Quest for understanding the fundamentals of Linux Operating System and Embedded Systems

C program to send signal (SIGTERM) from the signal handler(SIGINT) of one process to another process which is registered with SIGTERM

Neelkanth_98$ cat signal1.c

#include<stdio.h>

#include<signal.h>

#include<unistd.h>

void sig_handler(int signo)

{

int process_id;

char process_id_from_file[10];

FILE *pid_fd;

char command[100];

sprintf(command,"pidof %s", "test_process");

pid_fd = popen(command, "r");

if (pid_fd == NULL)

{

return;

}

if (fgets(process_id_from_file, sizeof process_id_from_file , pid_fd) == NULL)

{

return;

}

pclose(pid_fd);

process_id = atoi(process_id_from_file);

if (signo == SIGINT) {

printf("received SIGINT from another process\n");

kill(process_id, SIGTERM);

printf("send SIGTERM to another process\n");

return;

}

int main(void)

{ /* register signal handler */

if (signal(SIGINT, sig_handler) == SIG_ERR)

printf("\ncan't catch SIGINT\n");

while(1)

sleep(1);

return 0;

}

Neelkanth_98$ cat signal2.c

#include<stdio.h>

#include<signal.h>

#include<unistd.h>

void sig_handler(int signo)

{

if (signo == SIGTERM)

printf("received SIGTERM from test process\n");

}

int main(void)

{ /* register signal handler */

if (signal(SIGTERM, sig_handler) == SIG_ERR)

printf("\ncan't catch SIGTERM\n");

while(1)

sleep(1);

return 0;

}

C program to print userid and group id

#include <stdio.h>

#include <sys/types.h>

#include <pwd.h>

#include <unistd.h>

#include <sys/types.h>

#include <grp.h>

int main()

{

struct passwd *pwd;

struct group *grp;

int uid, gid;

pwd = getpwnam("nobody");

if (pwd == NULL) {

printf("Failed to get uid");

}

uid = pwd->pw_uid;

grp = getgrnam("root");

if (grp == NULL) {

printf("Failed to get gid");

}

gid = grp->gr_gid;

printf("\n %d %d \n", uid, gid);

return 0;

}

C program to implement error handling

#include <stdio.h>

#include <errno.h>

extern int errno ;

int main ()

{

FILE * pFile;

pFile = fopen ("unexist.ent","rb");

if (pFile == NULL)

{

perror ("The following error occurred");

printf( "Value of errno: %d\n", errno );

}

else

fclose (pFile);

return 0;

}

If file unexist.ent does not exit then it will produce following result:

The following error occurred: No such file or directory

Value of errno: 29

C program to implement software timer

#include <stdio.h>

#include <time.h>

int main()

{

time_t start = time(NULL);

for (;;)

{

if (time(NULL) > start + 10) {

printf("10s timeout!\n");

break;

}

return 0;

}

c program to parsing a string with delimiter in-between [using strtok]

C program to parsing a string with delimiter in-between [using strtok]

#include <stdio.h>

#include <string.h>

int main (void)

{

char str[] = "neelkanth<delimiter>google<delimiter>stallone";

char *temp = str;

char *delim = "<delimiter>";

char *token;

do {

printf("Before Tokenizing : %s\n", temp);

token = strstr(temp , delim);

if (token)

*token = '\0';

printf("After Tokenizing :%s\n", temp);

temp = token+strlen(delim);

}while(token!=NULL);

return 0;

}

Neelkanth_98$ ./a.out

Before Tokenizing : neelkanth<delimiter>google<delimiter>stallone

After Tokenizing :neelkanth

Before Tokenizing : google<delimiter>stallone

After Tokenizing :google

Before Tokenizing : stallone

After Tokenizing :stallone

C program to write environment variables and their values into file and read the variables and their values from the file (using strtok)

Neelkanth_39$ cat neel.c

#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>

#include <string.h>

void func_read();

void func_write(char **envp);

void func_write(char **envp)

{

FILE *fp = NULL;

char a[1000];

int i = 0;

int len = 0;

printf("\n WRITING ENVIRONMENT VARIABLES INTO FILE \n");

fp = fopen ("file.txt", "w");

if (fp == NULL){

printf("failed to open %s", "file.txt");

return ;

}

while(envp[i] != NULL) {

strcat(a , envp[i]);

strcat(a , "@");

i++;

}

printf("%s\n",a);

len = strlen(a);

a[len+1] = '\0';

fwrite( a, strlen(a), 1, fp);

fclose(fp);

func_read();

}

void func_read()

{

char **envp;

char a[1000];

FILE *fp;

char *pt;

char temp[10][100];

int i = 0;

printf("\n READING ENVIRONMENT VARIABLES AND THEIR VALUES FROM FILE \n");

fp = fopen ("file.txt", "r");

if (fp == NULL) {

printf("failed to open %s", "file.txt");

return ;

}

fscanf(fp, "%s", a);

pt = strtok (a,"@");

while (pt != NULL) {

printf("%s\n", pt);

strcpy(temp[i], pt);

pt = strtok (NULL, "@");

i++;

}

printf("temp[0]: %s\n", temp[0]);

printf("temp[1]: %s\n", temp[1]);

printf("temp[2]: %s\n", temp[2]);

char *name, *value;

name = strtok(temp[0], "=");

printf("name: %s\n", name);

value = strtok(NULL, "=");

printf("value: %s\n", value);

return;

}

int main()

{

char *envp[] = {"stallone=rambo" ,"google=god", "hrithik=me", NULL };

printf("\n PASS ENVIRONMENT VARIABLES TO WRITE TO FILE FUNCTION \n");

func_write(&envp[0]);

return 0;

}

Output:

Neelkanth_39$ gcc neel.c

Neelkanth_39$ ./a.out

PASS ENVIRONMENT VARIABLES TO WRITE TO FILE FUNCTION

WRITING ENVIRONMENT VARIABLES INTO FILE

stallone=rambo@google=god@hrithik=me@

READING ENVIRONMENT VARIABLES AND THEIR VALUES FROM FILE

stallone=rambo

google=god

hrithik=me

temp[0]: stallone=rambo

temp[1]: google=god

temp[2]: hrithik=me

name: stallone

c program to copy values from double pointer array into file

C program to copy values from double pointer array into file

#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>

#include <string.h>

void func(char **envp)

{

FILE *fp = NULL;

fp = fopen ("dhcp", "w");

if (fp == NULL){

printf("failed to open %s", "dhcp");

return ;

}

int i = 0;

while(envp[i] != NULL) {

fwrite( envp[i], strlen(envp[i])+1, 1, fp);

i++;

}

fclose(fp);

}

int main()

{

char *envp[] = {"stallone\0" ,"google\0", NULL };

func(&envp[0]);

return 0;

}