Question

I am having an issue with dynamic arrays and malloc. I am fairly new to C so please excuse (and advise on) any rookie mistakes.

The problem is I create an array (input_string in this case) and pass it to func2. Then in func2 I do a test, print out the first element of input_string.

This works as expected in the first printout before the malloc, but after the malloc it doesn’t print anything. This seems weird to me since in between the to printf statements I do nothing to the input_string.

I’m assuming that I am dealing with these arrays incorrectly, but I’m unsure.

Here is a snippet of the code in question:

Updated

... // includes not in snippet

/* CONSTANTS */
#define LINE_LEN 80

/* Function declarations */
char* func1(void);
char* func2(int tl, char* input_string);

int main(void) {
    char* input_string;
    int tab_length;
    char* output_string;

    input_string = func1();
    output_string = func2(tl, input_string);

    return 0;
}

char* func1(void) {
    char cur_char;
    char* input_ptr;
    char input_string[LINE_LEN];
    while ((cur_char = getchar()) != '\n' && chars_read < 80) {
        // iterate and create the array here
    }
    input_ptr = &input_string[0]; /* set pointer to address of 0th index */
    return input_ptr;
}

char* func2(int tl, char* input_string) {
    int n = 0, output_idx = 0;
    char* output_ptr;
    printf("\nBefore malloc: %c ", *(input_string));
    output_ptr = malloc(tab_length * chars_read+1);
    if (output_ptr == NULL) {
            printf("Failed to allocate memory for output_ptr.\nExiting");
            exit(1);
    }
    printf("\nAfter malloc: %c ", *(input_string));
    ...
    return output_ptr;
}

P.s.: Any undeclared variables have been declared outside of this snippet of code.

Update

Thanks for all the replies and advice. It is very much appreciated.

Answer 1

func1 Returns a pointer to a temporary string. You did not allocate it. This will produce undefined behaviour.

Instead you should do this:

char* func1(void) {
    char cur_char;
    char* input_ptr = (char*)malloc(LINE_LEN * sizeof(char));
    while ((cur_char = getchar()) != '\n' && chars_read < 80) {
        // iterate and create the array here
    }
    return input_ptr;
}

Interestingly, you did use malloc inside func2.

When you are finished with it, you need to call free to release the memory.

int main(void) {
    char* input_string;
    int tab_length;
    char* output_string;

    input_string = func1();
    output_string = func2(tl, input_string);

    free(input_string);
    free(output_string);    

    return 0;
}