CSC Execution Environment. Program Execution Environment

Transcription

1 CSC 1600 Program Execution Environment Execution Environment The main func*on is the entry point for C program execu*on int main( int argc, char * argv[]); The linker (called by the C compiler) specifies, in the executable program file, a special start- up rou*ne to be called before main The start- up rou*ne takes command line arguments and environment values from the kernel sets things up so that main is then called 2 1

2 Environment When a UNIX program is executed, it receives two pieces of data: the arguments the environment To C programs both are arrays of character pointers all but the last of the character pointers point to a NULL- terminated string the last pointer is a NULL pointer A count of the number of arguments (not including the NULL pointer) is also passed 3 Memory Layout of a C Program 4 2

3 Environment The global variable environ points to the array of environment strings (whose last entry is NULL) extern char **environ; /* environment array (not in any header) */ 5 Environment One way to access the environment strings is to use the environ variable directly. Each environment string is of the form name=value extern char **environ; int main(int argc, char *argv[]) { } int i; for (i=0; environ[i]!= 0; i++) printf("%s\n",environ[i]); return 0; 6 3

4 Environment Example PWD=/mnt/a/mdamian/Systems/Shell TZ=US/Eastern HOSTNAME=tanner USER=mdamian MACHTYPE=sparc-sun-solaris2.9 OLDPWD=/mnt/a/mdamian/Systems LOGNAME=mdamian SHLVL=1 SHELL=/bin/bash HOSTTYPE=sparc OSTYPE=solaris2.9 HOME=/mnt/a/mdamian TERM=vt100 PATH=/usr/ccs/bin:/home2/jdk1.5.0/bin:/usr/sbin:/ opt/gnu/bin:/usr/bin:/opt/local/bin:/usr/local/ bin:.:/usr/ccs/bin:/usr/bin:/bin:/usr/sbin:/sbin:/ usr/gnu/bin:/usr2/local/bin:/mnt/local/bin:/usr/ucb SSH_TTY=/dev/pts/6 7 getenv Call Lis*ng an en*re environment is an unusual requirement Typically, the value for one environment variable is needed #include <stdlib.h> char *getenv(const char * var); var is the variable to find returns value or NULL if not found 8 4

5 getenv Example #include <stdio.h> #include <stdlib.h> int main(void) { char *s; s = getenv("logname"); if (s == NULL) printf("variable not found\n"); else printf("value is \"%s\"\n",s); } exit(exit_success); 9 Some Environment variables Name HOME USER HOSTNAME PWD PATH Description your home directory your login ID the name of the host computer the current working directory a list of directories in which to look for executable commands 10 5

6 man Command: Your Best Friend Syntax is man command Shows information about the command Usually it takes multiple screens, so hit Space or Enter for more information When done, hit q to quit 11 C Standard Library Function: strtok 6

7 The strtok Function char * strtok(char *s, char * delim); delim contains all characters separating the tokens if s is non-null: return pointer to beginning of first token in s, and terminate token with NULL. If s is NULL: use remainder of untokenized string from the last call to strtok 13 strtok in Action for ( token = strtok(line,,; ); token!= NULL; token = strtok(null,,; ) ) printf( Token is [%s]\n, token); - d o g, c a t ; NULL line token 14 7

8 strtok in Action After first call token = strtok(line,,; ); - d o g NULL c a t ; NULL line token Output is Token is [-dog] 15 strtok in Action After second call token = strtok(null,,; ); - d o g NULL c a t NULL NULL line token Output is Token is [cat] 16 8

9 strtok in Action After third call token = strtok(null,,; ); the loop breaks. - d o g NULL c a t NULL NULL line NULL token 17 What is the Output? #define MAX 80 char buf[max]; char * token; sprintf(buf, "%s", "This.is.my.strtok.test\n"); token = strtok(buf, ".\n"); printf("[%s], [%s] ", buf, token); token = strtok(buf, ".\n"); printf("[%s], [%s] ", buf, token); token = strtok(null, ".\n"); printf("[%s], [%s] ", buf, token); token = strtok(null, ".\n"); printf("[%s], [%s] ", buf, token); 18 9

10 Hands On Write a program called pathenv.c that retrieves the value of the PATH variable from the program environment, and prints out all search paths, one per line. Sample output: /usr/ccs/bin /home2/jdk1.5.0/bin /usr/sbin /opt/gnu/bin /usr/bin /opt/local/bin /usr/local/bin 19 10