Debugging a C program using `gdb` (GNU debugger)

The biggest problem for beginning C programmers

Biggest headache for beginning C programmers:

Unlike Java, when a C program crashes, you will see:

How to find out the location of the program crash

Rule:

You should always compile your C program with the debug flag -g:

gcc -g -o yourProgName C-program.c

If your C program consists of multiple files:

gcc -g prog-file1.c gcc -g prog-file2.c .... gcc -g -o yourProgName prog-file1.o prog-file2.o ...

To find the crash location, run the debugger on your program:

gdb yourProgName
At the gdb prompt, type the run command: (gdb) run [optionally: arguments]

The program will stop at the crash location !!!

You can use the list command of gdb to print the program fragment at the crash location:
This may help you identify the segment of the source file

Finding out what caused the crash

Fact:

There is no scientific way to find out what caused the program to crash
There are commands in gdb that will help you find out the reason your program crashed.

Most common cause of a program crash:
You can print out the value in any variable at the crash location using the p command in gdb:

Example using gdb

Program: (crash1.c)

// Program file: /home/cs255001/demo/C/set1/crash1.c int main(int argc, char *argv[]) { int a[10]; int i = -872625577; a[i] = 1234; printf("i = %d\n", i); printf("a[i] = %d\n", a[i]); }

Compile with:

gcc -g crash1.c

Run with this command:

gdb a.out

Output:

cs255 (7072)> gdb a.out GNU gdb (GDB) Red Hat Enterprise Linux (7.2-64.el6_5.2) Copyright (C) 2010 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law. Type "show copying" and "show warranty" for details. This GDB was configured as "x86_64-redhat-linux-gnu". For bug reporting instructions, please see: ... Reading symbols from /home/cs255000/demo/c/set2/a.out...done. (gdb) run Starting program: /home/cs255000/demo/c/set2/a.out Program received signal SIGSEGV, Segmentation fault. 0x00000000004004df in main (argc=1, argv=0x7fffffffe9d8) at crash1.c:17 17 a[i] = 1234; (GDB tells us the program crashed at line 17 in the file crash1.c) (gdb) where #0 0x00000000004004df in main (argc=1, argv=0x7fffffffe9d8) at crash1.c:17 (Ask GDB to tell where the program stopped. GDB tells us: line 17 in the file crash1.c) (gdb) list 12 int main(int argc, char *argv[]) 13 { 14 int a[10]; 15 int i = -872625577; 16 17 a[i] = 1234; 18 printf("i = %d\n", i); 19 printf("a[i] = %d\n", a[i]); 20 } (Ask GDB to list the program source around the place where the program stopped. GDB gives us about 10 lines of text.... Conclusion: the execution of a[i] = 1234 caused the error.... Find out the value of these variables !!! (gdb) p i $1 = -872625577 (Ask GDB to show us the value of the variable i GDB gives us -872625577 (gdb) print a[i] Cannot access memory at address 0x7fff2ff3121c (Ask GDB to show us the value of the variable a[i] GDB gives us -872625577

Breakpoints

You can put a break point at any function in your program
Command to put a break proint at a function:
Example:

When your program has stopped at a break point, you can do these thing:

print the value in variables (to help you find error(s)

(gdb) p variableName

Add more break points or delete an existing breakpoint:

(gdb) b functionName (add breakpoint) (gdb) delete breakpointNumber

Find the breakpoint's number by using the info command:

(gdb) info breakpoints

Continue with your program execution:

(gdb) n (short for next) or: (gdb) c (short for continue)

I will discuss these command next....

Executing the program after it stopped at a breakpoint

After your program has stopped at an (any one) break point, you can choose to execute the program in several ways:

Execute one statement and stop:

(gdb) n (short for "next")

This command is like the STEP command in EGTAPI

Execute the program continuously until it stops at a breakpoint or crashes or terminates:
This command is like the RUN command in EGTAPI

Debugging a C program using gdb (GNU debugger)

Debugging a C program using `gdb` (GNU debugger)