Question

Possible Duplicate:
Difference between – returning a ‘local’ char* from a function vs returning a ‘local’ int* from a function

Here is a simple code, where in 3 different functions [ localStrPtr, localIntPtr, localCharPtr] return a pointer to their local variables [string, integer, char] in their respective functions.

CODE:

#include <stdio.h>

char*  localStrPtr (char*);
int*   localIntPtr (int, int);
char*  localCharPtr (char);

main()
{
    int *pInt;
    char *pChar;

    printf( "localStrPtr = %s\n", localStrPtr("abcd") );

    pInt = (int*) localIntPtr(3, 5);
    printf( "localIntPtr = %d\n", *pInt );

    pChar = (char*) localCharPtr('y');
    printf( "localCharPtr = %c\n", *pChar );
}

char* localStrPtr(char* argu)
{
    char str[20];
    // char* str = (char*) malloc (20);

    strcpy (str, argu);
    return str;
}

int* localIntPtr (int argu1, int argu2)
{
    int local;
    local = argu1 + argu2;
    return (&local);
}

char* localCharPtr (char argu)
{
    char local;
    local = argu;
    return (&local);
}

COMPILE LOG:

stringManip.c: In function `localStrPtr':
stringManip.c:27: warning: function returns address of local variable
stringManip.c: In function `localIntPtr':
stringManip.c:34: warning: function returns address of local variable
stringManip.c: In function `localCharPtr':
stringManip.c:41: warning: function returns address of local variable

RUN LOG:

localStrPtr =
localIntPtr = 8
localCharPtr = y

As you can see in the log file, localStrPtr returns “some garbage”, whereas localIntPtr and localCharPtr return “expected” values.

But, in the function localStrPtr, if I change char str[20] -to-> char* str = (char*) malloc (20), localStrPtr returns the string “abcd” correctly. Here is the RUN LOG, once the above change is made.

NEW RUN LOG:

localStrPtr = abcd
localIntPtr = 8
localCharPtr = y

QUESTIONS:

[1] In functions localIntPtr and localCharPtr, contents of the returned local variable addresses WORKED, but for the function localStrPtr, correct value is returned “only” with malloc, but will not with local char str[20]. Why doesn’t it work with str[20] while works with local variables char and int ?

[2] Why do we see in the COMPILE LOG, the lines below for all the 3 functions ?

• stringManip.c:27: warning: function returns address of local variable
• stringManip.c:34: warning: function returns address of local variable
• stringManip.c:41: warning: function returns address of local variable

Answer 1

As the compiler mentions you are returning an address on the stack. After you do the return the value in the stack may or may not be overwritten by some other function call coming in later. The fact that you get an int or a char is just chance. if you issue another printf( "localIntPtr = %d\n", *pInt ); just before your return again you are likely to see that the pInt has been overwritten already.