C: forking new process fails

Question

The fork in the following code does not work! It never goes into run function. Can you please tell me what is wrong here?

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
void run(char* a)
{
    char* k=a;
    int i;
    for(i=0;i<3;i++)
      k[i]='s';
    printf("hi");
    return;
}
int main()
{   
    char* a="cbd";
    pid_t j;
    j=fork();
    printf("%d\t",j);
    int y;
    if(j==0)
      run(a);
    int i;
    for(i=0;i<3;i++)
      printf("%c\t",a[i]);
 return 0;
}

The output is

24180  c b d

I would use `stderr` with error checking. like this: `if (j < 0) perror ("couldn't fork")` — tay10r, Jul 10 '13 at 04:47

score 4 · Accepted Answer · answered Jul 10 '13 at 04:50

4

Actually run() function is called. But the problem with k[i]='s'; statement. "cbd" is a string literal and read-only.

Allocate memory for string locally and pass the address.

like @Andy mentioned, child process crashed.

answered Jul 10 '13 at 04:50

Jeyaram

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No, because the "hi" is not printing – smo Jul 10 '13 at 04:52
Just put that printf() before for loop. – Jeyaram Jul 10 '13 at 04:52
1

Arrgh... was just typing my answer when you beat me to it. This is correct. "hi" does not print because **the child process crashes** trying to assign to k[0]. – Andy Ross Jul 10 '13 at 04:53
change `char *` to `char a[]` – tay10r Jul 10 '13 at 04:56
@smo refer this http://stackoverflow.com/questions/17111140/string-in-function-parameter gives you some idea. – Jeyaram Jul 10 '13 at 04:56
@TaylorFlores I want to use char* only. Some body mentioned allocating memory. – smo Jul 10 '13 at 05:02
@smo you should read up on the difference between arrays and pointers. You can either use an array or allocate memory. It's hard to see the point of using `char *` in this case. – tay10r Jul 10 '13 at 05:07

Greg Prisament · Answer 2 · 2013-07-10T05:12:43.313

1

The problem is with the k[i] = 's', which is causing the process to crash with a bus error. The reason is, you are trying to edit the string literal "cbd" which is placed in the RODATA section of the object file, which gets stored in read-only memory. If change the first line of main to char a[3] = {'c', 'b', 'd'}; your program works as expected.

edited Jul 10 '13 at 05:12

answered Jul 10 '13 at 04:56

Greg Prisament

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I wouldn't suggest adding any object file terminology, it makes your answer confusing to the OP who is still learning basic `C` – tay10r Jul 10 '13 at 04:57

score 1 · Answer 3 · answered Jul 10 '13 at 04:57

1

As for the fork(), there is no problem. It's your run() function. k is a pointer to a const string, so k[0]='s' tend to report access violation.

answered Jul 10 '13 at 04:57

Manas

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C: forking new process fails

3 Answers3