Undefined Behavior: File Pointer Use After Close

The application uses a file operation on a closed file pointer.

Performing file operations on a system FILE object after its associated stream is closed results in undefined behavior. Depending on the specific C compiler in use, the file operation can cause a system crash or even potentially result in modification or reading of the same or different file.

Example 1: The following code shows an attempt to read a system FILE object after the corresponding stream is closed.

FILE *sysfile = fopen(test.file, "r+");
res = fclose(sysfile);
if(res == 0){
    printf("%c", getc(sysfile));    
}

Because the getc() function runs after the file stream for sysfile is closed, getc() results in undefined behavior and can cause a system crash or potential modification or reading of the same or different file.