace output from simple assignment 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 ``Diagnostics'' section explains when to use this option.

Run the C preprocessor on the input before tracing it. You can also use the -D, -I, and -U options of cc(CP).

Use only basic functions in the trace code, that is, those in ctype(S), printf(S), and string(S). These are usually available even in cross-compilers for microprocessors. In particular, this option is needed when the traced program runs under an operating system that does not have signal(S), fflush(S), longjmp(S), or setjmp(S).

-p string
Change the trace print function from the default of printf(. For example, if string is fprintf(stderr,, output from ctrace is sent to standard error output.

-r f
Use file f in place of the runtime.c trace function package. This lets you change the entire print function, instead of just the name and leading arguments (see the -p option).

Prints version information on the standard error.

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 explicitly asks for no such information, which is the default behavior.

Execution-Time trace control

The default operation for ctrace is to trace the entire program 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)
You can also call these functions from sdb(CP) 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()

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.

Exit values

Upon successful completion, the exit status of ctrace is 0, otherwise the exit status is non-zero.


This section contains diagnostic messages from ctrace. The traced code often gets some cc warning messages. Refer to the cc(CP) man page for more information. You can also get cc error messages in some rare cases, all of which can be avoided.

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 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. If you still get the error message, check the ``Warning'' section below.


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 (for example, 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 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.


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 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, for example, "int EOF;", will cause a syntax error.


If the file lc.c contains this C program:
    1 #include <stdio.h>
    2 main()        /* count lines in input */
    3 {
    4         int c, nl;
    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

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

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' or "text" */
    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         /* repeating */

If you now enter an end-of-file character (<Ctrl>-D) the final output will be:

             /* repeated < 1 time */
    7            while ((c = getchar()) != EOF)
                 /* 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 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 your attention is drawn to this if statement, you will probably realize that you used the assignment operator (``='') in place of the equality operator (``==''). You can easily miss this error during code reading.


Run-time trace package.

See also

bfs(C), ctype(S), fclose(S), more(C), printf(S), setjmp(S), signal(S), string(S)

Standards conformance

ctrace(CP) is not part of any currently supported standard; it was developed by UNIX System Laboratories, Inc. and is used by permission.
© 2005 Commands for Programming (CP)
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