Returning a pointer to a char array inside a struct in C?

Question

I have the following struct in the header file:

static FILINFO fno;

Which looks like so:

typedef struct {
DWORD   fsize;      /* File size */
WORD    fdate;      /* Last modified date */
WORD    ftime;      /* Last modified time */
BYTE    fattrib;    /* Attribute */
char    fname[13];  /* File name */
} FILINFO;

Now I would like a certain function to return a pointer to fname[] which should be possible since the struct is defined static. Here is my function:

char* get_open_file_name (void)
{
    return fno.fname;
}

I'm calling this function from main like so:

char* temp;

int main (void)
{
    intialize_sd_card ();
    ...
    ...
    temp = get_open_file_name();

This results in conflicting types error message. However if I try to call it like so:

char* temp = get_open_file_name();

It works. What am I missing here?

EDIT: Here is the actual error msg I'm getting from GCC

Severity    Code    Description Project File    Line
Warning     assignment makes integer from pointer without a cast

I suggest you share the error message (although I think someone that's not in about to go to bed now like me, could probably spot that nevertheless). :) — gsamaras
– gsamaras, Commented Nov 16, 2019 at 1:17
Sounds like you did not declare the function before calling it? Please post a Minimal, Reproducible Example — M.M
– M.M, Commented Nov 16, 2019 at 2:35

S. Exchange Considered Harmful · Accepted Answer · 2019-11-16 02:25:51Z

1

Just a wild guess, but could something like this be at play here?

#include <stdio.h>

typedef struct {
    char fname[4];
} FILINFO;

static FILINFO fno;

char* temp;

char* get_open_file_name(void) {
    return fno.fname;
}

int main(void) {
    fno.fname[0] = 'L';
    fno.fname[1] = 'o';
    int temp; //my awesome hidden debug value, with regards, previous dev
    fno.fname[2] = 'l';
    fno.fname[3] = '\0';
    if(1 /*or whatever*/) {
        temp = get_open_file_name();
        printf("%s\n", temp);
    }
    return 0;
}

Owing to local-scope visibility rules, this frankenstein can be legal - that up to the point where the char pointer is to be casted to an integer, of course. Then if You replace temp = get_open_file_name(); with char* temp = get_open_file_name();, it will work fine.

Once the offending debug value is gone, the following compiles and runs flawlessly, using GCC 9.2.0 with -s -O3 -std=c99 -pedantic -Wall -Wextra:

#include <stdio.h>

typedef struct {
    char fname[4];
} FILINFO;

static FILINFO fno;

char* temp;

char* get_open_file_name(void) {
    return fno.fname;
}

int main(void) {
    fno.fname[0] = 'L';
    fno.fname[1] = 'o';
    fno.fname[2] = 'l';
    fno.fname[3] = '\0';
    temp = get_open_file_name();
    printf("%s\n", temp);
    return 0;
}

edited Nov 16, 2019 at 2:25

answered Nov 16, 2019 at 2:08

S. Exchange Considered Harmful

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1 Comment

user733606 Over a year ago

Your guess was right! The header file that included the static struct had an int temp variable for debugging and * I guess * since the header was included before the declaration of the char* temp in main.c it ended up never "seeing it".

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