Lecture 3

Programming in C: How to Get Things Done! Jan Wigginton Biostatistics 615/815 Last Lecture z Anatomy of a C program ? A collection of short functions z Built-in data types available in C z The C standard library Executing C Code z C is a high level language ? Relatively easy to understand z Computer CPUs execute much more detailed, "lower-level" instructions z A compiler performs the necessary translation? Working in a UNIX Environment z GCC ? Compile code z GDB ? Debug and test code z GPROF ? Collect profiling information GCC z GCC is a free C compiler ? GNU C Compiler ? G++ is a companion tool for C++ z Versions available for ? Linux ? Unix ? Mac ? Windows z Developed by Free Software Foundation Simple Usage gcc my_program.c ?o my_program -lm z Uses text file with C/C++ code as input z Produces executable program as output Compiling with multiple source files z All ?.cpp? files in current directory gcc *.cpp ?o my_program ?lm z All ?.cpp? files in ?my_directory? gcc ?Imy_directory my_program.cpp my_directory/*.cpp ?o my_program ?lm A shortcut z On Unix, it can be a time saver to create a ?build.csh? script #!/bin/tcsh gcc *.c ?o my_program ?lm z Make it executable chmod +x build.csh z To compile ./build.csh Programming errors z Compile time ? Program does not compile. ? Compiler reports a ?best guess? of the problem z Run time ? Executable crashes or has unexpected behavior ? May not appear for all conditions or all data sets Compiler output z Errors or warnings z Executable isn?t created until all errors are corrected z Program will still compile if warnings are not dealt with z Good practice to correct compiler warnings as well Common compiler errors z Undeclared variables or functions z Missing semicolon or brace z Typos z Missing files or libraries z Type ambiguities Common compiler errors Undeclared variable #include int main(int argc, char * argv[]) { printf("I am a computer program\n"); printf(?Value for variable %d", var); return 0; } Compiler message ./build.csh basic.cpp: In function `int main(int, char**)': basic.cpp:14: error: `var' undeclared (first use this function) basic.cpp:14: error: (Each undeclared identifier is reported only once for each function it appears in.) Common compiler errors z Missing semicolon #include void print_something() int main(int argc, char * argv[]) { printf("I am a computer program\n"); print_something(); return 0; } void print_something() { printf("Printing from a subroutine\n\n"); } z Compiler message ./build.csh print_something.cpp: In function `void print_something()': print_something.cpp:5: error: parse error before `int' print_something.cpp: In function `void print_something()': print_something.cpp:17: error: redefinition of `void print_something()' print_something.cpp:5: error: `void print_something()' previously defined here print_something.cpp:17: error: redefinition of `void print_something()' print_something.cpp:5: error: `void print_something()' previously defined here Common compiler errors z Missing brace #include void print_something(); void print_something_else(); int main(int argc, char * argv[]) { printf("I am a computer program\n"); return 0; } void print_something() { printf("Printing from a subroutine\n"); // } void print_something_else() { printf("Printing from subroutine II\n"); } z Compiler message ./build.csh basic.cpp: In function `void print_something()': basic.cpp:18: error: parse error before `{' token Common compiler errors z Missing library //include void print_something(); int main(int argc, char * argv[]) { printf("I am a computer program\n"); return 0; } void print_something() { printf("Printing from a subroutine\n"); } z Compiler message ./build.csh basic.cpp: In function `int main(int, char**)': basic.cpp:7: error: `printf' undeclared (first use this function) basic.cpp:7: error: (Each undeclared identifier is reported only once for each function it appears in.) basic.cpp: In function `void print_something()': basic.cpp:14: error: `printf' undeclared (first use this function) Common compiler errors z Type ambiguity #include #include int main(int argc, const char *argv[]) { printf("I am a computer program"); int var = log(10); return 0; } z Compiler message Visual C++ c:\Documents and Settings\Visual tudioProjects\Example\MyApplication.cp p (9): error C2668: 'log' : ambiguous call to overloaded function GCC ./build.csh overload.cpp: In function `int main(int, const char**)': overload.cpp:9: warning: converting to `int' from `double' Tips for working through compiler errors z Start at the top z Compiler may be ?confused? by missing semicolons or braces and produce many error messages that don?t seem logical. z Recompile as you work out errors z Check for missing header file declarations and code libraries Common run-time errors z Uninitialized variables z Memory errors z Numeric errors z Type errors in print statements z Closing a NULL file pointer z Accessing a NULL pointer z Variables out of scope Tips for tracking down run-time errors z Use a debugger z Debug data not code z Isolate a small case z Understand what?s happening before making changes z For updated code with new run-time errors ? Suspect recent changes ? Can also be a pre-existing bug that was uncovered by code changes Avoiding errors z Declare and initialize new variables as you code z Write paired coding constructs at the same time ? Open paren/ close paren ? Allocate/deallocate ? Constructor / destructor ? fopen/fclose z Always consider end cases z Pay attention to variable scope z Use error check code GDB z A simple debugger z Helps test and evaluate programs by: ? Stopping program at specific points ? Running program one line at a time ? Displaying the values of specific variables Simple Usage z Compile program with debug information gcc ?pg my_program.c ?o my_program -lm z Load program into debugger gdb my_program z Use debugger commands to control execution of program ? Essential GDB Commands z run ? Start execution of the program z next ? Execute a single line of code z step ? Execute a single line of code, stop at entry point of any called functions z continue ? Execute program until next breakpoint z where ? Reports line of code where crash occurs More Essential GDB Commands z break main ? Stop program when main function is called z break n ? Stop program when line n is reached z print x ? Print contents of variable x z info locals ? Print all local variables (gdb) break 1 Breakpoint 1 at 0x8048444: file example2.cpp, line 1. (gdb) list 2 3 #define UPPER 10 4 5 int vals[UPPER]; 6 7 void print_something(); 8 9 int main(int argc, const char * argv[]) 10 { 11 printf("I am a computer program\n"); (gdb) run Starting program: /home/wiggie/powerpoint/biostat851/example Breakpoint 1, main (argc=134513732, argv=0x1) at example2.cpp:10 10 { (gdb) n 11 printf("I am a computer program\n"); (gdb) n I am a computer program 13 for (int i = 0; i < UPPER; i++) (gdb) n 15 vals[i] = i + 1; . (gdb) info locals i = 0 (gdb) print vals[i] $1 = 0 (gdb) s 13 for (int i = 0; i < UPPER; i++) (gdb) print vals[i] $2 = 1 (gdb) s 15 vals[i] = i + 1; (gdb) break print_something() Breakpoint 2 at 0x8048496: file example2.cpp, line 24. (gdb) cont Continuing. Breakpoint 2, print_something () at example2.cpp:24 24 int var = 5, count; (gdb) list 19 return 0; 20 } 21 22 void print_something() 23 { 24 int var = 5, count; 25 printf("\n\nPrinting from a subroutine\n"); 26 var ++; 27 } 28 (gdb) n 25 printf("\n\nPrinting from a subroutine\n"); (gdb) print count $3 = -1076624204 (gdb) print var var A syntax error in expression, near `var'. (gdb) print var $4 = 5 (gdb) n Printing from a subroutine 26 var ++; (gdb) n Debugging with printf int printf(char * format, ?); z Writes formatted output z Help localize problems, particularly for tight loops or when array variables are suspect z Output is buffered, so printf should be followed call to fflush(stdout) printf int printf(char * format, ?); z Format string controls how arguments are converted to text z Parameters are printed as specified in % fields z %[flags][width][precision]type z Otherwise, string is quoted printf fields %[flags][width][precision]type Flags: z -? to left justify result ? ?+? to show sign in positive numbers Width z Minimum number of characters to print Precision z Number of digits after decimal (for floating point) ? Maximum number of characters (for strings) Type z ?s? for strings z ?d? for integers, ?x? to print hexadecimal integers ? z ?f? for floating point, ?e? for exponential notation, ?g? for automatic About compiler flags z Running GDB on executable that isn?t compiled with debug flags, will produce confusing output (gdb) run Starting program: /home/wiggie/powerpoint/biostat851/basic Breakpoint 1, 0x0804844a in main () (gdb) list 1 in ../sysdeps/i386/elf/start.S (gdb) print The history is empty. . (gdb) info locals No symbol table info available. z Recompile finished code without debug flags z Optimization flags (-O2 ?O3) can make executable faster z -Wall for ?picky? compilation GPROF z Works with GCC to collect information about a program's execution ? How often is each function called? ? How much time is spent in each function? z Works in three steps ? Compile code ? Execute program ? Tabulate profile information Simple Usage z Compile program with profile information gcc ?pg my_program.c ?o my_program -lm z Execute program ./my_program z Summarize profile information gprof my_program time fantasia:~/powerpoint/biostat851> time ./example I am a computer program I can count 1 potato 2 potato 3 potato 4 potato 5 potato 6 potato 7 potato 8 potato 9 potato 10 potato Printing from a subroutine 0.004u 0.000s 0:00.02 0.0% 0+0k 0+0io 0pf+0w fantasia:~/powerpoint/biostat851> Getting Help z From the UNIX prompt, you can use the man command to get help ? z For example: ?man gcc - info on how to run gcc ?man printf - info on using printf() Working in a Windows Environment z Good integrated toolsets exist z Good options include: ? Microsoft Visual Studio / Visual C++ ? Discounted version available through the University ? Turbo C++ Explorer ? Free C/C++ compiler, www.turboexplorer.com Microsoft Visual Studio z ?Student Tools for Visual Studio? ? Included in Academic Edition ? Simplifies environment slightly z Each application you develop is a project ? Can include a collection of source files ? But only one for most of our examples z Many types of applications possible ? ? Our examples will be ?Console Applications? ?All-In-One? z Edit and manage source files z Compile code z Run, debug and test File | New | Project ? Project | Add New Item | C++ File F1: Online Help System z Visual Studio includes an extensive integrated help system z Using the F1 key, you can get info on standard library functions: ? Including examples of how the function should be used. Important Commands z Build | Build MyApplication ? Compiles code, identifies problems with source z Debug | Start, Step Into, Step Over ? Manage execution of program z Debug | QuickWatch ? Examine contents of variables as program runs z Click to the left of particular source lines to set breakpoints Build | Build MyProject.cpp Debug | Start Debug | Start Stepping through code Adding a watch variable Call stack Today z How to compile, run and debug C code z GNU tool collection ? Unix / Linux / newer Macs z Integrated environments, such as Visual Studio ? Windows Questions/Clarification? z Feel free to drop me an e-mail : Janis Wigginton wiggie@umich.edu Goncalo Abecasis Microsoft PowerPoint - 615.03 -- Jan's Programming Tools