pty/pty_master_open_bsd.c

This is pty/pty_master_open_bsd.c (Listing 64-4, page 1396), an example from the book, The Linux Programming Interface.

The source code file is copyright 2024, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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In the listing below, the names of Linux system calls and C library functions are hyperlinked to manual pages from the Linux man-pages project, and the names of functions implemented in the book are hyperlinked to the implementations of those functions.

  Cover of The Linux Programming Interface
+/* pty_master_open_bsd.c
+
+   Implement our ptyMasterOpen() function, based on BSD pseudoterminals.
+   See comments below.
+
+   Note: BSD pseudoterminals are not specified in SUSv3, and are considered
+   obsolete on Linux.
+
+   See also pty_master_open.c.
+*/
 #include <fcntl.h>
 #include "pty_master_open.h"            /* Declares ptyMasterOpen() */
 #include "tlpi_hdr.h"
 
 #define PTYM_PREFIX     "/dev/pty"
 #define PTYS_PREFIX     "/dev/tty"
 #define PTY_PREFIX_LEN  (sizeof(PTYM_PREFIX) - 1)
 #define PTY_NAME_LEN    (PTY_PREFIX_LEN + sizeof("XY"))
 #define X_RANGE         "pqrstuvwxyzabcde"
 #define Y_RANGE         "0123456789abcdef"
 
+/* Open a BSD pty master, returning file descriptor, or -1 on error.
+   On successful completion, the name of the corresponding pty slave
+   is returned in 'slaveName'. 'snLen' should be set to indicate the
+   size of the buffer pointed to by 'slaveName'. */
+
 int
 ptyMasterOpen(char *slaveName, size_t snLen)
 {
     int masterFd, n;
     char *x, *y;
     char masterName[PTY_NAME_LEN];
 
     if (PTY_NAME_LEN > snLen) {
         errno = EOVERFLOW;
         return -1;
     }
 
     memset(masterName, 0, PTY_NAME_LEN);
     strncpy(masterName, PTYM_PREFIX, PTY_PREFIX_LEN);
 
+    /* Scan through all possible BSD pseudoterminal master names until
+       we find one that we can open. */
+
     for (x = X_RANGE; *x != '\0'; x++) {
         masterName[PTY_PREFIX_LEN] = *x;
 
         for (y = Y_RANGE; *y != '\0'; y++) {
             masterName[PTY_PREFIX_LEN + 1] = *y;
 
             masterFd = open(masterName, O_RDWR);
 
             if (masterFd == -1) {
                 if (errno == ENOENT)    /* No such file */
                     return -1;          /* Probably no more pty devices */
                 else                    /* Other error (e.g., pty busy) */
                     continue;
 
             } else {            /* Return slave name corresponding to master */
                 n = snprintf(slaveName, snLen, "%s%c%c", PTYS_PREFIX, *x, *y);
                 if (n >= snLen) {
                     errno = EOVERFLOW;
                     return -1;
                 } else if (n == -1) {
                     return -1;
                 }
 
                 return masterFd;
             }
         }
     }
 
     return -1;                  /* Tried all ptys without success */
 }

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