yang@nff.ncl.omron.co.jp (YANG Liqun) (01/23/91)
In arcticle <15715hh2x@vax5.cit.cornell.edu> writes: >Could someone please explain the functions freopen() and access()? I have some >source that calls these functions but they aren't included with my compiler. >If someone could tell me what they do I may be able to write them myself. >Replies through Email would be greatly appreciate. access() is a system call of Unix OS, it investigate the access permision of given file: #include <sys/file.h> #define R_OK 4/* test for read permission */ #define W_OK 2/* test for write permission */ #define X_OK 1/* test for execute (search) permission */ #define F_OK 0/* test for presence of file */ accessible = access(path, mode) int accessible; char *path; int mode; As far I know, it is written in assembler in System V. First it looks for the inode number of given file(use ls -i, you can do it), and then read the inode information from disk and then find what access permission the file has. For freopen(), you can easily rewrite it in C. FILE *freopen(filename, type, stream) char *filename, *type; FILE *stream; First it closes "stream" by calling fclose. Then calls "open" system call with flag information supplied in "type" to open a file with name "filename"(it may be a different file). The file descriptor returned by "open" system call and flag information in "type" will be used to replace fields in FILE struct: ... oflag = O_RDWR /* O_RDONLY, O_WRONLY, and etc., according to type */ if ((fd = open(filename, oflag, 0666)) <0) return (NULL); stream->_cnt = 0; /* File read & write offset */ stream-> _file = fd; stream->_flag = _IORW; /* or _IOREAD, _IOWRT, according to type*/ ... /* If file is opened as "a" type, you need to use lseek to move file pointer to the end of the file */ Finally, set _bufend(stream) = stream->_base = stream->ptr = NULL; return(stream); So freopen() open another file but use the old FILE struct by closing the old file and replacing contents of FILE struct. -- ; Li-qun Yang OMRON Computer Technology R&D lab ; yang@nff.ncl.omron.co.jp tel: 075-951-5111 fax: 075-956-7403
gwyn@smoke.brl.mil (Doug Gwyn) (01/24/91)
In article <YANG.91Jan23145507@newyork.nff.ncl.omron.co.jp> yang@nff.ncl.omron.co.jp (YANG Liqun) writes: >In arcticle <15715hh2x@vax5.cit.cornell.edu> writes: >>Could someone please explain the functions freopen() and access()? My reaction was, somebody should tell this guy that if he expects to deal with UNIX source code he should go to his local technical bookstore and buy a copy of some UNIX Programmer Reference Manual. >As far I know, [access] is written in assembler in System V. No, while all the system-call hooks in the C library are tiny assembly- language modules that merely cause a trap with the proper subcode, the actual implementation of system calls in the UNIX kernel originates as C source code. I suspect that whatever code is using access() already has a bug, since access() rarely does what any sensible application would want it to do. (This is a design deficiency in the semantics for access().) >For freopen(), you can easily rewrite it in C. But only if you know enough about the specific implementation of stdio for the system in question. The main constraint is that the same FILE structure must be used for the newly opened file. Odds are good that the application in question can be readily changed to use fclose()/fopen() rather than relying on freopen(). That would be preferable to trying to implement one's own freopen().
ch@dce.ie (Charles Bryant) (02/01/91)
In article <14944@smoke.brl.mil> gwyn@smoke.brl.mil (Doug Gwyn) writes: >I suspect that whatever code is using access() already has a bug, since >access() rarely does what any sensible application would want it to do. >(This is a design deficiency in the semantics for access().) Is this bug that the programs use it when checking effective UID access, or that they are setuid programs which have a window between checking permissions and opening the file in which the file could be switched? -- Charles Bryant (ch@dce.ie) -- /usr/ch/.signature: Block device required