#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <errno.h>
int main() {
FILE *f = fopen("stories.txt", "w");
if (!f) {
error("Can't open stories.txt");
}
pid_t pid = fork();
if (pid == -1) {
error("Can't fork process");
}
if (!pid) {
fprintf(f, "f---- child process wrote\n");
printf("---- child process wrote\n");
if (execl("/bin/ls", "/bin/ls", NULL) == -1) {
error("Can't run script");
}
}
fprintf(stdout, "parent process wrote it after fork!\n");
fprintf(f, "parent process wrote it before return main!\n");
return 0;
}
When I run the above code in Ubuntu Linux 64-bit, this
fprintf(f, "f---- child process wrote\n");
is not written in the stories.txt file.
Can you help me explain why this happens?
When I comment out the execl then the write to the file from the child process is done OK.
use fclose(f); before run execl
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <errno.h>
#include <string.h>
#include <errno.h>
int main() {
FILE *f = fopen("stories.txt", "w");
if (!f) {
error("Can't open stories.txt");
}
pid_t pid = fork();
if (pid == -1) {
error("Can't fork process");
}
if (!pid) {
fprintf(f, "f---- child process wrote\n");
printf("---- child process wrote\n");
fclose(f);
//--^^^^^^^^^^--//
if (execl("/bin/ls", "/bin/ls", NULL) == -1) {
error("Can't run script");
}
exit(0);
}
fprintf(stdout, "parent process wrote it after fork!\n");
fprintf(f, "parent process wrote it before return main!\n");
return 0;
}
Related
I would like to create a named pipe in the parent process and after write a string to it in the child process and finally read this string in the parent process. When run the program I dont get back the prompt like still waiting for end of child process. Why the child process not finished?
Current output:
Expected output:
(picture created without multiprocesses)
My source code:
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <sys/file.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
#include <sys/wait.h>
int main() {
int pipefd[2];
pid_t cpid;
char szoveg[32];
int fd, ret;
char buf[32];
buf[0]=0;
cpid = fork();
if (cpid == -1) {
perror("fork");
exit(-1);
}
if (cpid == 0) {
printf("%d: Child process\n",getpid());
strcpy(buf,"Some text \0");
printf("%d:write to fifo: %s:%ld\n",getpid(),buf,strlen(buf));
write(fd,buf,strlen(buf));
exit(0);
} else {
printf("%d: Parent process\n",getpid());
ret=mkfifo("FifoName",00666);
if (ret == -1) {
perror("mkfifo()");
exit(-1);
}
fd=open("FifoName",O_RDWR);
if (fd == -1) {
perror("open() error!");
exit(-1);
}
wait(NULL);
ret=read(fd,buf,32);
printf("%d:read() Read %d bytes: %s\n",getpid(),ret,buf);
close(fd);
unlink("FifoName");
exit(0);
}
}
William Pursell right. The problem was the missing fd=open("FifoName",O_RDWR); line from child process.
I am trying to write a program that will fork, then open a file and execute it. The file it should execute is called child and it has been compiled. When I type ./child, it runs. However, when I run this program it does not execute the child program and I am prompted with the error message I put in "Execution failed". What I am doing wrong?
This is my parent class
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/wait.h>
int main (int argc, char **argv)
{
pid_t parent = getpid();
pid_t pid = fork();
if (pid == -1)
{
// error, failed to fork()
}
else if (pid > 0)
{
int status;
waitpid(pid, &status, 0);
}
else
{
int var = execvp("./child", NULL);
if(var < 0)
{
printf("Execution failed");
}
}
exit(0); // exec never returns
}
This is the child
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main (int argc, char **argv)
{
printf ("Im the child");
exit (0);
}
I actually don't know what you are doing wrong. After a copy and a compilation (and several warning complains) your code runs fine (GCC 7.2).
Obviously, child must be in the same working directory in which you run your main executable (the one that forks).
But probably I would write that code in this way, but I'm not an expert in forking:
#include <stdio.h>
#include <string.h>
#include <unistd.h>
#include <sys/wait.h>
#include <errno.h>
extern int errno;
int main () {
pid_t pid = fork();
if (pid < 0) {
fprintf(stderr, "%s\n", strerror(errno));
return 1;
}
if (pid == 0) {
int ret = execl("./child", "", (char *)NULL);
if(ret < 0) {
fprintf(stderr, "%s\n", strerror(errno));
return 1;
}
} else {
wait(NULL);
}
return 0;
}
At least it tells you which error execl has encountered.
Question asked again and code modified...
I need to create three programs named as program0 program1 and program2 in linux.
Program0:Creates a parent with two child processes and executes program 1 and program 2 with its childs waits them to finish and close.
Program1:Takes a file name from the user and writes text to the file.It finishes writing when CTNL+D pressed and creates a pipe.After that by using cat command it writes file to stdout and uses dup() to create pipe which has file in it.
Program2:It reads filename from the pipe with the help of dup() and then executes wc command.
So far I managed to create all programs and I have no compling errors.Program 0 executes both programs.Program1 is also working and sends file to the pipe but program2 cannot read it from the pipe is prints weird symbols..
When I try to read from the pipe within the program1 it works(see the deactivated code in program1) but same code is not working if I put it inside program2.
So what how can I make program2 to read from the pipe after that I will try to execute wc command in program2 but first I should be able to see that its taking file input from the stdout so how?
I know its kinda long but please help me guys...
Program 0
#include <stdio.h>
#include <sys/types.h>
#include <sys/wait.h>
#include <unistd.h>
#include <stdlib.h>
#define MAX 999
int main()
{
pid_t pid1, pid2;
pid1 = fork();
if(pid1<0)
{
fprintf(stderr,"Fork basarisiz");
exit(-1);
}
else if (pid1 ==0)/*child prosesleri*/
{
printf("program1\n");
execlp("./program1","program1",NULL);
execlp("./program2","program2",NULL);
}
else /*parent procsesleri */
{
wait(NULL);
pid2 = fork();
if(pid2<0)
{
fprintf(stderr,"Fork basarisiz");
exit(-1);
}
else if (pid2 ==0)/*child prosesleri*/
{
printf("\n");
printf("Program 2\n");
printf("\n");
execlp("./program2","program2",NULL);
//printf("\n");
}
else
{
}
/////////////////////////////////////////////////////////////////////////
wait(NULL);
printf("\n");
printf("Parent:Two child processes have successfully been created\n");
printf("Parent:Two child processes have successfully been terminated\n");
printf("Parent:This process will now terminate\n");
printf("\n");
exit(0);
}
}
Program 1
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
#define MAX 999
int main()
{
char c[10000];
char file[10000];
int words;
printf("Child1:A text file will be created\n");
printf("Child1:Enter the name of the file\n");
scanf("%123s",file);
strcat(file,".txt");
FILE * pf;
pf = fopen(file, "w" );
if (!pf)
fprintf( stderr, "I couldn't open the file.\n" );
else
{
printf("Child1: Input a number of text lines ended, each ended by a CR (carriage return).\n");
/////////////////////////////
do
{
if (NULL != fgets(c, sizeof(c), stdin))
{
if (0 == strcmp(c, ".\n"))
{
break;
}
fprintf(pf, "%s", c);
}
else
{
if (0 != ferror(stdin))
{
fprintf(stderr, "An error occured while reading from stdin\n");
}
else
{
printf("Child1: Finish the input by CNTL^D\n");
}
break;
}
} while (1);
/////////////////////////////
}
printf("\nChild1:The file %s is succesfully created and saved in the current dictionary\n",file);
//////////////////////////////////////////////
/////////////////////////pipe///////////////
fclose(pf); // close file
char ch;
int outcount = 0;
int fd[2], nbytes;
pid_t childpid;
int i;
char f2[2];
char readbuffer[80];
pipe(fd);
if((childpid = fork()) == -1)
{
perror("fork");
exit(1);
}
if(childpid == 0)
{ printf("\nChild1:The file written to pipe with cat\n");
close(1) ;
dup(fd[1]);
close(fd[0]);
execlp("/bin/cat", "cat", file,NULL);
}
else
{
wait(NULL);
//close(0) ;
//dup(fd[0]) ;
//close(fd[1]);
//nbytes = read(fd[0], readbuffer, sizeof(readbuffer));
//printf("%s\n",readbuffer);
}
return(0);
}
Program 2
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <unistd.h>
int main()
{
int fd[2],nbytes;
pid_t childpid;
char readbuffer[80];
pipe(fd);
if((childpid = fork()) == -1)
{
perror("fork");
exit(1);
}
if(childpid == 0)
{
}
else
{
close(0) ;
dup(fd[0]) ;
close(fd[1]);
nbytes = read(fd[0], readbuffer, sizeof(readbuffer));
printf("%s\n",readbuffer);
}
return(0);
}
You may want to check the man pages for execve(2) (for starting cat) and dup2(2) (for overriding stdin and stdout as needed) for this one. execve will overwrite the currently executing program by a different one (same PID, same file descriptors), while dup2 will allow you re-define any of the standard file descriptors to point into any file descriptor you provide to it (such as any of the ends of your pipe).
Program::
#include <sys/types.h>
#include <stdio.h> //standard input output header file
#include <unistd.h> // POSIX operating system API
#include <stdlib.h>
int main()
{
pid_t pid;
pid = fork();
system("clear");
if(pid < 0)
{
fprintf(stderr, "Fork failed to create process\n");
return 1;
}
else if(pid == 0)
{
printf("This is the child process \'%d\' of \'%d\'\n\n",getpid(),getppid());
execlp("/bin/ls","ls",NULL);
}
else
{
printf("This is parent process \'%d\' my parent is %d\n\n",getpid(),getppid());
wait(NULL);
printf("DONE\n\n");
exit(0);
}
return 0;
}
Output:
This is the child process '3979' of '3978'
This is parent process '3978' my parent is '3681'
DONE
vijay#workspace:~/Documents/os$ a.out os_fork.c
I am unable to understand why the execlp() got executed after parent process not along with child process
I'm trying to redirect the stdin from the parent to the child through the pipe "my_pipe", but when I run my program, I do not see the expected results.
When I execute the program, it expects input from the stdin, so why didn't it redirect stdin in dup2?
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>
int main(int argc, char* argv[])
{
char* arguments[] = {"sort", NULL};
int my_pipe[2];
if(pipe(my_pipe) == -1)
{
fprintf(stderr, "Error creating pipe\n");
}
pid_t child_id;
child_id = fork();
if(child_id == -1)
{
fprintf(stderr, "Fork error\n");
}
if(child_id == 0) // child process
{
close(my_pipe[1]); // child doesn't write
dup2(0, my_pipe[0]); // redirect stdin
execvp(argv[0], arguments);
fprintf(stderr, "Exec failed\n");
}
else
{
close(my_pipe[0]); // parent doesn't read
char reading_buf[1];
write(my_pipe[1], "hello", strlen("hello"));
write(my_pipe[1], "friend", strlen("friend"));
close(my_pipe[1]);
wait();
}
}
Your arguments to dup2 are backwards. Try dup2(my_pipe[0], STDIN_FILENO)