Question

Full disclosure: This is my first time doing any significant programming in C, and my first post on Stack Overflow.

I’m working on code that will eventually be used with Bison to implement a small subset of the Scheme/Racket language. All of this code is in a single C file. I have three structs: Binding, Lambda, and SymbolEntry. I’m not using the Lambda struct yet, it’s just there for completeness. I also have a symbol table that holds symbol entries. printSymbolTable() does exactly what the name implies:

typedef struct
{
    char* name;
    char* value;
} Binding;

typedef struct
{
    int numBindings;
    Binding** bindings;
    char* functionBody; 
} Lambda;

typedef struct
{
    Binding* binding;
    Lambda* function;
} SymbolEntry;

SymbolEntry* symbolTable = NULL;
int numSymbols = 0;

void printSymbolTable()
{
    if (symbolTable)
    {
        int i = 0;
        for (i; i < numSymbols; i++)
        {
            printf("\tsymbolTable[%i]: %s = %s\n", i, symbolTable[i].binding->name, symbolTable[i].binding->value);
        }
    }
}

I’m currently trying to work out the logic for defining and looking up variables. The 2 relevant functions:

// Takes a name and an exprssion and stores the result in the symbol table
void defineVar(char* name, char* expr)
{
    printf("\nSetting %s = %s\n", name, expr);
    printf("Previous number of symbols: %i\n", numSymbols);
    Binding props;
    props.name = name;
    props.value = expr;

    SymbolEntry entry;
    entry.binding = &props;
    entry.function = NULL;

    symbolTable = realloc(symbolTable, sizeof(SymbolEntry) * ++numSymbols);
    if (!symbolTable)
    {
        printf("Memory allocation failed. Exiting.\n");
        exit(1);
    }
    symbolTable[numSymbols - 1] = entry;
    printf("New number of symbols: %i\n", numSymbols);
    printf("defineVar result:\n");
    printSymbolTable();
}

// Test storing and looking up at least 4 variables, including one that is undefined
void testVars()
{
    printf("Variable tests\n");

    defineVar("foo", "0");
    printf("After returning from defineVar:\n");
    printSymbolTable();

    defineVar("bar", "20");
    printf("After returning from defineVar:\n");
    printSymbolTable();
}

main() calls testVars(). I get no warnings or errors when compiling, and the program executes successfully. However, this is the result:

Variable tests

Setting foo = 0
Previous number of symbols: 0
New number of symbols: 1
defineVar result:
    symbolTable[0]: foo = 0
After returning from defineVar:
    symbolTable[0]: 1�I��^H��H���PTI��@ = �E

Setting bar = 20
Previous number of symbols: 1
New number of symbols: 2
defineVar result:
    symbolTable[0]: bar = 20
    symbolTable[1]: bar = 20
After returning from defineVar:
    symbolTable[0]: 1�I��^H��H���PTI��@ = �E
    symbolTable[1]: 1�I��^H��H���PTI��@ = �E���

Not only am I getting junk values when outside of the defineVar() function, but the call to define bar shows incorrect non-junk values as well. I’m not sure what I’m doing wrong, but I assume it’s probably something with realloc(). However, a similar strategy worked when parsing a string into individual tokens, so that’s what I was trying to emulate. What am I doing wrong?

Answer 1

Because it’s pointing to variables (or variable — at least props, haven’t read further) local to functions and the stack frame is discarded (and soon overwritten) after you return.