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CITS2002 - Lecture 16

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CITS2002 - Lecture 15
System-Calls, Structures and File Systems - Slides

System Calls & System- Defined Structures

  • @ A system call is a controlled request from a user program to the OS kernel to perform a task that the program cannot do directly
    • Most system calls accept integers and pointers to characters as parameters, and return an integer to indicate success or failure
  • When more complex data is needed, structures are used defined in system header files
    • e.g. <time.h>, <sys/stat.h>

Accessing Structures via Pointers

  • Normally to access fields of a structure we use a dot: structure.field
  • When using pointers to structures, use the arrow operator ->
struct tm *tm = localtime(&NOW);
printf("%i/%i/%i", tm->tm_mday, tm->tm_mon + 1, tm->tm_year + 1900);

Accessing System Data

  • The /etc/passwd file historically stores local user info
  • Programs can access it through system-provided structures and functions like: {C}struct passwd *getpwent(void)

Defining Our Own Datatypes

  • Use typedef with struct to define custom types for clarity and reusability:
typedef struct {
    char teamname[31];
    int played;
} TEAM;
TEAM team[MAX_TEAMS];
  • Access using -> if using pointers:
TEAM *tp = &team[t];
printf("%s %d\n", tp->teamname, tp->played);
  • Instead of using parallel arrays, group related variables into a single structure:
typedef struct {
    char *stopid;
    char *name;
    int metres;
} VIABLE;
VIABLE *home_stops = NULL;
VIABLE *dest_stops = NULL;

Investigating Files - Using stat()

  • The POSIX function stat() retrieves file attributes stored in a struct stat:
struct stat stat_buffer;
stat(filename, &stat_buffer);
printf("Size: %d\n", (int)stat_buffer.st_size);
  • Can determine if a path is a file or directory:
if S_ISREG(stat_buffer.st_mode) -> file
if S_ISDIR(stat_buffer.st_mode) -> directory
  • This is important as there are differences between the code needed to open and read a text file, versus opening and reading a directory

Reading Directory Contents

  • Open and read directories like files using:
DIR *dirp = opendir(dirname);
struct dirent *dp;
while((dp = readdir(dirp)) != NULL)
    printf("%s\n", dp->d_name);
closedir(dirp);
  • These are POSIX functions, not part of C11

Combining readdir() with stat()

  • To find whether each directory entry is another directory or file:
sprintf(fullpath, "%s/%s", dirname, dp->d_name);
stat(fullpath, &stat_buffer);
if(S_ISDIR(stat_buffer.st_mode)) ...
else if(S_ISREG(stat_buffer.st_mode)) ...

CITS2002 - Lecture 17

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