A Tutorial Introduction to ADB

J. F. Maranzano, S. R. Bourne

May 5, 1977

1. Introduction

ADB is a new debugging program that is available on UNIX. It provides capabilities to look at “core” files resulting from aborted programs, print output in a variety of formats, patch files, and run programs with embedded breakpoints. This document provides examples of the more useful features of ADB. The reader is expected to be familiar with the basic commands on UNIX with the C language, and with References 1, 2 and 3.

2. A Quick Survey

2.1. Invocation

ADB is invoked as:

adb objfile corefile

where objfile is an executable UNIX file and corefile is a core image file. Many times this will look like:

adb a.out core

or more simply:

adb

where the defaults are a.out and core respectively. The filename minus (-) means ignore this argument as in:

adb - core

ADB has requests for examining locations in either file. The ? request examines the contents of objfile, the / request examines the corefile. The general form of these requests is:

address ? format

or

address / format

2.2. Current Address

ADB maintains a current address, called dot, similar in function to the current pointer in the UNIX editor. When an address is entered, the current address is set to that location, so that:

0126?i

sets dot to octal 126 and prints the instruction at that address. The request:

.,10/d

prints 10 decimal numbers starting at dot. Dot ends up referring to the address of the last item printed. When used with the ? or / requests, the current address can be advanced by typing newline; it can be decremented by typing ^.

Addresses are represented by expressions. Expressions are made up from decimal, octal, and hexadecimal integers, and symbols from the program under test. These may be combined with the operators +, -, *, % (integer division), & (bitwise and), | (bitwise inclusive or), # (round up to the next multiple), and ~ (not). (All arithmetic within ADB is 32 bits.) When typing a symbolic address for a C program, the user can type name or _name; ADB will recognize both forms.

2.3. Formats

To print data, a user specifies a collection of letters and characters that describe the format of the printout. Formats are “remembered” in the sense that typing a request without one will cause the new printout to appear in the previous format. The following are the most commonly used format letters.

b one byte in octal
c one byte as a character
o one word in octal
d one word in decimal
f two words in floating point
i PDP 11 instruction
s a null terminated character string
a the value of dot
u one word as unsigned integer
n print a newline
r print a blank space
^ backup dot

(Format letters are also available for “long” values, for example, `D' for long decimal, and `F' for double floating point.) For other formats see the ADB manual.

2.4. General Request Meanings

The general form of a request is:

address,count command modifier

which sets `dot' to address and executes the command count times.

The following table illustrates some general ADB command meanings:

Command Meaning
? Print contents from a.out file
/ Print contents from core file
= Print value of “dot”
: Breakpoint control
$ Miscellaneous requests
; Request separator
! Escape to shell

ADB catches signals, so a user cannot use a quit signal to exit from ADB. The request $q or $Q (or cntl-D) must be used to exit from ADB.

3. Debugging C Programs

3.1. Debugging A Core Image

Consider the C program in Figure 1. The program is used to illustrate a common error made by C programmers. The object of the program is to change the lower case “t” to upper case in the string pointed to by charp and then write the character string to the file indicated by argument 1. The bug shown is that the character “T” is stored in the pointer charp instead of the string pointed to by charp. Executing the program produces a core file because of an out of bounds memory reference.

ADB is invoked by:

adb a.out core

The first debugging request:

$c

is used to give a C backtrace through the subroutines called. As shown in Figure 2 only one function (main) was called and the arguments argc and argv have octal values 02 and 0177762 respectively. Both of these values look reasonable; 02 = two arguments, 0177762 = address on stack of parameter vector. The next request:

$C

is used to give a C backtrace plus an interpretation of all the local variables in each function and their values in octal. The value of the variable cc looks incorrect since cc was declared as a character.

The next request:

$r

prints out the registers including the program counter and an interpretation of the instruction at that location.

The request:

$e

prints out the values of all external variables.

A map exists for each file handled by ADB. The map for the a.out file is referenced by ? whereas the map for core file is referenced by /. Furthermore, a good rule of thumb is to use ? for instructions and / for data when looking at programs. To print out information about the maps type:

$m

This produces a report of the contents of the maps. More about these maps later.

In our example, it is useful to see the contents of the string pointed to by charp. This is done by:

*charp/s

which says use charp as a pointer in the core file and print the information as a character string. This printout clearly shows that the character buffer was incorrectly overwritten and helps identify the error. Printing the locations around charp shows that the buffer is unchanged but that the pointer is destroyed. Using ADB similarly, we could print information about the arguments to a function. The request:

main.argc/d

prints the decimal core image value of the argument argc in the function main. The request:

*main.argv,3/o

prints the octal values of the three consecutive cells pointed to by argv in the function main. Note that these values are the addresses of the arguments to main. Therefore:

0177770/s

prints the ASCII value of the first argument. Another way to print this value would have been

*”/s

The ” means ditto which remembers the last address typed, in this case main.argc ; the * instructs ADB to use the address field of the core file as a pointer.

The request:

.=o

prints the current address (not its contents) in octal which has been set to the address of the first argument. The current address, dot, is used by ADB to “remember” its current location. It allows the user to reference locations relative to the current address, for example:

.-10/d

3.2. Multiple Functions

Consider the C program illustrated in Figure 3. This program calls functions f, g, and h until the stack is exhausted and a core image is produced.

Again you can enter the debugger via:

adb

which assumes the names a.out and core for the executable file and core image file respectively. The request:

$c

will fill a page of backtrace references to f, g, and h. Figure 4 shows an abbreviated list (typing DEL will terminate the output and bring you back to ADB request level).

The request:

,5$C

prints the five most recent activations.

Notice that each function (f,g,h) has a counter of the number of times it was called.

The request:

fcnt/d

prints the decimal value of the counter for the function f. Similarly gcnt and hcnt could be printed. To print the value of an automatic variable, for example the decimal value of x in the last call of the function h, type:

h.x/d

It is currently not possible in the exported version to print stack frames other than the most recent activation of a function. Therefore, a user can print everything with $C or the occurrence of a variable in the most recent call of a function. It is possible with the $C request, however, to print the stack frame starting at some address as address$C.

3.3. Setting Breakpoints

Consider the C program in Figure 5. This program, which changes tabs into blanks, is adapted from Software Tools by Kernighan and Plauger, pp. 18-27.

We will run this program under the control of ADB (see Figure 6a) by:

adb a.out -

Breakpoints are set in the program as:

address:b [request]

The requests:

settab+4:b
fopen+4:b
getc+4:b
tabpos+4:b

set breakpoints at the start of these functions. C does not generate statement labels. Therefore it is currently not possible to plant breakpoints at locations other than function entry points without a knowledge of the code generated by the C compiler. The above addresses are entered as symbol+4 so that they will appear in any C backtrace since the first instruction of each function is a call to the C save routine (csv). Note that some of the functions are from the C library.

To print the location of breakpoints one types:

$b

The display indicates a count field. A breakpoint is bypassed count -1 times before causing a stop. The command field indicates the ADB requests to be executed each time the breakpoint is encountered. In our example no command fields are present.

By displaying the original instructions at the function settab we see that the breakpoint is set after the jsr to the C save routine. We can display the instructions using the ADB request:

settab,5?ia

This request displays five instructions starting at settab with the addresses of each location displayed. Another variation is:

settab,5?i

which displays the instructions with only the starting address.

Notice that we accessed the addresses from the a.out file with the ? command. In general when asking for a printout of multiple items, ADB will advance the current address the number of bytes necessary to satisfy the request; in the above example five instructions were displayed and the current address was advanced 18 (decimal) bytes.

To run the program one simply types:

:r

To delete a breakpoint, for instance the entry to the function settab, one types:

settab+4:d

To continue execution of the program from the breakpoint type:

:c

Once the program has stopped (in this case at the breakpoint for fopen), ADB requests can be used to display the contents of memory. For example:

$C

to display a stack trace, or:

tabs,3/8o

to print three lines of 8 locations each from the array called tabs. By this time (at location fopen) in the C program, settab has been called and should have set a one in every eighth location of tabs.

3.4. Advanced Breakpoint Usage

We continue execution of the program with:

:c

See Figure 6b. Getc is called three times and the contents of the variable c in the function main are displayed each time. The single character on the left hand edge is the output from the C program. On the third occurrence of getc the program stops. We can look at the full buffer of characters by typing:

ibuf+6/20c

When we continue the program with:

:c

we hit our first breakpoint at tabpos since there is a tab following the “This” word of the data.

Several breakpoints of tabpos will occur until the program has changed the tab into equivalent blanks. Since we feel that tabpos is working, we can remove the breakpoint at that location by:

tabpos+4:d

If the program is continued with:

:c

it resumes normal execution after ADB prints the message

a.out:running

The UNIX quit and interrupt signals act on ADB itself rather than on the program being debugged. If such a signal occurs then the program being debugged is stopped and control is returned to ADB. The signal is saved by ADB and is passed on to the test program if:

:c

is typed. This can be useful when testing interrupt handling routines. The signal is not passed on to the test program if:

:c 0

is typed.

Now let us reset the breakpoint at settab and display the instructions located there when we reach the breakpoint. This is accomplished by:

settab+4:b settab,5?ia

It is also possible to execute the ADB requests for each occurrence of the breakpoint but only stop after the third occurrence by typing:

getc+4,3:b main.c?C

This request will print the local variable c in the function main at each occurrence of the breakpoint. The semicolon is used to separate multiple ADB requests on a single line.

Warning: setting a breakpoint causes the value of dot to be changed; executing the program under ADB does not change dot. Therefore:

settab+4:b .,5?ia
fopen+4:b

will print the last thing dot was set to (in the example fopen+4) not the current location (settab+4) at which the program is executing.

A breakpoint can be overwritten without first deleting the old breakpoint. For example:

settab+4:b settab,5?ia; ptab/o

could be entered after typing the above requests.

Now the display of breakpoints:

$b

shows the above request for the settab breakpoint. When the breakpoint at settab is encountered the ADB requests are executed. Note that the location at settab+4 has been changed to plant the breakpoint; all the other locations match their original value.

Using the functions, f, g and h shown in Figure 3, we can follow the execution of each function by planting non-stopping breakpoints. We call ADB with the executable program of Figure 3 as follows:

adb ex3 -

Suppose we enter the following breakpoints:

h+4:b hcnt/d; h.hi/; h.hr/
g+4:b gcnt/d; g.gi/; g.gr/
f+4:b fcnt/d; f.fi/; f.fr/
:r

Each request line indicates that the variables are printed in decimal (by the specification d). Since the format is not changed, the d can be left off all but the first request.

The output in Figure 7 illustrates two points. First, the ADB requests in the breakpoint line are not examined until the program under test is run. That means any errors in those ADB requests is not detected until run time. At the location of the error ADB stops running the program.

The second point is the way ADB handles register variables. ADB uses the symbol table to address variables. Register variables, like f.fr above, have pointers to uninitialized places on the stack. Therefore the message “symbol not found”.

Another way of getting at the data in this example is to print the variables used in the call as:

f+4:b fcnt/d; f.a/; f.b/; f.fi/
g+4:b gcnt/d; g.p/; g.q/; g.gi/
:c

The operator / was used instead of ? to read values from the core file. The output for each function, as shown in Figure 7, has the same format. For the function f, for example, it shows the name and value of the external variable fcnt. It also shows the address on the stack and value of the variables a, b and fi.

Notice that the addresses on the stack will continue to decrease until no address space is left for program execution at which time (after many pages of output) the program under test aborts. A display with names would be produced by requests like the following:

f+4:b fcnt/d; f.a/“a=“d; f.b/“b=“d; f.fi/“fi=“d

In this format the quoted string is printed literally and the d produces a decimal display of the variables. The results are shown in Figure 7.

3.5. Other Breakpoint Facilities

  • Arguments and change of standard input and output are passed to a program as:
:r arg1 arg2 … <infile >outfile

This request kills any existing program under test and starts the a.out afresh.

  • The program being debugged can be single stepped by:
:s

If necessary, this request will start up the program being debugged and stop after executing the first instruction.

  • ADB allows a program to be entered at a specific address by typing:
address:r
  • The count field can be used to skip the first n breakpoints as:
,n:r

The request:

,n:c

may also be used for skipping the first n breakpoints when continuing a program.

  • A program can be continued at an address different from the breakpoint by:
address:c
  • The program being debugged runs as a separate process and can be killed by:
:k

4. Maps

UNIX supports several executable file formats. These are used to tell the loader how to load the program file.

File type 407 is the most common and is generated by a C compiler invocation such as cc pgm.c. A 410 file is produced by a C compiler command of the form cc -n pgm.c, whereas a 411 file is produced by cc -i pgm.c. ADB interprets these different file formats and provides access to the different segments through a set of maps (see Figure 8). To print the maps type:

$m

In 407 files, both text (instructions) and data are intermixed. This makes it impossible for ADB to differentiate data from instructions and some of the printed symbolic addresses look incorrect; for example, printing data addresses as offsets from routines.

In 410 files (shared text), the instructions are separated from data and ?* accesses the data part of the a.out file. The ?* request tells ADB to use the second part of the map in the a.out file. Accessing data in the core file shows the data after it was modified by the execution of the program. Notice also that the data segment may have grown during program execution.

In 411 files (separated I & D space), the instructions and data are also separated. However, in this case, since data is mapped through a separate set of segmentation registers, the base of the data segment is also relative to address zero. In this case since the addresses overlap it is necessary to use the ?* operator to access the data space of the a.out file. In both 410 and 411 files the corresponding core file does not contain the program text.

Figure 9 shows the display of three maps for the same program linked as a 407, 410, 411 respectively. The b, e, and f fields are used by ADB to map addresses into file addresses. The “f1” field is the length of the header at the beginning of the file (020 bytes for an a.out file and 02000 bytes for a core file). The “f2” field is the displacement from the beginning of the file to the data. For a 407 file with mixed text and data this is the same as the length of the header; for 410 and 411 files this is the length of the header plus the size of the text portion.

The “b” and “e” fields are the starting and ending locations for a segment. Given an address, A, the location in the file (either a.out or core) is calculated as:

b1⇐A⇐e1 ⇒ file address = (A-b1)+f1
b2⇐A⇐e2 ⇒ file address = (A-b2)+f2

A user can access locations by using the ADB defined variables. The $v request prints the variables initialized by ADB:

b base address of data segment
d length of the data segment
s length of the stack
t length of the text
m execution type (407,410,411)

In Figure 9 those variables not present are zero. Use can be made of these variables by expressions such as:

<b

in the address field. Similarly the value of the variable can be changed by an assignment request such as:

02000>b

that sets b to octal 2000. These variables are useful to know if the file under examination is an executable or core image file.

ADB reads the header of the core image file to find the values for these variables. If the second file specified does not seem to be a core file, or if it is missing then the header of the executable file is used instead.

5. Advanced Usage

It is possible with ADB to combine formatting requests to provide elaborate displays. Below are several examples.

5.1. Formatted dump

The line:

<b,-1/4o4^8Cn

prints 4 octal words followed by their ASCII interpretation from the data space of the core image file. Broken down, the various request pieces mean:

<b The base address of the data segment.
<b,-1 Print from the base address to the end of file. A negative count is used here and elsewhere to loop indefinitely or until some error condition (like end of file) is detected.

The format 4o4^8Cn is broken down as follows:

4o Print 4 octal locations.
4^ Backup the current address 4 locations (to the original start of the field).
8C Print 8 consecutive characters using an escape convention; each character in the range 0 to 037 is printed as @ followed by the corresponding character in the range 0140 to 0177. An @ is printed as @@.
n Print a newline.

The request:

<b,<d/4o4^8Cn

could have been used instead to allow the printing to stop at the end of the data segment (<d provides the data segment size in bytes).

The formatting requests can be combined with ADB's ability to read in a script to produce a core image dump script. ADB is invoked as:

adb a.out core < dump

to read in a script file, dump, of requests. An example of such a script is:

    120$w
    4095$s
    $v
    =3n
    $m
    =3n"C Stack Backtrace"
    $C
    =3n"C External Variables"
    $e
    =3n"Registers"
    $r
    0$s
    =3n"Data Segment"
    <b,-1/8ona

The request 120$w sets the width of the output to 120 characters (normally, the width is 80 characters). ADB attempts to print addresses as:

symbol + offset

The request 4095$s increases the maximum permissible offset to the nearest symbolic address from 255 (default) to 4095. The request = can be used to print literal strings. Thus, headings are provided in this dump program with requests of the form:

=3n”C Stack Backtrace”

that spaces three lines and prints the literal string. The request $v prints all non-zero ADB variables (see Figure 8). The request 0$s sets the maximum offset for symbol matches to zero thus suppressing the printing of symbolic labels in favor of octal values. Note that this is only done for the printing of the data segment. The request:

<b,-1/8ona

prints a dump from the base of the data segment to the end of file with an octal address field and eight octal numbers per line.

Figure 11 shows the results of some formatting requests on the C program of Figure 10.

5.2. Directory Dump

As another illustration ( Figure 12) consider a set of requests to dump the contents of a directory (which is made up of an integer inumber followed by a 14 character name):

adb dir -
=n8t”Inum”8t”Name”
0,-1? u8t14cn

In this example, the u prints the inumber as an unsigned decimal integer, the 8t means that ADB will space to the next multiple of 8 on the output line, and the 14c prints the 14 character file name.

5.3. Ilist Dump

Similarly the contents of the ilist of a file system, (e.g. /dev/src, on UNIX systems distributed by the UNIX Support Group; see UNIX Programmer's Manual Section V) could be dumped with the following set of requests:

adb /dev/src -
02000>b
?m <b
<b,-1?”flags”8ton”links,uid,gid”8t3bn”,size”8tbrdn”addr”8t8un”times”8t2Y2na

In this example the value of the base for the map was changed to 02000 (by saying ?m<b) since that is the start of an ilist within a file system. An artifice (brd above) was used to print the 24 bit size field as a byte, a space, and a decimal integer. The last access time and last modify time are printed with the 2Y operator. Figure 12 shows portions of these requests as applied to a directory and file system.

5.4. Converting values

ADB may be used to convert values from one representation to another. For example:

072 = odx

will print

072 58 #3a

which is the octal, decimal and hexadecimal representations of 072 (octal). The format is remembered so that typing subsequent numbers will print them in the given formats. Character values may be converted similarly, for example:

'a' = co

prints

a 0141

It may also be used to evaluate expressions but be warned that all binary operators have the same precedence which is lower than that for unary operators.

6. Patching

Patching files with ADB is accomplished with the write, w or W, request (which is not like the ed editor write command). This is often used in conjunction with the locate, l or L request. In general, the request syntax for l and w are similar as follows:

?l value

The request l is used to match on two bytes, L is used for four bytes. The request w is used to write two bytes, whereas W writes four bytes. The value field in either locate or write requests is an expression. Therefore, decimal and octal numbers, or character strings are supported.

In order to modify a file, ADB must be called as:

adb -w file1 file2

When called with this option, file1 and file2 are created if necessary and opened for both reading and writing.

For example, consider the C program shown in Figure 10. We can change the word “This” to “The ” in the executable file for this program, ex7, by using the following requests:

adb -w ex7 -
?l 'Th'
?W 'The '

The request ?l starts at dot and stops at the first match of “Th” having set dot to the address of the location found. Note the use of ? to write to the a.out file. The form ?* would have been used for a 411 file.

More frequently the request will be typed as:

?l 'Th'; ?s

and locates the first occurrence of “Th” and print the entire string. Execution of this ADB request will set dot to the address of the “Th” characters.

As another example of the utility of the patching facility, consider a C program that has an internal logic flag. The flag could be set by the user through ADB and the program run. For example:

adb a.out -
:s arg1 arg2
flag/w 1
:c

The :s request is normally used to single step through a process or start a process in single step mode. In this case it starts a.out as a subprocess with arguments arg1 and arg2. If there is a subprocess running ADB writes to it rather than to the file so the w request causes flag to be changed in the memory of the subprocess.

7. Anomalies

Below is a list of some strange things that users should be aware of.

  1. Function calls and arguments are put on the stack by the C save routine. Putting breakpoints at the entry point to routines means that the function appears not to have been called when the breakpoint occurs.
  2. When printing addresses, ADB uses either text or data symbols from the a.out file. This sometimes causes unexpected symbol names to be printed with data (e.g. savr5+022). This does not happen if ? is used for text (instructions) and / for data.
  3. ADB cannot handle C register variables in the most recently activated function.

8. Acknowledgements

The authors are grateful for the thoughtful comments on how to organize this document from R. B. Brandt, E. N. Pinson and B. A. Tague. D. M. Ritchie made the system changes necessary to accommodate tracing within ADB. He also participated in discussions during the writing of ADB. His earlier work with DB and CDB led to many of the features found in ADB.

9. References

  1. D. M. Ritchie and K. Thompson, “The UNIX Time-Sharing System”, CACM, July, 1974.
  2. B. W. Kernighan and D. M. Ritchie, “The C Programming Language”, Prentice-Hall, 1978.
  3. K. Thompson and D. M. Ritchie, UNIX Programmer's Manual - 7th Edition, 1978.
  4. B. W. Kernighan and P. J. Plauger, “Software Tools”, Addison-Wesley, 1976.