Advanced Programming in the UNIX® Environment

ptg10805159

Section 2.5 Limits 51

errno = 0;

if ((pathmax = pathconf("/", _PC_PATH_MAX)) < 0) {

if (errno == 0)

pathmax = PATH_MAX_GUESS; /* it’s indeterminate */

else

err_sys("pathconf error for _PC_PATH_MAX");

}else {

pathmax++; /* add one since it’s relative to root */

}

}

/*

*Before POSIX.1-2001, we aren’t guaranteed that PATH_MAX includes

*the terminating null byte. Same goes for XPG3.

*/

if ((posix_version < 200112L) && (xsi_version < 4))

size = pathmax + 1;

else

size = pathmax;

if ((ptr = malloc(size)) == NULL)

err_sys("malloc error for pathname");

if (sizep != NULL)

*sizep = size;

return(ptr);

}

Figure 2.16Dynamically allocate space for a pathname

Maximum Number of Open Files

Acommon sequence of code in a daemon process — a process that runs in the

background, not connected to a terminal—isone that closes all open files. Some

programs have the following code sequence, assuming the constantNOFILE was

defined in the<sys/param.h>header:

#include <sys/param.h>

for (i = 0; i < NOFILE; i++)

close(i);

Other programs use the constant_NFILEthat some versions of<stdio.h>provide as

the upper limit. Some hardcode the upper limit as 20. However,none of these

approaches is portable.

We would hope to use the POSIX.1 value OPEN_MAXto determine this value

portably,but if the value is indeterminate, we still have a problem. If we wrote the

following code and ifOPEN_MAXwas indeterminate, the loop would never execute,

sincesysconfwould return−1: