segmentation fault when accessing pointer element

June 18, 2023

I have been attempting to parse a string into separate tokens for a command line interface for a project of mine, I have created this function to do it:

char **string_parser(char *input) {
    char **output = (char **) malloc(sizeof(input));
    int word_num = 0;
    int word_index = 0;

    for(int i = 0; i < strlen(input); i++) {
        if(input[i] == ' ') {
            output[word_num][word_index] = '\0';
            word_index = 0;
            word_num++;
            continue;
        }

        if(input[i] == '\0') {
            output[word_num][word_index] = '\0';
            break;
        }

        output[word_num][word_index] = input[i];
        word_index++;
    }

    return output;
}

but it segmentation faults after 1 iteration,

I have been calling the function on:
char *input = "this is a parser test.";
any help is greatly appreciated.

>Solution :

This memory allocation

char **output = (char **) malloc(sizeof(input));

is incorrect. It allocates memory only for one object of the type char *.

Moreover the allocated memory is uninitialized. So at least this statement (and similar statements)

output[word_num][word_index] = '\0';

invokes undefined behavior.

You need to allocate as many pointers as there are words in the source string. And for each word you also need to allocate a character array to store the extracted word.

And calling the function strlen in the for loop

for(int i = 0; i < strlen(input); i++) {

is redundant and inefficient.

Pay attention to that the function parameter should be declared with qualifier const because the source string is not changed within the function.

Here is a demonstration program that shows an approach to the function implementation. The function has drawbacks because it does not check that memory allocations were successfull. You will need to do that yourself. The last pointer in the allocated array of pointers is set to NULL to allow to determine the number of valid extracted words.

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

char ** string_parser( const char *input ) 
{
    char **output = NULL;
    size_t word_num = 0;

    for (const char *delim = " \t"; *input != '\0'; )
    {
        input += strspn( input, delim );

        if (*input)
        {
            size_t n = strcspn( input, delim );

            output = realloc( output, ( word_num + 1 ) * sizeof( char * ) );

            output[word_num] = malloc( n + 1 );

            memcpy( output[word_num], input, n );
            output[word_num][n] = '\0';
            ++word_num;

            input += n;
        }
    }

    output = realloc( output, ( word_num + 1 ) * sizeof( char * ) );

    output[word_num] = NULL;

    return output;
}

int main( void )
{
    const char *input = "this is a parser test.";

    char **output = string_parser( input );

    for (char **p = output; *p != NULL; ++p)
    {
        puts( *p );
    }

    for (char **p = output; *p != NULL; ++p)
    {
        free( *p );
    }
    free( output );
}

The program output is