Some Notes on Debugging C Using gdb

gcc -g -o myprog myprog.c

gdb myprog

Debugging Concepts

You are probably familiar with the Java debugger in Eclipse. gdb can do much the same thing for a C program, on the command line. A debugger allows you to control the execution of a program by setting breakpoints (places in the source code where the program will pause automatically) and watchpoints (which cause the program to paused when the value of a variable or expression changes). A breakpoint can be conditional, meaning that the execution will pause only if some specified condition evaluates to true.

When the program is paused, it is possible to check the values of variables and expressions and even to set the value of a variable. You can view information about the function call stack. You can resume execution, or you can execute the program one step at a time.

The rest of this document discusses gdb commands to do all these things. Remember that these commands are given at the (gdb) prompt while the debugger is running. The most common commands have one or two letter equivalents (such as "c", "s", and "n" for "continue", "step", and "next"). Any command can be abbreviated to the shortest unambiguous prefix.

Program Execution

• run — Start running the program. If the program is already running, you will be asked if you want to kill the current execution and restart from the beginning.
• quit — Quit the debugger. If the program is running, you will be asked if you want to kill it.
• kill — Kill the current execution of the program. You can then use run to restart.
• continue — This command and the next four are used when the program is paused, such as by a breakpoint. The continue command will resume automatic execution, until the end of the program or something else happens to make it pause again, such as encountering a breakpoint.
• step — Execute the next line of the program. If the line contains a function call, this will step into the function, stopping at the first line of the function. ("Step into" in the Eclipse Java debugger.)
• next — Execute the next line of the program. If the line contains a function call, the entire function call is executed (unless it contains a breakpoint or other reason for stopping). ("Step over" in the Eclipse Java debugger.)
• finish — Resume automatic execution until the current function returns (or something else happens to stop the execution). ("Step out" in the Java debugger.)
• until <location> — Resume automatic execution until the <location> is reached (or something else happens to stop the execution).

Here and in the rest of this document, <location> can be a line number in the current source code file or a function name, or it can be of the form file_name:line_number to mean a line number in the specified file.

Breakpoints and Watchpoints

• break <location> — Set a breakpoint at <location> (such as a line number or function name).
• break <location> if <expression> — Set a conditional breakpoint at <location>. That is, the break will only occur if the <expression> is true. (For a numeric expression, true means non-zero.)
• clear <location> — Remove any breakpoint at the specified <location>.
• info break — List information about all breakpoints, including breakpoint numbers. Breakpoint numbers can be used in the following three commands.
• disable <break-number> — Disable breakpoint number <break-number>. Execution will not pause at a disabled breakpoint.
• enable <break-number> — Enable breakpoint number <break-number>.
• delete <break-number> — Remove breakpoint number <break-number>.
• continue <number> — Can be used when the program is paused at a breakpoint. Resumes program execution, ignoring the current breakpoint <number> times.
• watch <expression> — Set a watchpoint for <expression>. The program will pause execution whenever the value of the expression changes.

A watchpoint is actually a kind of breakpoint. The info break command will list watchpoints as well as breakpoints, including watchpoint numbers. Watchpoint numbers can be used with the disable, enable, and delete commands.

A watchpoint will automatically be deleted if the expression contains local variables, and one of those variables goes out of scope.

View Information and Set Values

• where — Tells you where in the program you are. Often, there will be several active functions that have been called, each with its own "frame" (or activation record). The where command prints a numbered list of active frames, with information about the current line number in that frame. In addition to functions in your program, this can include system calls and calls to library functions. In general, you want to look for the first frame for a function in your own program.
• frame <number> — Select frame number <number>, where the number is the frame number from the where command. The print and info locals commands are in the context of the currently selected frame.
• print <expression> — Prints the value of <expression>. The expression can include function calls.
• print <variable> = <expression> — This is really just a special case of the print command, to emphasize that it works with assignment statements. Using an assignment statement in the print command allows you change the value of a variable.
• print/x <expression> — Prints the value of <expression> in hexadecimal, after type conversion to an unsigned long int if necessary.
• info locals — Prints the name and value of all local variables in the currently selected frame.
• backtrace full — Prints information about all frames, including local variables. Can be abbreviated to bt f. Without the "f" or "full", it's equivalent to where.
• list — Print ten lines from the program around the current line.
• list <location> — Print ten lines from the program around the specified <location>, such as a line number or function name.
• help — Print general help on using gdb.
• help <command> — Print help about using the specified gdb <command>.

Note that it is possible to print arrays using the print command. If the array A has been statically allocated, such as "int A[10]", then print A will print a list containing all of the elements of A. However, often you are working with pointers instead of arrays, and in that case, gdb doesn't know how long the array is. If p is a pointer, and you want to treat it as an array of 10 elements, you can say print *p@10. This will print out the 10 items starting with *p.