Set child pid to 0 but output others
I read such a snippet of code from an instruction
#include "apue.h"
int globvar = 6; /* external variable in initialized data */
char buf = "a write to stdoutn";
int main(void)
{
int var;
pid_t pid; /* automatic variable on the stack */
var = 88; if (write(STDOUT_FILENO, buf, sizeof(buf) - 1) != sizeof(buf) - 1)
err_sys("write error");
printf("before forkn"); /* we don’t flush stdout */
if ((pid = fork()) < 0) {
err_sys("fork error");
} else if (pid == 0) { /* child */
globvar++; /* modify variables */
var++;
} else {
sleep(2); /* parent */
}
printf("pid = %ld, glob = %d, var = %d, bufsize = %lun", (long)getpid(), globvar, var, sizeof(buf));
exit(0);
}
Run it and get outputs
$ ./a.out
a write to stdout
before fork
pid = 7310, glob = 7, var = 89, bufsize = 19 #child’s variables were changed
pid = 7309, glob = 6, var = 88, bufsize = 19 #parent’s copy was not changed
I am confused about the child's pid
if ((pid = fork()) < 0)
the pid is set as 0, but in the output it's pid is 7310.
How could that happen?
does the pid here is just a number rather than a process?
c
asked Nov 15 '18 at 3:44
JawSawJawSaw
4,57811837
add a comment |
I read such a snippet of code from an instruction
#include "apue.h"
int globvar = 6; /* external variable in initialized data */
char buf = "a write to stdoutn";
int main(void)
{
int var;
pid_t pid; /* automatic variable on the stack */
var = 88; if (write(STDOUT_FILENO, buf, sizeof(buf) - 1) != sizeof(buf) - 1)
err_sys("write error");
printf("before forkn"); /* we don’t flush stdout */
if ((pid = fork()) < 0) {
err_sys("fork error");
} else if (pid == 0) { /* child */
globvar++; /* modify variables */
var++;
} else {
sleep(2); /* parent */
}
printf("pid = %ld, glob = %d, var = %d, bufsize = %lun", (long)getpid(), globvar, var, sizeof(buf));
exit(0);
}
Run it and get outputs
$ ./a.out
a write to stdout
before fork
pid = 7310, glob = 7, var = 89, bufsize = 19 #child’s variables were changed
pid = 7309, glob = 6, var = 88, bufsize = 19 #parent’s copy was not changed
I am confused about the child's pid
if ((pid = fork()) < 0)
the pid is set as 0, but in the output it's pid is 7310.
How could that happen?
does the pid here is just a number rather than a process?
c
asked Nov 15 '18 at 3:44
JawSawJawSaw
4,57811837
add a comment |
I read such a snippet of code from an instruction
#include "apue.h"
int globvar = 6; /* external variable in initialized data */
char buf = "a write to stdoutn";
int main(void)
{
int var;
pid_t pid; /* automatic variable on the stack */
var = 88; if (write(STDOUT_FILENO, buf, sizeof(buf) - 1) != sizeof(buf) - 1)
err_sys("write error");
printf("before forkn"); /* we don’t flush stdout */
if ((pid = fork()) < 0) {
err_sys("fork error");
} else if (pid == 0) { /* child */
globvar++; /* modify variables */
var++;
} else {
sleep(2); /* parent */
}
printf("pid = %ld, glob = %d, var = %d, bufsize = %lun", (long)getpid(), globvar, var, sizeof(buf));
exit(0);
}
Run it and get outputs
$ ./a.out
a write to stdout
before fork
pid = 7310, glob = 7, var = 89, bufsize = 19 #child’s variables were changed
pid = 7309, glob = 6, var = 88, bufsize = 19 #parent’s copy was not changed
I am confused about the child's pid
if ((pid = fork()) < 0)
the pid is set as 0, but in the output it's pid is 7310.
How could that happen?
does the pid here is just a number rather than a process?
c
asked Nov 15 '18 at 3:44
JawSawJawSaw
4,57811837
I read such a snippet of code from an instruction
#include "apue.h"
int globvar = 6; /* external variable in initialized data */
char buf = "a write to stdoutn";
int main(void)
{
int var;
pid_t pid; /* automatic variable on the stack */
var = 88; if (write(STDOUT_FILENO, buf, sizeof(buf) - 1) != sizeof(buf) - 1)
err_sys("write error");
printf("before forkn"); /* we don’t flush stdout */
if ((pid = fork()) < 0) {
err_sys("fork error");
} else if (pid == 0) { /* child */
globvar++; /* modify variables */
var++;
} else {
sleep(2); /* parent */
}
printf("pid = %ld, glob = %d, var = %d, bufsize = %lun", (long)getpid(), globvar, var, sizeof(buf));
exit(0);
}
Run it and get outputs
$ ./a.out
a write to stdout
before fork
pid = 7310, glob = 7, var = 89, bufsize = 19 #child’s variables were changed
pid = 7309, glob = 6, var = 88, bufsize = 19 #parent’s copy was not changed
I am confused about the child's pid
if ((pid = fork()) < 0)
the pid is set as 0, but in the output it's pid is 7310.
How could that happen?
does the pid here is just a number rather than a process?
c
c
asked Nov 15 '18 at 3:44
JawSawJawSaw
4,57811837
asked Nov 15 '18 at 3:44
JawSawJawSaw
4,57811837
asked Nov 15 '18 at 3:44
JawSawJawSaw
4,57811837
asked Nov 15 '18 at 3:44
JawSawJawSaw
4,57811837
asked Nov 15 '18 at 3:44
JawSawJawSaw
4,57811837
4,57811837
add a comment |
add a comment |
1 Answer
1
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votes
fork() returns 0 in the child process, but in the printf statement you are printing the pid using getpid() which is the actual pid of the child process.
If you had used pid in the printf statement instead of getpid() you will see that it prints 0.
answered Nov 15 '18 at 4:15
P.WP.W
15k31452
add a comment |
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1 Answer
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votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
fork() returns 0 in the child process, but in the printf statement you are printing the pid using getpid() which is the actual pid of the child process.
If you had used pid in the printf statement instead of getpid() you will see that it prints 0.
answered Nov 15 '18 at 4:15
P.WP.W
15k31452
add a comment |
fork() returns 0 in the child process, but in the printf statement you are printing the pid using getpid() which is the actual pid of the child process.
If you had used pid in the printf statement instead of getpid() you will see that it prints 0.
answered Nov 15 '18 at 4:15
P.WP.W
15k31452
add a comment |
fork() returns 0 in the child process, but in the printf statement you are printing the pid using getpid() which is the actual pid of the child process.
If you had used pid in the printf statement instead of getpid() you will see that it prints 0.
answered Nov 15 '18 at 4:15
P.WP.W
15k31452
fork() returns 0 in the child process, but in the printf statement you are printing the pid using getpid() which is the actual pid of the child process.
If you had used pid in the printf statement instead of getpid() you will see that it prints 0.
answered Nov 15 '18 at 4:15
P.WP.W
15k31452
answered Nov 15 '18 at 4:15
P.WP.W
15k31452
answered Nov 15 '18 at 4:15
P.WP.W
15k31452
answered Nov 15 '18 at 4:15
P.WP.W
15k31452
15k31452
add a comment |
add a comment |
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