Advanced Programming in the UNIX® Environment

ptg10805159

Section 4.18 Creating and Reading Symbolic Links 123

$ln -s /no/such/file myfile create a symbolic link

$ls myfile

myfile lssays it’sthere

$cat myfile so we try to look at it

cat: myfile: No such file or directory

$ls -l myfile try-loption

lrwxrwxrwx 1 sar 13 Jan 22 00:26 myfile -> /no/such/file

The filemyfiledoes exist, yetcatsays there is no such file, becausemyfileis a

symbolic link and the file pointed to by the symbolic link doesn’t exist. The-loption

tolsgives us two hints: the first character is anl,which means a symbolic link, and

the sequence->also indicates a symbolic link. Thelscommand has another option

(-F)that appends an at-sign (@) to filenames that aresymbolic links, which can help us

spot symbolic links in a directory listing without the-loption.

4.18 Creating and Reading Symbolic Links

Asymbolic link is created with either thesymlinkorsymlinkatfunction.

#include <unistd.h>

int symlink(const char *actualpath,const char *sympath);

int symlinkat(const char *actualpath,int fd,const char *sympath);

Both return: 0 if OK,−1 on error

Anew directory entry,sympath, is created that points toactualpath.It is not required

thatactualpathexist when the symbolic link is created. (Wesaw this in the example at

the end of the previous section.) Also,actualpathandsympathneed not reside in the

same file system.

Thesymlinkatfunction is similar to symlink,but thesympath argument is

evaluated relative to the directory referenced by the open file descriptor for that

directory (specified by thefdargument). If thesympathargument specifies an absolute

pathname or if thefdargument has the special valueAT_FDCWD,then symlinkat

behaves the same way assymlink.

Because theopenfunction follows a symbolic link, we need a way to open the link

itself and read the name in the link. Thereadlinkandreadlinkatfunctions do this.

#include <unistd.h>

ssize_t readlink(const char* restrictpathname,char *restrict buf,

size_t bufsize);

ssize_t readlinkat(intfd,const char* restrictpathname,

char *restrictbuf,size_tbufsize);

Both return: number of bytes read if OK,−1 on error