11Simple C Programs2Goals for this Lecture• Help you learn about:• Simple C programs• Program structure• Defining symbolic constants• Detecting and reporting failure• Functionality of the gcc command• Preprocessor, compiler, assembler, linker• Memory layout of a Linux process• Text section, rodata section, stack section23“Circle” Program• File circle.c:#include <stdio.h>int main(void)/* Read a circle's radius from stdin, and compute and write itsdiameter and circumference to stdout. Return 0. */{int radius;int diam;double circum;printf("Enter the circle's radius:\n");scanf("%d", &radius);diam = 2 * radius;circum = 3.14159 * (double)diam;printf("A circle with radius %d has diameter %d\n",radius, diam);printf("and circumference %f.\n", circum);return 0;}4Building and Running• To build (preprocess, compile, assemble, and link):• To run:$ gcc217 circle.c –o circle$ circleEnter the circle's radius:5A circle with radius 5 has diameter 10and circumference 31.415900.Typed byuser35Steps in the Build Process• To build one step at a time:• Why build one step at a time?• Helpful for learning how to interpret error messages• Permits partial builds (described later in course)$ gcc217 –E circle.c > circle.i$ gcc217 –S circle.i$ gcc217 –c circle.s$ gcc217 circle.o –o circlePreprocess:circle.c → circle.iCompile:circle.i → circle.sAssemble:circle.s → circle.oLink:circle.o → circle6• File circle.c:The Preprocessor’s View#include <stdio.h>int main(void)/* Read a circle's radius from stdin, and compute and write itsdiameter and circumference to stdout. Return 0. */{int radius;int diam;double circum;printf("Enter the circle's radius:\n");scanf("%d", &radius);diam = 2 * radius;circum = 3.14159 * (double)diam;printf("A circle with radius %d has diameter %d\n",radius, diam);printf("and circumference %f.\n", circum);return 0;}CommentPreprocessor removesPreprocessor directivePreprocessor replaces with contentsof file /usr/include/stdio.h47• File circle.i:Results of Preprocessingint printf(char*, …);int scanf(char*, …);…int main(void){int radius;int diam;double circum;printf("Enter the circle's radius:\n");scanf("%d", &radius);diam = 2 * radius;circum = 3.14159 * (double)diam;printf("A circle with radius %d has diameter %d\n",radius, diam);printf("and circumference %f.\n", circum);return 0;}Declarations of printf(), scanf(),and other functions; compiler willhave enough information to check subsequent function callsNote: Definitions of printf() and scanf()are not present8• File circle.i:The Compiler’s Viewint printf(char*, …);int scanf(char*, …);…int main(void){int radius;int diam;double circum;printf("Enter the circle's radius:\n");scanf("%d", &radius);diam = 2 * radius;circum = 3.14159 * (double)diam;printf("A circle with radius %d has diameter %d\n",radius, diam);printf("and circumference %f.\n", circum);return 0;}FunctiondefinitionCompoundstatementalias blockFunction declarationsCompiler notes return typesand parameter types so it cancheck your function callsReturn type of main()should be int59• File circle.i:The Compiler’s View (cont.)int printf(char*, …);int scanf(char*, …);…int main(void){int radius;int diam;double circum;printf("Enter the circle's radius:\n");scanf("%d", &radius);diam = 2 * radius;circum = 3.14159 * (double)diam;printf("A circle with radius %d has diameter %d\n",radius, diam);printf("and circumference %f.\n", circum);return 0;}Declaration statementsMust appear before any otherkind of statement in block;variables must be declaredbefore useFunction callstatementsString constants& (“address of”) operatorExplained later in course,with pointers10• File circle.i:The Compiler’s View (cont.)int printf(char*, …);int scanf(char*, …);…int main(void){int radius;int diam;double circum;printf("Enter the circle's radius:\n");scanf("%d", &radius);diam = 2 * radius;circum = 3.14159 * (double)diam;printf("A circle with radius %d has diameter %d\n",radius, diam);printf("and circumference %f.\n", circum);return 0;}ExpressionstatementsCast operatorUnnecessary here, but good style to avoidmixed-type expressionsConstant of type doubleConstant of type int611• File circle.i:The Compiler’s View (cont.)int printf(char*, …);int scanf(char*, …);…int main(void){int radius;int diam;double circum;printf("Enter the circle's radius:\n");scanf("%d", &radius);diam = 2 * radius;circum = 3.14159 * (double)diam;printf("A circle with radius %d has diameter %d\n",radius, diam);printf("and circumference %f.\n", circum);return 0;}Function call statementsprintf() can be called with1 or more actual parametersReturn statementConvention: 0 returned from main() means success; non-0means failure12• File circle.s:• Still missing definitions of printf() and scanf()Results of CompilingAssembly language.section .rodata.LC0:.string "Enter the circle's radius:\n".LC1:.string "%d"….text.globl main.type main, @functionmain:pushl %ebpmovl %esp, %ebp…pushl $.LC0call printfaddl $16, %espsubl $8, %espleal -4(%ebp), %eaxpushl %eaxpushl $.LC1call scanf…713• File circle.s:• Assembler translates assembly language into machine language• Details provided in 2nd half of courseThe Assembler’s View.section .rodata.LC0:.string "Enter the circle's radius:\n".LC1:.string "%d"….text.globl main.type main, @functionmain:pushl %ebpmovl %esp, %ebp…pushl $.LC0call printfaddl $16, %espsubl $8, %espleal -4(%ebp), %eaxpushl %eaxpushl $.LC1call scanf…Assembly language14• File circle.o:• Object file• Still missing definitions of printf() and scanf()Results of AssemblingMachine languageListing omittedNot human-readable815• File circle.o:• The linker:• Observes that• Code in circle.o calls printf() and scanf()• Code in circle.o does not define printf() or scanf()• Fetches machine language definitions of printf() and scanf() from standard C library (/usr/lib/libc.a on hats)• Merges those definitions with circle.o to create…The Linker’s ViewMachine languageListing omittedNot human-readable16• File circle:• Complete executable binary fileResults of LinkingListing omittedNot human-readableMachine language917Run-Time View• At run-time, memory devoted to program is divided into sections:TEXTRODATADATABSSHEAPSTACK• TEXT (read-only)• Stores executable machine language instructions•RODATA(read-only)• Stores read-only data, esp. string constants•STACK(read/write)• Stores
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