Партнёры:
Хостинг:
access - determine accessibility of a file
#include <unistd.h>
int access(const char *path, int amode);
The access() function shall check the file named by the pathname pointed to by the path argument for accessibility according to the bit pattern contained in amode, using the real user ID in place of the effective user ID and the real group ID in place of the effective group ID.
The value of amode is either the bitwise-inclusive OR of the access permissions to be checked (R_OK, W_OK, X_OK) or the existence test (F_OK).
If any access permissions are checked, each shall be checked individually, as described in the Base Definitions volume of IEEE Std 1003.1-2001, Chapter 3, Definitions. If the process has appropriate privileges, an implementation may indicate success for X_OK even if none of the execute file permission bits are set.
If the requested access is permitted, access() succeeds and shall return 0; otherwise, -1 shall be returned and errno shall be set to indicate the error.
The access() function shall fail if:
The access() function may fail if:
The following sections are informative.
The following example tests whether a file named myfile exists in the /tmp directory.
#include <unistd.h> ... int result; const char *filename = "/tmp/myfile"; result = access (filename, F_OK);
Additional values of amode other than the set defined in the description may be valid; for example, if a system has extended access controls.
In early proposals, some inadequacies in the access() function led to the creation of an eaccess() function because:
Historical implementations of access() do not test file access correctly when the process' real user ID is superuser. In particular, they always return zero when testing execute permissions without regard to whether the file is executable.
The superuser has complete access to all files on a system. As a consequence, programs started by the superuser and switched to the effective user ID with lesser privileges cannot use access() to test their file access permissions.
However, the historical model of eaccess() does not resolve problem (1), so this volume of IEEE Std 1003.1-2001 now allows access() to behave in the desired way because several implementations have corrected the problem. It was also argued that problem (2) is more easily solved by using open(), chdir(), or one of the exec functions as appropriate and responding to the error, rather than creating a new function that would not be as reliable. Therefore, eaccess() is not included in this volume of IEEE Std 1003.1-2001.
The sentence concerning appropriate privileges and execute permission bits reflects the two possibilities implemented by historical implementations when checking superuser access for X_OK.
New implementations are discouraged from returning X_OK unless at least one execution permission bit is set.
chmod() , stat() , the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>
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