Why does cURL resources still occupy memory after curl_close? How to solve it?

In PHP, cURL is a powerful library for data exchange with other servers. It allows developers to send and receive data via HTTP requests. After completing a cURL request, we usually use the curl_close function to close the cURL session and free up the resources.

 $ch = curl_init("https://gitbox.net/some-api-endpoint");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
$response = curl_exec($ch);
curl_close($ch);

In this example, after curl_close($ch) is called, we expect the resources occupied by the $ch variable to be released.

Why is the memory not fully released?

If the memory is still occupied after using curl_close , there may be multiple aspects of the reason. Here are some common reasons:

1. PHP memory management mechanism
   PHP uses a mechanism that delays garbage collection. Even if curl_close is called, the release of memory may not happen immediately. PHP may wait until the script execution is over before recycling these resources.

2. Curl_init and curl_close multiple times
   If you initialize and close cURL resources multiple times in the same script, PHP may not release the previous resources immediately, especially when there are a large number of requests. Each time a new cURL session is created, it takes up a certain amount of memory and will not be thoroughly cleaned until the script execution is finished.

3. cURL resource is not completely closed <br> If an error occurs before calling curl_close , or you do not make sure that the cURL session has been executed correctly, the resource may not be fully released. For example, curl_exec returns false without obtaining the error message through curl_error .

How to completely free cURL memory?

Although curl_close is used to free resources, sometimes it is not enough to ensure full freeing of memory. Here are some ways to try:

1. Use unset to clear variables

Using unset can help clear PHP variables, which helps free memory faster. Even if curl_close has been executed, unset ensures that the variable no longer references the resource.

 curl_close($ch);
unset($ch);

2. Force garbage collection using gc_collect_cycles

In PHP, garbage collection is automatically performed, but it can be forced to perform a garbage collection by manually calling gc_collect_cycles to ensure that resources are recycled in time.

 curl_close($ch);
unset($ch);
gc_collect_cycles();

3. Analyze memory usage

If you find that the memory usage is still too high, you can use memory_get_usage and memory_get_peak_usage to analyze memory usage. This will help you understand more clearly whether the memory is actually freed.

 echo "Current memory usage: " . memory_get_usage() . " bytes";
echo "Maximum memory usage: " . memory_get_peak_usage() . " bytes";

4. Process multiple requests in batches

If you are executing a large number of cURL requests, consider processing the requests in batches instead of executing all requests at once. This not only helps memory management, but also reduces server load.

 // Process multiple requests in batches
$urls = [
    "https://gitbox.net/api1",
    "https://gitbox.net/api2",
    "https://gitbox.net/api3",
];

foreach ($urls as $url) {
    $ch = curl_init($url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_exec($ch);
    curl_close($ch);
    unset($ch);
}

5. Use cURL to multithread

For situations where multiple requests need to be executed, using cURL's multi-threaded functions (such as curl_multi_* series functions) can effectively reduce memory usage and accelerate requests. Multithreading can execute multiple requests concurrently, reducing the memory usage of a single request.

 $multiHandle = curl_multi_init();
$handles = [];

foreach ($urls as $url) {
    $ch = curl_init($url);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_multi_add_handle($multiHandle, $ch);
    $handles[] = $ch;
}

do {
    curl_multi_exec($multiHandle, $running);
} while ($running > 0);

foreach ($handles as $ch) {
    curl_multi_remove_handle($multiHandle, $ch);
    curl_close($ch);
}

curl_multi_close($multiHandle);