access() - Linux man page
access - check real user's permissions for a file
int access(const char *pathname, int mode);
access() checks whether the calling process can access the file pathname. If pathname is a symbolic link, it is dereferenced.
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.
On success (all requested permissions granted), zero is returned. On error (at least one bit in mode asked for a permission that is denied, or some other error occurred), -1 is returned, and errno is set appropriately.
access() shall fail if:
The requested access would be denied to the file, or search permission is denied for one of the directories in the path prefix of pathname. (See also path_resolution(7).)
Too many symbolic links were encountered in resolving pathname.
pathname is too long.
A component of pathname does not exist or is a dangling symbolic link.
A component used as a directory in pathname is not, in fact, a directory.
Write permission was requested for a file on a read-only file system.
access() may fail if:
pathname points outside your accessible address space.
mode was incorrectly specified.
An I/O error occurred.
Insufficient kernel memory was available.
Write access was requested to an executable which is being executed.