TruncationAssumedToRewind

warning native on by default

Group: Probable bugs

Rewind after truncating the stream.

Example

Finding:

<?php
$fp = fopen($path, "w+"); if ($fp === false) { return; } if (fseek($fp, 10) !== 0) { return; } if (ftruncate($fp, 0)) { fwrite($fp, "new"); }

Reported:

• line 2: Rewind after truncating the stream.

Detection

• Track a successful writable stream, a proven positive current position, ftruncate(handle,0), and subsequent fwrite/fputs before repositioning. Report the write because ftruncate does not reset position. Positive position may come from a successful fseek or known-length prior write.
• Emit one finding per violating operation. Flow facts require a reachable path with no intervening mutation, escaped alias, unknown call involving the tracked value or analysis-budget exhaustion. Uncertain facts cannot prove a violation.

Exceptions (no report)

• Rewind/fseek to zero, unknown position, failed truncation branches and intentional sparse-file construction explicitly outside this rule through suppression are excluded.
• A stream opened in an append mode (the mode string starts with a, with or without +, b or t) is never reported: every write goes to the end of the stream regardless of the stale position, so no zero-filled gap appears.
• A stream opened on php://memory is never reported: truncating it pulls a position past the new end back to that end. php://temp stays covered because it can spill to a real file.
• Exclude shadowed builtin symbols, unresolved required types, incomplete syntax and unsupported PHP versions.
• A value the shared flow analysis proves is not the failure value at the use is never reported, whatever form the proof takes: a test in an enclosing branch, loop condition or short-circuit operand, an early exit, a statement that leaves on the failing outcome ($v !== false or exit;, $v or throw …;, $v ?: throw …;), a failing branch that ends in a call that never returns (a callee declared never, or a function or a final or private method every path of which throws or exits), a test stored in a boolean and tested later while neither the boolean nor the tested variable has been written, and, on PHP 8.0 and later outside any try with a catch, a builtin call that returned after receiving the value for a parameter that only accepts arrays, objects or callables: such a call throws a TypeError for false. A switch (true) case is read like the condition of an if/elseif chain; in a switch on the variable itself with constant labels, the cases after a false or null label and the default hold a value that is neither, as does a true case. Positions PHP may skip prove nothing: the right side of ??=, the arguments of a null-safe call and anything chained after ?->. A null-safe call to a never-returning method does not end the path, and f(...) creates a closure without calling f. An arm of match (true) is read like a switch (true) case; an arm that throws, exits or calls a function that never returns takes no part in what holds after the match. | and & between two tests are read like || and && for the outcome that fixes both operands (a false |, a true &), and a strict in_array($v, [...], true) over literals that include neither false nor null excludes both where it holds. goto jumps are followed, including a jump out of a try or catch block, which runs the finally blocks it leaves on its way. Where a goto label is written before the use, and after the place where its scope first names the checked variable, a variable that can hold the same value, or code that may write any variable (extract, compact, get_defined_vars, parse_str given one argument, a variable variable, $GLOBALS, eval, an included file), a jump can land between a guard and the use: only what the flow analysis establishes counts there, and a guard recognized by where it is written proves nothing. Two variables can hold the same value when one is written from the other by a plain assignment, an assignment by reference or ??=, when a foreach takes one out of the other, or through a chain of those; in the value assigned, the receiver and the arguments of a call, the operands of an operator other than ??, the variables a string embeds and an element key do not count, and a statement that only reads (echo $prefix . $parts['host'];) ties nothing. A parameter is named from the start of the function, and a checked expression that reads $this or no variable counts every label before it. In a scope the flow analysis does not follow (one that evaluates or includes code, which may write any variable) a guarded use after such a label is therefore reported. A generator is followed: while it is suspended at a yield, the code that consumes it may write only what an unknown call may write, and, when the generator returns by reference, the variable it yields. A label written before all of that, such as a retry: label ahead of the try that produces and checks the value, changes nothing, whatever else the function reads beside the checked value; at file level every label written before the use counts. What was established of a static local holds until the next call, or code that runs without a call being written (an object converted to text, a magic property or offset access, the destructor of an object a variable held when it is given another value), that may enter the function again and assign the variable. A call of PHP's own counts as such when it may run code of the project: it is given a callback other than the name of a PHP function that only computes on text and numbers and raises no warning whatever text it is given (a closed list: strlen, trim, intval and the like, not hexdec, abs or chr, which warn about text that is no number), an object it may convert, a class name it loads (method_exists($name, 'm'), is_callable('A::b'), defined('A::B'), class_alias($name, 'B'), new ReflectionClass($name), $arrayObject->setIteratorClass($name)), a class to construct ($statement->fetchObject('Row')) or a filter that one of PHP's own FILTER_… constants does not name, which may be the one that calls back, or it runs what was registered earlier (an error, session or output-buffer handler, the callbacks of a cURL transfer or an XML parser, an iterator wrapped in another, a filter added to a stream). A warning that a PHP function raises by itself when called directly (hex2bin given an odd number of digits, a file that cannot be read) is not counted, although the error handler of the project receives it. This limits every guard form of this specification, not only the ones the flow analysis reads.

Fix

No automatic fix. Choosing a repair requires runtime information, error-handling policy or a semantic decision.

All edits use exact byte ranges, preserve comments and evaluation order, and require syntax supported by the target PHP version. A fix must never add an evaluation or silently change unrelated arguments.

PHP versions

Available throughout PHP 5.3–8.5.

Configure

In custos.json:

{
  "rules": {
    "TruncationAssumedToRewind": {
      "enabled": false
    }
  }
}

Suppress

Before the statement or declaration (or the first statement of the file):

// @custos-ignore TruncationAssumedToRewind