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POPointers getting junk values outside of functions, but regular values inside them
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  1. POPointers getting junk values outside of functions, but regular values inside them
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    copied!<p>Full disclosure: This is my first time doing any significant programming in C, and my first post on Stack Overflow.</p> <p>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: <code>Binding</code>, <code>Lambda</code>, and <code>SymbolEntry</code>. I'm not using the <code>Lambda</code> struct yet, it's just there for completeness. I also have a symbol table that holds symbol entries. <code>printSymbolTable()</code> does exactly what the name implies:</p> <pre><code>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 &lt; numSymbols; i++) { printf("\tsymbolTable[%i]: %s = %s\n", i, symbolTable[i].binding-&gt;name, symbolTable[i].binding-&gt;value); } } } </code></pre> <p>I'm currently trying to work out the logic for defining and looking up variables. The 2 relevant functions:</p> <pre><code>// 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 = &amp;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(); } </code></pre> <p><code>main()</code> calls <code>testVars()</code>. I get no warnings or errors when compiling, and the program executes successfully. However, this is the result:</p> <pre><code>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��� </code></pre> <p>Not only am I getting junk values when outside of the <code>defineVar()</code> function, but the call to define <code>bar</code> shows incorrect non-junk values as well. I'm not sure what I'm doing wrong, but I assume it's probably something with <code>realloc()</code>. 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?</p>
 

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