Making a terminal in c

Question

When I run this c snippet, it outputs something really random every time, and then segfaults... Code:

#include <stdio.h>
#include <stdlib.h>

int parse(void) {
  int i = 0;
  int system(const char *command);
  char line[1024];
  scanf("%[^\n]", line);
  system(line);
  do {
    line[i] = "\0";
    i++;
  } while (i != 1024);
  parse();
}

int main(void) {
  parse();
  return 0;
}

What I expected was a prompt, and when any shell command is entered (I used pwd for my testing), the output of the command prints and the prompt returns. And this is what actually happened:

Output:

> pwd
/home/runner/c-test
sh: 1: �: not found
sh: 1: : not found
sh: 1: ׀: not found
signal: segmentation fault (core dumped)

First of all, why do you provide your own prototype for the `system` function, instead of using the one provided by ``? — Some programmer dude, Nov 03 '22 at 12:29
Secondly, `line[i]` is a single character, a single `char`. `"\0"` is a null.terminated string, an array of two elements (both being the null-terminator) which decays to a pointer to its first element. Why do you assign this pointer to the character `line[i]`? Doesn't the compiler issue a warning about this? Then you need to enable more warnings, and treat them as errors. — Some programmer dude, Nov 03 '22 at 12:30
Fourthly, you don't output any prompt, so why do you expect one? — Some programmer dude, Nov 03 '22 at 12:35
And you're not really making a terminal, but rather a program that works somewhat similar to a *shell*. — Some programmer dude, Nov 03 '22 at 12:37
you call yourself - your stack consumption will increase with every iteration — 0___________, Nov 03 '22 at 12:38
@0___________ it is tail recursion, which gets optimized to a loop anyway. Not saying it is good, but it will not consume more stack memory. — Tenobaal, Nov 03 '22 at 13:45
@Tenobaal no it will not,as it has to add stack space for local variables on every call https://godbolt.org/z/T3Peqh45n Stack consumption will grow — 0___________, Nov 03 '22 at 13:51
@0___________ gcc does, I tried it. The lifetime of the local variables values end all end with the recursive call. "-foptimize-sibling-calls Optimize sibling and tail recursive calls. Enabled at levels -O2, -O3, -Os." from the gcc docs — Tenobaal, Nov 03 '22 at 14:05
Also it it explained [here](https://stackoverflow.com/questions/15518882/how-exactly-does-tail-recursion-work) — Tenobaal, Nov 03 '22 at 14:10
@Tenobaal gcc does not in this case. This case can't be tail optimized out. see my link. — 0___________, Nov 03 '22 at 14:46
@Tenobaal because it has to keep copy of the local variables on every call. — 0___________, Nov 03 '22 at 16:08
@0___________ but why? They are not needed anymore at the point of the recursive call, which should allow the compiler to optimize it — Tenobaal, Nov 03 '22 at 18:39

0___________ · Accepted Answer · 2022-11-03T16:09:31.537

1

Print prompt
Do not use scanf
Use static storage duration buffer

#include <stdio.h>
#include <stdlib.h>


int parse(void) 
{
  static char line[1024];
  while(1)
  {
      printf(">>>");
      if(!fgets(line, 1024, stdin)) return 1;
      system(line);
  }
}

int main(void) {
  parse();
}

https://www.onlinegdb.com/wmPB3ZGNQ

edited Nov 03 '22 at 16:09

answered Nov 03 '22 at 13:13

0___________

60,014
4
34
74

1

Are you really proposing a solution using recursion? – Support Ukraine Nov 03 '22 at 13:18
@SupportUkraine Does not matter as recurson will be optimized out https://godbolt.org/z/qjqEnrnYT – 0___________ Nov 03 '22 at 13:54
2

This optimization is just a random effect, you teach bad habits. -- Oh, and you do not explain your solution. – the busybee Nov 03 '22 at 14:51

Support Ukraine · Answer 2 · 2022-11-03T13:26:43.913

The reason for your crash is most likely explained by the following:

scanf("%[^\n]", line);

means keep reading until there is a newline in the input stream. So once it completes the next character in the input stream is a newline.

The next time you do

scanf("%[^\n]", line);

that newline is still the first character, so the scanf will return without waiting for the user to type any input.

Since you are doing recursion this will happen again and again and again.... Then most likely the system crashes due to stack overflow after a while.

Solution: Read that newline from the input stream before calling scanf again.

Besides that you should:

Remove int system(const char *command);
Put a maximum field width on scanf
Check the return value of scanf
Change "\0" to '\0' (or delete the whole do-while loop as you don't need it)
Use a while(1) { ... } loop instead of recursive calls

Solution: do not use `scanf` with an unbounded buffer like that. — Dúthomhas, Nov 03 '22 at 13:14
@Dúthomhas The real solution would be to delete all the code and do it completely different. But the point of this answer is to explain why OPs code behaves the way it does. I do not intend to rewrite OPs code. We are not a free coding service — Support Ukraine, Nov 03 '22 at 13:18

Making a terminal in c

2 Answers2