Introduction

Systems Programming Overview Systems programming is the

programming activity to create software used by other programmers and users. System programming examples include the creation of a compiler, a database, an operating system, a game, or a word processor.

System programming is in contrast with application programming which processes data for end-users.

Systems programming often involves extensive use of resources from operating systems and programming libraries. 1-1

Introduction

Systems Calls and Library Calls System calls on Linux are function

calls to the functions provided by the Linux operating systems. These calls are described in Section 2 of the Linux manual pages. E.g., open is a system call to open a file

handler Library calls on Linux are function

calls to various C libraries. These calls are described in Section 3 of the Linux manual pages. E.g., fopen is a C library call to open a file

(stream) handler.1-2

Systems Call Example

Introduction 1-3

#include <sys/types.h>#include <sys/stat.h>#include <fcntl.h>#include <stdlib.h> // for exit(3)#define BUFFSIZE 512int main(int argc, char* argv[]) {

int f = open("syscall.c", O_RDONLY); char buf[BUFFSIZE + 1]; int read_size = 0; int stdout = 1; // number one is the standard outputif (f < 0) {

perror("Open failed -- "); exit(1); // abnormal exit

} else { read_size = read(f, buf, BUFFSIZE); while (read_size > 0) {

buf[read_size] = 0; // terminate the buffer write(stdout, buf, read_size); // print it to the screen

read_size = read(f, buf, BUFFSIZE);

} // end of while } // end of if-else close(f); return 0;

}

Library Call Example

Introduction 1-4

#include <stdio.h>#include <stdlib.h> // for exit(3)

int main(int argc, char* argv[]) {

char c; FILE * f = fopen("libcall.c", "r"); if (f == NULL) {

fprintf(stderr, "open failed.\n"); exit(1);

} else { fscanf(f, "%c", &c); while (!feof(f)) {

fprintf(stdout, "%c", c); fscanf(f, "%c", &c);

} // end of while } // end of if-else fclose(f); return 0;

}

Library and System Call Facts Where do the manual pages reside?

/usr/share/man What is the format of text of the manual

pages? nroff (take a look at an example, e.g.,

write.2.tgz) How many system calls are there?

Roughly 430+ How many library calls (c, X, …) are

there? Roughly 8,000

Introduction 1-5

C Program Common Mistakes(1) Off-by-one // example 1.

char * src = “hello”;char * dst = (char*)malloc(strlen(src));strcpy(dst, src); // or strncpy(dst, src, strlen(src));

// example 2.int max = 16;char buf[max];… // file opened properlyint size_read = read(f, buf, max);if (size_read < max)

buf[size_read] = 0; // terminate the stringelse

buf[max] = 0; // terminate the stringIntroduction 1-6

C Program Common Mistakes(2) Forgot size differences // example 1.

long int x; // in some old programs, long == int, 32 bits

/* new use in gnu c compiler */#include <stdint.h>uint32_t x; // 32-bit intuint16_t y; // short, 16-bit intuint64_t z; // 64-bit int on 64-bit systems

// example 2.struct x { … }; // structure x is defined properly hereprintf( “size of struct x %d\n”, sizeof(struct x)); //

correct wayprintf( “size of struct x %d\n”, sizeof(struct x *)); //

pointer to struct Introduction 1-7

C Program Common Mistakes(3) Forgot to initialize variables before using them // example 1.

char * a;strcpy(a, “hello”);

// example 2.struct x { … };struct x *y;y-> member = 5;

Introduction 1-8

C Program Common Mistakes(4) Deallocate variable memory when still in use char * list_remove(struct list_t * listptr) {

struct node_t * toremove = listptr->head; char * retval = listptr->head->name; listptr->head = listptr->head->next; free(toremove); return retval;

}

Introduction 1-9