Returning a char pointer in C

Question

I am working on some kind of file sharing program which is written in C. There is a function that can read a data file and store the data into a string and return this string to main function and the main function send back to client. Codes are shown below

char* ListFiles(){
    FILE *fp;
    char file[30];
    char *f;
    if((fp=fopen("list","r"))==NULL)  
    {
        ...
    }
    while (!feof(fp))
    {
        fgets(file,50,fp);
    }
    fclose(fp);
    f=file;
    printf("%s",f); //get display!!!
    return f;
}
int main(){
      char *files;
      ...
      ...
      files=ListFiles();
      printf("%s",files); //nothing display!!
      sent();
}

However, this method doesn't work. There is nothing display and of course nothing is sent. But I do get the correct display in function ListFiles(). I don't know what happen. I also use strcpy() and it still fail to work.

Solution 1

Follow George Skoptsov recommendations. But If you don't have the strdup() function,then use this:

char* strdup(const char* org)
{
    if(org == NULL) return NULL;

    char* newstr = malloc(strlen(org)+1);
    char* p;

    if(newstr == NULL) return NULL;

    p = newstr;

    while(*org) *p++ = *org++; /* copy the string. */
    return newstr;
}

And then:

#include <string.h> /* strlen() call */
#include <stdlib.h> /* NULL, malloc() and free() call */

/* do something... */

char* ListFiles() {
        /* .... */ 
         return strdup(f);
}

Or instead of char file[30]; do a dynamic memory allocation: char* file = malloc(30); then you can do return f; and it will work fine because f now is not a pointer to a local variable.

Solution 2

What you're doing is returning a pointer to a local variable, which used to be allocated on stack.

Change your return statement to

return strdup(file);

Solution 3

file is a stack variable in ListFiles() and you're returning a pointer to it. Once you return from that function, the variable will cease to exist, so the returned pointer will be invalid.

If you want to return a string, you should allocate it on the heap, return it, use it, then free it once you're done using it.

Solution 4

You are returning a pointer points to the stack variable. When the function returns, the stack gets popped off. The stack variable no longer exists, the pointer become a dangling pointer.

One solution is to allocate the appropriate amount of memory in the main function, and pass the pointer to the memory to the helper function.

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Comments

[Mr Lister]

strdup is not a standard function, it's only defined in MSVC.

[ouah]

scope and storage duration are two different beasts.

[Jack]

@MrLister: It's available in POSIX environment as well.

[Mr Lister]

Even if you don't have strdup, you always have strcpy when including <string.h>...

[Joniale]

Here a good example on how to do different things with strings and pointers in C stackoverflow.com/a/46344713/5842403