Error Segmentation fault without indentification

Question

Hello, my friends!

I am creating a program that will read a text. This text should be allocated one line at a time (up to 75 characters per line).

The program receives, in the input , the user’s text until the string "the end!" is typed.

However when running the program, I am having the error: Segmentation fault (core dumped). (Detail: This error is common to me, and several times I have tried to solve it, but without success). Please tell us why this error happens, not only in this code but also its common causes.)

Later, I will use the stringUpper function to capitalize my text.

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>
#define MAX_CHAR 75

const char THE_END[] = "the end!";
void stringUpper(char*, int);

void main() {
    char **texto;
    int i = 0;
    int j;
    texto = NULL;
    for( ; ; ) {
        texto = (char**)realloc(texto,(i+1)*sizeof(char*));
        texto[i] = (char*)malloc(MAX_CHAR*sizeof(char));
        fgets(texto[i],MAX_CHAR,stdin);
        texto[strlen(texto[i]-1)] = '\0'; // troca o '\n' (úlimo dígito da string) pelo terminador nulo
        if(strcmp(THE_END,texto[i]) == 0) break;
        i++;
    }

    for(j = 0; j < i; i++) {
        free(texto[j]);
    }

    free(texto);






}

void stringUpper(char *s, int tam) {
    int i = 0;
    for(i = 0; i < tam; i++) {
        s[i] = toupper(s[i]);
    }
}

Answer 1

There are some problems in your program. strcmp, in the case of equality, returns 0, not the other way around. In addition, the fgets has the drawback of not eliminating the \n at the end of the input. You can fix this by replacing it with a \0 or by putting it in the comparison. A possible solution then would be:

if (strcmp("the end!\n", texto[i]) == 0) break;

Also, don’t forget that you need to give free not only at the address of texto, but on each of the lines as well.

I couldn’t recreate your Segfault, but the problem could be in this realloc which is called when the pointer texto initially contains trash. Try making a malloc for him before.

Answer 2

Friend Emoon found the first part of the problem in his answer, that the fgets puts a \n at the end of the string read and need to give free in each of the strings allocated.

As for this \n, the solution would be to replace it with \0 and that’s what you did. Or rather, that’s what you tried to make!

You made this code to replace the \n for \0:

texto[strlen(texto[i]-1)] = '\0';

That is wrong. That would be right:

texto[i][strlen(texto[i]) - 1] = '\0';

Then, at the time of shifting the strings, see your loop:

for(j = 0; j < i; i++)

That tie is wrong! It was meant to be j++ instead of i++.

Fixing these little problems, the code worked for me as expected.

Answer 3

I finished the code. It’s like this now:

 #include <stdio.h> 
 #include <stdlib.h>
 #include <ctype.h>
 #include <string.h>
 #define MAX_CHAR 75

const char THE_END[] = "the end!";
void stringUpper(char*, int);
void clearBuffer();
void textUpper(char**,int);
void printText(char**,int);
char **desalocaTexto(char**,int);

void main() {
    char **texto;
    int i = 0;
    int j;
    texto = NULL;
    for( ; ; ) {
        texto = (char**)realloc(texto,(i+1)*sizeof(char*));
        texto[i] = (char*)malloc(MAX_CHAR*sizeof(char));
        //clearBuffer();
        fgets(texto[i],MAX_CHAR,stdin);
        texto[i][strlen(texto[i]) - 1] = '\0';// troca o '\n' (úlimo dígito da string) pelo terminador nulo

        if(strcmp(THE_END,texto[i]) == 0) break;
        i++;
    }

    textUpper(texto, i);
    printText(texto,i);

    texto = desalocaTexto(texto,i);
}

void stringUpper(char *s, int tam) {
    int i = 0;
    for(i = 0; i < tam; i++) {
        s[i] = toupper(s[i]);
    }
}


void clearBuffer() {
    char c;
    while((c = getchar()) != '\n' && c != EOF);

}

void textUpper(char **texto ,int tam) {
    int i;
    for(i = 0; i < tam; i++) {
        stringUpper(texto[i],MAX_CHAR);
    }
}

void printText(char **texto,int tam) {
    int i;
    for(i = 0; i < tam; i++) {
        printf("%s\n",texto[i]);
    }

}

char **desalocaTexto(char **t ,int linhas) {
    int j;
    for(j = 0; j < linhas; j++) {
        free(t[j]);
    }

    free(t);
    return NULL;
}

Guys, considering I’m a beginner, is the code well done in your opinion? How long on average can I become a C professional?