fcloseInLoopCondition

Message: fclose() used as loop condition may skip loop body or double-close file handle.

Category: Resource Management

Severity: Warning

Language: C and C++

Description

Using fclose() as a loop condition leads to two unwanted outcomes:

• On success, the condition is false and the loop body never executes. The intent was likely to process the file inside the loop, but it is already closed.
• On failure, the condition is true and the loop body executes but fclose() is called again on the already-closed file handle on the next iteration, which is undefined behaviour.

This pattern is almost always a misunderstanding of what fclose() returns, or confusion with a function that reads/processes data incrementally (like fgets or fread). Unlike those functions, fclose() is a one-shot teardown operation and has no meaningful retry or loop-until-done semantic.

How to fix

Call fclose() outside the loop condition. If you need to check whether the close succeeded, store the return value and test it separately.

Before:

FILE *fp = fopen("data.txt", "r");
while (fclose(fp)) {
    /* process file */
}

After:

FILE *fp = fopen("data.txt", "r");
/* process file */
if (fclose(fp) != 0) {
    /* handle close error */
}

Related checkers

• useClosedFile - for using a file handle that has already been closed