How can I access a (const char*) from a function within other function?

Question

I am trying to access a const char* created in function1 in function2 in order to use it for some operations. It has to be in a different function for several reasons.

I've tried using this code in function2:

const char* str[1024] = { function1() };

but I had no success whatsoever. If I try

printf("%s\n", str[1]);

in function2, it just prints (null).

I've also tried using malloc, but I had no success.

//main function {it does its thing, it wouldn't interest us}
//function1 {it creates a const char* var[1024];)
//function2 {here I want to use the const char* var[1024]; from function1}

Observation: In function1, the const char* prints just fine what it needs to print.

I wish I could find a way to solve this. Thank you for your patience!

Later edit for code:

const char* function1()
{
    lang = fopen("lang.csv", "r");
    int i = 0;
    char line[1024];
    const char* word[1024];
    char num[] = { 1 , 2 };
    while (fgets(line, 1024, lang))
    {
        char* tmp = _strdup(line);
        printf("Field 1 would be %s\n", getfield(tmp, num[0])); // NOTE strtok clobbers tmp
        word[i] = getfield(tmp, num[0]);
        i++; 
        free(tmp);
    }
    printf("%s\n", word[1]); //prints successfully
    fclose(lang);
    return NULL;
}

int function2() {
    const char* word[1024] = { function1() };
    printf("%s\n", word[1]); // failure, prints (null)
}

Answer 1

This is a stand-alone example that shows you how to allocate your word list from the heap and return it to another function:

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

#define WORD_COUNT 1024

const char **function1(void) {
    const char **word = malloc(sizeof(const char *) * WORD_COUNT);
    for (int index = 0; index < WORD_COUNT; index++) {
        // You would replace these lines with your getfield code
        char number[20];
        snprintf(number, sizeof(number), "%d", index);
        word[index] = strdup(number);
    }
    // We return word here because that is how we pass the result to the caller
    return word;
}

void function2(void) {
    const char **word = function1();
    printf("word[1] = %s\n", word[1]);
}

int main(void) {
    function2();
    return 0;
}

Output

word[1] = 1

Warning

The program allocates memory via malloc and strdup but does not call free to de-allocate it. This could result in memory leaks.

Answer 2

You have a number of problems.

• word is not static so does not exist outside of function1()
• Even if word were static, its elements point to content in tmp which is free()'d before function1() returns.
• function1() returns NULL - presumably the intent was to return the pointer word? But that would be incorrect in any event for all the reasons above.
• function2() assigns as many word pointers as there are lines in the file, which may be more than the allocated 1024.

This problem can be solved in a number of ways. One way is to pass the array of pointers to words to function1() and have function1() allocate space for each word. This then requires function2() to take responsibility for freeing the allocated words when done. Not a method I'd normally recommend, but the alternative is to define some arbitrary fixed maximum length for each word.

I have made some assumptions about the semantics of getfield() and removed the array num as it seems to serve no purpose in the original code.

int function1( const char** words, size_t max_words )
{
    FILE* lang = fopen( "lang.csv", "r" );
    char line[1024];
    size_t i = 0;
    while( i < max_words && fgets( line, 1024, lang ) != NULL )
    {
        printf( "Field 1 would be %s\n", getfield( line, 1 ) ) ;
        const char* tmp = getfield( line, 1 );
        char* word = malloc( strlen( tmp) + 1 ) ;
        strcpy( word, tmp ) ;
        i++ ;
    }
    printf( "%s\n", words[1] ); //prints successfully
    fclose( lang );

    return i ;
}

int function2() 
{
    char* word[1024] = {0} ;

    // Fill the word list
    int count = function1( word, sizeof(word) / sizeof(*word) ) ;

    // Print first word
    printf( "%s\n", word[1] ); //failure, prints (null)

    // Delete list elements allocated by function2()
    for( int i = 0; i < count; i++ )
    {
        free( word[i] ) ;
    }

    return count ;
}