#include <unistd.h> int access(const char *pathname, int mode);
The mode specifies the accessibility check(s) to be performed, and is either the value F_OK, or a mask consisting of the bitwise OR of one or more of R_OK, W_OK, and X_OK. F_OK tests for the existence of the file. R_OK, W_OK, and X_OK test whether the file exists and grants read, write, and execute permissions, respectively.
The check is done using the calling process's real UID and GID, rather than the effective IDs as is done when actually attempting an operation (e.g., open(2)) on the file. This allows set-user-ID programs to easily determine the invoking user's authority.
If the calling process is privileged (i.e., its real UID is zero), then an X_OK check is successful for a regular file if execute permission is enabled for any of the file owner, group, or other.
access() may fail if:
Warning: Using access() to check if a user is authorized to, for example, open a file before actually doing so using open(2) creates a security hole, because the user might exploit the short time interval between checking and opening the file to manipulate it. For this reason, the use of this system call should be avoided.
access() returns an error if any of the access types in mode is denied, even if some of the other access types in mode are permitted.
If the calling process has appropriate privileges (i.e., is superuser), POSIX.1-2001 permits implementation to indicate success for an X_OK check even if none of the execute file permission bits are set. Linux does not do this.
A file is only accessible if the permissions on each of the directories in the path prefix of pathname grant search (i.e., execute) access. If any directory is inaccessible, then the access() call will fail, regardless of the permissions on the file itself.
Only access bits are checked, not the file type or contents. Therefore, if a directory is found to be writable, it probably means that files can be created in the directory, and not that the directory can be written as a file. Similarly, a DOS file may be found to be "executable," but the execve(2) call will still fail.
access() may not work correctly on NFS file systems with UID mapping enabled, because UID mapping is done on the server and hidden from the client, which checks permissions.
In kernels before 2.6.20, access() ignored the effect of the MS_NOEXEC flag if it was used to mount(2) the underlying file system. Since kernel 2.6.20, access() honors this flag.