The document provides an overview of various Unix programming concepts including passing program arguments, using environment variables, obtaining time and date, and retrieving user and host information. It explains functions for handling these aspects such as getuid(), getlogin(), and syslog for logging. Additionally, it discusses resource limits that programs may encounter on Unix systems, emphasizing the use of the limits.h header file.

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The linux environment
 Passing arguments to programs
 Environment variables
 Finding out what the time is
 Getting information about the user and the host computer
 Causing and configuring log messages
 Discovering the limits imposed by the system

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Program Arguments
 When a UNIX program written in C runs, it starts at the function main.
For UNIX programs, main is declared as:
 int main(int argc, char *argv[])
 where argc is a count of the program arguments and argv is an
array of character strings representing the arguments themselves.
 Whenever the operating system starts a new program, the
parameters argc and argv are set up and passed to main.

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Program Arguments
 Here's a program, args.c, that examines its own arguments:
#include <stdio.h>
int main(int argc, char *argv[])
{
int arg;
for(arg = 0; arg < argc; arg++) {
if(argv[arg][0] == '−')
printf("option: %sn", argv[arg]+1);
else
printf("argument %d: %sn", arg, argv[arg]);
} exit(0);
}

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Output
 When we run this program, it just prints out its arguments and
detects options.
$ ./args −i −lr 'hi there' −f fred.c
argument 0: args
option: i
option: lr
argument 3: hi there
option: f
argument 5: fred.c

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Environment Variables
These are variables that can be used to control the behavior of
shell scripts and other programs.
$ echo $HOME
/home/Anupam
 A C program may gain access to environment
 variables using the putenv and getenv functions.
 #include <stdlib.h>
 char *getenv(const char *name);
 int putenv(const char *string);

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Environment variables.
 The getenv function searches the environment for a string with the
given name and returns the value associated with that name,null
otherwise.
 The putenv function takes a string of the form name=value and
adds it to the current environment. It will fail and return −1 if it can't
extend the environment due to lack of available memory.

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Time and Date
 Times are handled using a defined type, a time_t. This is an integer
type large enough to contain dates and times in seconds.
 On UNIX and Linux systems using a 32−bit time_t type the time will
rollover in the year 2038!
 You can find the low level time value by calling the time function,
which returns the number of seconds since the start of the epoch. It
will also write the returned value to a location pointed to by tloc, if
this isn't a null pointer.
#include <time.h>
time_t time(time_t *tloc);

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User Information
 All UNIX programs, with the notable exception of init, are started by
other programs or by users.
 Each program that UNIX runs is run on behalf of a user and has an
associated UID.
 UID:-unique user identifier
 The UID has its own type uid_t defined in sys/types.h.
 UID is a small integer.
 Normally, users usually have UID values larger than 100.

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getuid() and getlogin() functions.
 The getuid function returns the UID with which the program is
associated. This is usually the UID of the user who started the
program.
 The getlogin function returns the login name associated with the
current user.
#include <sys/types.h>
#include <unistd.h>
uid_t getuid(void);
char *getlogin(void);

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Host Information
 gethostname function:
 The gethostname function writes the machine's network name into the
string name. This string is assumed to be at least namelen characters
long. gethostname returns 0 if successful, −1 otherwise.
#include <unistd.h>
int gethostname(char *name, size_t namelen);
 For more detailed information about the host computer:
#include <sys/utsname.h>
int uname(struct utsname *name);
The uname function writes host information into the structure pointed to by
the name parameter.

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LOGGING
 Many applications need to record their activities. System programs
will very often write messages to the console, or a log file.
 On a typical Linux installation, the file /var/log/messages contains all
 system messages, /var/log/maillog contains other log messages
from the mail system and /var/log/debug may
 contain debug messages.
 You can check your system's configuration in the file
/etc/syslog.conf.

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Syslog function.
The UNIX specification provides an interface for all programs to
produce logging messages, using the syslog function:
 #include <syslog.h>
 void syslog(int priority, const char *message, arguments...);
The syslog function sends a logging message to the logging facility.
Each message has a priority argument
which is a bitwise OR of a severity level and a facility value.

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Resources and Limits
 Programs running on a UNIX system are subject to resource
limitations.
 physical limits imposed by hardware (such as memory).
 limits imposed by system policies (for example, allowed CPU time).
 implementation limits (such as the size of an integer or the maximum
number of characters allowed in a filename).
 The header file limits.h defines many manifest constants that
represent the constraints imposed by the operating system.

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limits.h header file