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Why does my fork() doesn't fork a child and output the print statements?

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0

I am writing a c program in Ubuntu, and in the code I am using fork() to generate 5 different children. But, when i compile and run my program, no children are created, and the printf("Test") that I put inside the three if statements (for case fork() == 0, > 0, < 0), only in > 0, there in the output of the printf statements.

Actually a while ago, the fork() runs just fine, but after I continue my work on the program, it suddenly does not work.

Why is this happening and how should I fix it?

for (i = 0; i < proc; ++i)
 printf("In for %d",i);
 // TODO
 int fork_result = fork();
 if (fork_result == 0) // Create child process
 child_pids[i] = getpid();
 printf("Test 5");
 printf("In if %d",i);
 break;
 
 else if(fork_result < 0)
 printf("Fork failed");
 
 else if (fork_result > 0)
 printf("Parent");
 

Well, the expected outcome is that it would contain "Test 5" or "In if"

Actual output:

In for 0In for 1In for 2In for 3In for 4

Which the actual output does not contain any "Test 5"

c linux fork

share|improve this question

edited Mar 27 at 17:47

Jonathan Leffler

591k9797 gold badges709709 silver badges10651065 bronze badges

asked Mar 27 at 17:37

Just A Bad ProgrammerJust A Bad Programmer

4366 bronze badges

• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  End your printing statements with a newline; it maximizes the chance that the printed information will appear. Consider using fflush(stdout) liberally. But newlines are key: printf("Test 5n"): etc. (When debugging such code, consider printing the PID of each process in each output: printf("%d: Test 5n", (int)getpid());.) . Also think about whether break; is appropriate or whether exit(EXIT_SUCCESS); would be better.
  
  – Jonathan Leffler
  Mar 27 at 17:42
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I am unable to reproduce your problem. Your code runs for me and prints Test 5 the correct number of times.
  
  – larsks
  Mar 27 at 17:45
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  We need enough code to replicate the problem. Your code only shows the child buffering some output. If later code doesn't actually flush it, then of course it won't show up.
  
  – David Schwartz
  Mar 27 at 18:11
  

  
  
  
  

add a comment | 

0

I am writing a c program in Ubuntu, and in the code I am using fork() to generate 5 different children. But, when i compile and run my program, no children are created, and the printf("Test") that I put inside the three if statements (for case fork() == 0, > 0, < 0), only in > 0, there in the output of the printf statements.

Actually a while ago, the fork() runs just fine, but after I continue my work on the program, it suddenly does not work.

Why is this happening and how should I fix it?

for (i = 0; i < proc; ++i)
 printf("In for %d",i);
 // TODO
 int fork_result = fork();
 if (fork_result == 0) // Create child process
 child_pids[i] = getpid();
 printf("Test 5");
 printf("In if %d",i);
 break;
 
 else if(fork_result < 0)
 printf("Fork failed");
 
 else if (fork_result > 0)
 printf("Parent");
 

Well, the expected outcome is that it would contain "Test 5" or "In if"

Actual output:

In for 0In for 1In for 2In for 3In for 4

Which the actual output does not contain any "Test 5"

c linux fork

share|improve this question

edited Mar 27 at 17:47

Jonathan Leffler

591k9797 gold badges709709 silver badges10651065 bronze badges

asked Mar 27 at 17:37

Just A Bad ProgrammerJust A Bad Programmer

4366 bronze badges

• 
  
  
  

  
  4
  

  
  
  
  
  
  

  
  
  End your printing statements with a newline; it maximizes the chance that the printed information will appear. Consider using fflush(stdout) liberally. But newlines are key: printf("Test 5n"): etc. (When debugging such code, consider printing the PID of each process in each output: printf("%d: Test 5n", (int)getpid());.) . Also think about whether break; is appropriate or whether exit(EXIT_SUCCESS); would be better.
  
  – Jonathan Leffler
  Mar 27 at 17:42
  
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  I am unable to reproduce your problem. Your code runs for me and prints Test 5 the correct number of times.
  
  – larsks
  Mar 27 at 17:45
  

  
  
  
  


• 
  
  
  

  
  

  
  
  
  
  
  

  
  
  We need enough code to replicate the problem. Your code only shows the child buffering some output. If later code doesn't actually flush it, then of course it won't show up.
  
  – David Schwartz
  Mar 27 at 18:11
  

  
  
  
  

add a comment | 

I am writing a c program in Ubuntu, and in the code I am using fork() to generate 5 different children. But, when i compile and run my program, no children are created, and the printf("Test") that I put inside the three if statements (for case fork() == 0, > 0, < 0), only in > 0, there in the output of the printf statements.

Actually a while ago, the fork() runs just fine, but after I continue my work on the program, it suddenly does not work.

Why is this happening and how should I fix it?

for (i = 0; i < proc; ++i)
 printf("In for %d",i);
 // TODO
 int fork_result = fork();
 if (fork_result == 0) // Create child process
 child_pids[i] = getpid();
 printf("Test 5");
 printf("In if %d",i);
 break;
 
 else if(fork_result < 0)
 printf("Fork failed");
 
 else if (fork_result > 0)
 printf("Parent");
 

Well, the expected outcome is that it would contain "Test 5" or "In if"

Actual output:

In for 0In for 1In for 2In for 3In for 4

Which the actual output does not contain any "Test 5"

c linux fork

share|improve this question

edited Mar 27 at 17:47

Jonathan Leffler

591k9797 gold badges709709 silver badges10651065 bronze badges

asked Mar 27 at 17:37

Just A Bad ProgrammerJust A Bad Programmer

4366 bronze badges

for (i = 0; i < proc; ++i)
 printf("In for %d",i);
 // TODO
 int fork_result = fork();
 if (fork_result == 0) // Create child process
 child_pids[i] = getpid();
 printf("Test 5");
 printf("In if %d",i);
 break;
 
 else if(fork_result < 0)
 printf("Fork failed");
 
 else if (fork_result > 0)
 printf("Parent");
 

In for 0In for 1In for 2In for 3In for 4

share|improve this question

edited Mar 27 at 17:47

Jonathan Leffler

591k9797 gold badges709709 silver badges10651065 bronze badges

asked Mar 27 at 17:37

Just A Bad ProgrammerJust A Bad Programmer

4366 bronze badges

share|improve this question

edited Mar 27 at 17:47

Jonathan Leffler

591k9797 gold badges709709 silver badges10651065 bronze badges

asked Mar 27 at 17:37

Just A Bad ProgrammerJust A Bad Programmer

4366 bronze badges

edited Mar 27 at 17:47

Jonathan Leffler

591k9797 gold badges709709 silver badges10651065 bronze badges

edited Mar 27 at 17:47

Jonathan Leffler

591k9797 gold badges709709 silver badges10651065 bronze badges

asked Mar 27 at 17:37

Just A Bad ProgrammerJust A Bad Programmer

4366 bronze badges

asked Mar 27 at 17:37

Just A Bad ProgrammerJust A Bad Programmer

4366 bronze badges

1 Answer
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1

The child process continues to execute the code of the caller. The caller is most likely not expecting to execute the parent and all the children. You should exit the child before that happens:

for (i = 0; i < proc; ++i)
 printf("In for %d",i);
 fflush(stdout); // <--- here
 // TODO
 int fork_result = fork();
 if (fork_result == 0) // Create child process
 child_pids[i] = getpid();
 printf("Test 5");
 printf("In if %d",i);
 exit(0); // <---- must have this
 break;
 
 else if(fork_result < 0)
 printf("Fork failed");
 
 else if (fork_result > 0)
 printf("Parent");
 

// without the exit above the child will execute this code:
Some code

You must flush stdout, otherwise the children and the parent share the same prefix text in buffer, and both wil print the same prefix, when it finally flushes it. Eventually this sharing will confuse anybody trying to analyze the output.

Note: once there is an exit call, stdout will be flushed correctly.

Also, note that the different printouts might be interleaved. Putting them on different lines, with a distinguishing prefix (like i and/or pid) can help debugging.

Look at this run example

share|improve this answer

edited Mar 27 at 20:01

answered Mar 27 at 19:44

Michael VekslerMichael Veksler

6,22211 gold badge99 silver badges2626 bronze badges

add a comment | 

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1

The child process continues to execute the code of the caller. The caller is most likely not expecting to execute the parent and all the children. You should exit the child before that happens:

for (i = 0; i < proc; ++i)
 printf("In for %d",i);
 fflush(stdout); // <--- here
 // TODO
 int fork_result = fork();
 if (fork_result == 0) // Create child process
 child_pids[i] = getpid();
 printf("Test 5");
 printf("In if %d",i);
 exit(0); // <---- must have this
 break;
 
 else if(fork_result < 0)
 printf("Fork failed");
 
 else if (fork_result > 0)
 printf("Parent");
 

// without the exit above the child will execute this code:
Some code

You must flush stdout, otherwise the children and the parent share the same prefix text in buffer, and both wil print the same prefix, when it finally flushes it. Eventually this sharing will confuse anybody trying to analyze the output.

Note: once there is an exit call, stdout will be flushed correctly.

Also, note that the different printouts might be interleaved. Putting them on different lines, with a distinguishing prefix (like i and/or pid) can help debugging.

Look at this run example

share|improve this answer

edited Mar 27 at 20:01

answered Mar 27 at 19:44

Michael VekslerMichael Veksler

6,22211 gold badge99 silver badges2626 bronze badges

add a comment | 

1

The child process continues to execute the code of the caller. The caller is most likely not expecting to execute the parent and all the children. You should exit the child before that happens:

for (i = 0; i < proc; ++i)
 printf("In for %d",i);
 fflush(stdout); // <--- here
 // TODO
 int fork_result = fork();
 if (fork_result == 0) // Create child process
 child_pids[i] = getpid();
 printf("Test 5");
 printf("In if %d",i);
 exit(0); // <---- must have this
 break;
 
 else if(fork_result < 0)
 printf("Fork failed");
 
 else if (fork_result > 0)
 printf("Parent");
 

// without the exit above the child will execute this code:
Some code

You must flush stdout, otherwise the children and the parent share the same prefix text in buffer, and both wil print the same prefix, when it finally flushes it. Eventually this sharing will confuse anybody trying to analyze the output.

Note: once there is an exit call, stdout will be flushed correctly.

Also, note that the different printouts might be interleaved. Putting them on different lines, with a distinguishing prefix (like i and/or pid) can help debugging.

Look at this run example

share|improve this answer

edited Mar 27 at 20:01

answered Mar 27 at 19:44

Michael VekslerMichael Veksler

6,22211 gold badge99 silver badges2626 bronze badges

add a comment | 

The child process continues to execute the code of the caller. The caller is most likely not expecting to execute the parent and all the children. You should exit the child before that happens:

for (i = 0; i < proc; ++i)
 printf("In for %d",i);
 fflush(stdout); // <--- here
 // TODO
 int fork_result = fork();
 if (fork_result == 0) // Create child process
 child_pids[i] = getpid();
 printf("Test 5");
 printf("In if %d",i);
 exit(0); // <---- must have this
 break;
 
 else if(fork_result < 0)
 printf("Fork failed");
 
 else if (fork_result > 0)
 printf("Parent");
 

// without the exit above the child will execute this code:
Some code

You must flush stdout, otherwise the children and the parent share the same prefix text in buffer, and both wil print the same prefix, when it finally flushes it. Eventually this sharing will confuse anybody trying to analyze the output.

Note: once there is an exit call, stdout will be flushed correctly.

Also, note that the different printouts might be interleaved. Putting them on different lines, with a distinguishing prefix (like i and/or pid) can help debugging.

Look at this run example

share|improve this answer

edited Mar 27 at 20:01

answered Mar 27 at 19:44

Michael VekslerMichael Veksler

6,22211 gold badge99 silver badges2626 bronze badges

for (i = 0; i < proc; ++i)
 printf("In for %d",i);
 fflush(stdout); // <--- here
 // TODO
 int fork_result = fork();
 if (fork_result == 0) // Create child process
 child_pids[i] = getpid();
 printf("Test 5");
 printf("In if %d",i);
 exit(0); // <---- must have this
 break;
 
 else if(fork_result < 0)
 printf("Fork failed");
 
 else if (fork_result > 0)
 printf("Parent");
 

// without the exit above the child will execute this code:
Some code

share|improve this answer

edited Mar 27 at 20:01

answered Mar 27 at 19:44

Michael VekslerMichael Veksler

6,22211 gold badge99 silver badges2626 bronze badges

answered Mar 27 at 19:44

Michael VekslerMichael Veksler

6,22211 gold badge99 silver badges2626 bronze badges

answered Mar 27 at 19:44

Michael VekslerMichael Veksler

6,22211 gold badge99 silver badges2626 bronze badges