Function's pointer return keeps changing previous values in an array

Question

im trying to fill a 2d array with strings, problem is that i manage to fill the first index, however when i proceed to the next string it keeps changing the previous indexes. probably an issue with the pointer, this is the relevant code.

char* get_song_name(const char* song)
{
    strip(song);
    FILE* fp = fopen(song, "r");
    char str[9999];
    while(!feof(fp))
    {
        fgets(str,9999,fp);
        puts(str);
        strip(str);
        char* s = str;
        return s;
    }

` DIFFERENT FUNCTION:
for(i=0;i<lines;i++)
    {
        char *st = fgets(buff, 250, fp);
        st = create_path("c:\\Users\\Marian\\Desktop\\beatles", st);
        name[i] = get_song_name(st); //WORKS HOWEVER CHANGES PRVIOUS INDEXES VALUE TOO
    }`

The `get_song_name` opens the same file over and over again, every time reading one single line from the beginning and returning it. And what's worse, it returns a pointer to the local `str` array, a variable whose life-time will end when the function returns making the returned pointer invalid. — Some programmer dude, Dec 03 '21 at 14:07

tstanisl · Accepted Answer · 2021-12-03T14:25:00.140

2

You need to dynamically allocate the string so its lifetime does not end then the functions exits.

Just replace

return s;

with

return strdup(s);

EDIT

As OP is not allowed to use string.h here one can find an implementation of strdup() found in https://stackoverflow.com/a/37132824/4989451

#include <stdlib.h>

char *ft_strdup(char *src)
{
    char *str;
    char *p;
    int len = 0;

    while (src[len])
        len++;
    str = malloc(len + 1);
    p = str;
    while (*src)
        *p++ = *src++;
    *p = '\0';
    return str;
}

edited Dec 03 '21 at 14:25

answered Dec 03 '21 at 14:16

tstanisl

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And if im not allowed to use ? i tried to do dynamic allocation but i couldn't quite make it work. – noor napso Dec 03 '21 at 14:21
1

@noornapso, see updated answer – tstanisl Dec 03 '21 at 14:25

Vlad from Moscow · Answer 2 · 2021-12-03T14:13:04.083

This function

char* get_song_name(const char* song)

can invoke undefined behavior because it returns an invalid pointer that points to a local array of the function that will not be alive after exiting the function

char str[9999];
//...
char* s = str;
return s;

Moreover the function always returns the same pointer (the address of the first element of the local array). So this loop

for(i=0;i<lines;i++)
    {
        char *st = fgets(buff, 250, fp);
        st = create_path("c:\\Users\\Marian\\Desktop\\beatles", st);
        name[i] = get_song_name(st); //WORKS HOWEVER CHANGES PRVIOUS INDEXES VALUE TOO
    }`

does not make a sense.

You need to allocate dynamically a character array within the function get_song_name a pointer to which will be returned from the function.

Function's pointer return keeps changing previous values in an array

2 Answers2

EDIT