Given the code below, under what circumstances will Lines A, B, and C be reached
ID: 3844573 • Letter: G
Question
Given the code below, under what circumstances will Lines A, B, and C be reached (look to the comments for line identification)?
#include <sys/types.h>
#include <stdio.h>
#include <unistd.h>
int main()
{
pid t pid; /* fork a child process */
pid = fork();
if (pid < 0)
{ /* error occurred */
fprintf(stderr, "Fork Failed");
return 1; /* Line A */ }
else if (pid == 0) { /* child process */
execlp("/bin/ls","ls",NULL);
printf("Hello World "); /* Line B */ }
else { /* parent process */
/* parent will wait for the child to complete */ wait(NULL);
printf("Child Complete"); /* Line C */ }
return 0;
}
Explanation / Answer
First we need to understand what fork() is doing. It a build-in system call that results creation of the child process with its PID. Its caller function becomes this newly created process's Parent. It has to be understood that usage of fork() causes 2 values to be returned one to the child and one to the parent.
Below are different values returned by fork().
Negative Value: creation of a child process failed.
Zero: Returned to the newly created child process.
Positive: Returned to parent or caller.
Keeping this in mind, we see that
Line A is reached with fork() process failed to generate a child process.
Line B is reached when a new child process is created and then is replaced with execlp command with the selected executable /bin/ls. This replacement does not causes any changes in PID of the newly created child process. Once that replacement happens Line B is reached.
Line C is reached when fork() command created PID > 0 i.e. process has returned to parent is waiting with wait(NULL) command for the child process to complete execution. Child process in this case is the one enclosed within (pid == 0) condition. Once child completes Line C is reached.
Based on the above information the program can have only 2 possible scenarios:
1) Program fails to execute, fails to generate a child process with fork() system call. In this case only Line A is the output.
2) Program runs normally, in that case it outputs LINE B and then LINE C.
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