Segmentation fault (core dumped)

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There is something wrong with the pointer in this program, which aims to test a function that counts the number of characters in a string. For some reason he can’t access the memory he points to. For this program I created a source and header:

Main code(source):

#include <stdio.h>
#include "mystring.h"

 int main() {

 char palavra[20];

 puts("Digite uma paravra qualquer:");
 fgets(palavra, sizeof(palavra), stdin);
 printf("%d", mystrlen(palavra));

 return 0;
}

mystring. h code containing the function mystrlen(header):

int mystrlen(char *str) {
    int i = 0; // Contadora
    for (; (*str != '\0' || *str != '\n'); str++, i++);
    // Ponteiro percorre a string até o '\0' ou '\n'

    return i;
}

Below is the code of a similar example I did. This example, unlike the above program, works here.

int main() {

    char str[8] = "Exemplo", *ptr = str;
    int i;

    for (; *ptr != '\0'; ptr++, i++);

    printf("%d", i);

    return 0;
}

What could be the problem? Thank you.

c

  • To debug segmentation fault I strongly recommend that you use a debugger that allows you to run the program one step at a time, such as ddd or gdb. These debuggers will also inform you right the line number with the segmentation failure, the status of the call stack and the value of your variables.

  • In the second example the variable i has not been initialized. This may be a source of unpredictable behavior. btw, I’m not a fan of this compact style of writing C, a-la K&R. I think things get clearer with a command per line.

  • Well, the curious thing is that the second example works...

  • 1

    I don’t understand... what is the program that gives dick? Please include the entire program that you are running, without breaking it in parts.

  • I edited the question to try to make it clearer

  • 1

    in English: this line: (*str != '\0' || *str != '\n'); has a problem. with the || one or the other of the Expressions will Always be true the line should be: (*str != '\0' && *str != '\n');

  • 1

    in English: in the Second posted code, the variable i is not initialized Suggest replace: int i; with int i=0;

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There is an error in your loop. You are using OR instead of E:

// versão atual
(*str != '\0' || *str != '\n')

// versão correta
(*str != '\0' && *str != '\n')

The result is that the condition of the loop is always true.

  • Funny, I had already thought about replacing the operator OR by the E, but I thought that in this case it would not make a difference (continue in the loop if the address value is different from \0 OR OTHER THAN \n). I didn’t think that would be the problem because the error in accessing the memory address was exactly on the loop line, so I thought the problem was that somehow he couldn’t get access to the first address of the string. Anyway, I made that change and it worked out here, thank you.

  • I would say that this is an argument for writing a command per line rather than trying to write these super smart loops. : ) Error messages become clearer.

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