Close file in C

Question

Under Linux I use this code to redirect stdout and stderr on a file, as shown in the code the file is opened using fopen(f) and is it closed using close(fd).

int         fd;
FILE        *f;   

f = fopen("test.txt", "rb+");

fd = fileno(f);
dup2(fd,STDOUT_FILENO);
dup2(fd,STDERR_FILENO);
close(fd);

My question is whether the close(fd) statement closes all file descriptors, or is it necessary to use fclose(f) as well ?

Answer 1

The rule is to close the outermost level. Here the FILE object pointed to by f contains the fd file handle but also internal data like a possible buffer and various pointers to it.

When you use close(fd), you free the kernel structures related to the file, but all the data structures provided by the standard library are not released. On the other hand, fclose(f) will internally close the fd file handle, but it will also release all the resources that were allocated by fopen.

TL/DR: if you use fd= open(...); to open a file, you should use close(fd); to close it, but if you use f = fopen(...);, then you should use fclose(f);.

Answer 2

C FILE* streams use buffered I/O internally. fclose() flushes this buffer and then closes the file descriptor at OS level. close()'ing a FILE* stream may not flush this internal buffer and you may lose data. So for C streams always use C fxxx() functions.

Answer 3

As already pointed out in the other answers, you should use fclose(f); instead of close(fd);.

My question is whether the close(fd) statement closes all file descriptors [...]

No, it won't close all file descriptors. The file descriptors STDOUT_FILENO and STDERR_FILENO will still remain open and will now refer to the opened file test.txt. However, these file descriptors should probably not be closed, as it is good programming practice for STDOUT_FILENO and STDERR_FILENO to remain valid until the end of the program. They will be automatically closed by the kernel on process termination.