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System Interface

System Interface. Interface that provides services from the OS (Higher than BIOS) Memory Scheduler File/Storage System Inter-process Communication and Network, etc. program. libc. Operating System. BIOS. File System. A popular but also complex subsystem

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System Interface

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  1. System Interface • Interface that provides services from the OS (Higher than BIOS) • Memory • Scheduler • File/Storage System • Inter-process Communication and Network, etc program libc Operating System BIOS

  2. File System • A popular but also complex subsystem • File descriptors (pseudo files, like stdin and stdout) • Low level IO • File Management • Examples file pointer / usr bin home file EOF

  3. A little about File Attributes • File attributes (srwxrwxrwx) • read, write and execute • an octal digit, rwx, desribes the permission to a set of users • Three sets of differents users, owner, group and others • Some unix commands: umask, chmod, etc to change the attributes

  4. File Accesses • Read and Write • getc() and putc(); getchar() and putchar() • int read(int fd, char *buf, int n); • int write(int fd, char *buf, int n) #include “syscalls.h” /* Should be stdio.h */ int getchar () { char c; return (read(0, &c, 1) == 1 ) ? c : EOF; } #include “syscalls.h” /* Should be stdio.h */ int main () { char buf[1024]; int n; while ((n = read(0, buf, 1024)) > 0) write ( 1, buf, n); return 0; }

  5. File Accesses -- continued • Open, creat, close and unlink • int open(char *name, int flags, int perms); • Persm can be O_RONLY, O_RDWR, O_WRONLY and others • int creat(char *name, int perms); /* within open, O_CREAT */ • int close(int fd); • int unlink(char *path); #include “stdio.h” /* A copy program*/ int main () { char *f1 =“file1”, *f2=“file2”; int fd1, fd2, n; if (fd1 = open(f1, O_RDONLY, 0) == -1) usage(“uanble to opening source file %s\n”, f1); if (fd2 = creat(f2, 0666) == -1) usage(“unable to creating new file %s \n”, f2); while ((n = read(fd1, buf, 1024) > 0) write ( fd2, buf, n); return 0; } #include <stdio.h> void usage(char *fmt, …) { va_list args; va_start(args, fmt); fprintf(stderr, “error: “); vfprintf(stderr, fmt, args); va_end(args); fprint(“usage: srcfile destfile\n”); }

  6. File seek -- access with fix location • lseek • long lseek(int fd, long offset, itn origin); • Origin: 0, 1, 2, determines where to start offset. • 0: beginning; 1: current location; 2: the end. #include “stdio.h” /* A copy program*/ int main () { char *f1 =“file1”, *f2=“file2”; int fd1, fd2, n; if (fd1 = open(f1, O_RDONLY, 0) == -1) usage(“uanble to opening source file %s\n”, f1); if (fd2 = creat(f2, 0666) == -1) usage(“unable to creating new file %s \n”, f2); else lseek(fd2, 0L, 2); while ((n = read(fd1, buf, 1024) > 0) write ( fd2, buf, n); return 0; }

  7. Recap on lab 4 • File access alternatives • File * fp = fopen(); • fgets() • Int fd = open(name, flags, perms); • read(fd, buff, n); #include <stdio.h> /* lab 4 */ int main () { char *maibox = argv[1]; int fd; File *fp; #if 1 fp = fopen(mailbox, “r”); #else fd = open (mailbox, O_RDONLY, 0); #endif for ( ) { /* keep reading every line and process the HEADER and BODIES */ } return 0; }

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