Q44033: DOS Append Interfaces with execvp() in C

Article: Q44033
Product(s): See article
Version(s): 5.10
Operating System(s): MS-DOS
Keyword(s): ENDUSER | | mspl13_c
Last Modified: 25-MAY-1989

The versions of the exec and spawn (with P_OVERLAY) functions that
search the path for the child program to be run, fail with run-time
error "R6005 not enough memory on exec" if the child is not in the
current directory and the DOS APPEND command points to the child's
directory.

The DOS Versions 3.30 and later APPEND command is used to specify
directories for locating data files.

APPEND incorrectly finds .EXE and .COM files when DOS interrupt 21
hex, function 3D hex (open file) is called. This can cause the C
library function sopen() to mistakenly report that it has succeeded in
finding a .COM or .EXE file in the current directory when, in fact,
the file is in another directory that is pointed to by APPEND.

The path-searching versions of the exec and spawn (with P_OVERLAY)
functions use sopen() to first try to open the .COM or .EXE file in
the current working directory. If sopen() fails because the file is
not there, successive directories specified by the PATH are prepended
to the child's filename, and sopen() is called again until it
successfully opens the child's file in one of the directories
specified by the PATH.

If the child is not in the current working directory, but is pointed
to by APPEND, sopen() mistakenly reports that it has successfully
opened the child in the current directory and the necessary path is
not prepended to the child's filename. When the exec/spawn functions
call DOS interrupt 21 hex, function 4B hex, subfunction 3 (load and
execute overlay program), the child can't be found in the current
working directory, and the exec fails. (Unlike DOS function 3D hex to
open a file, DOS function 4B hex is not affected by APPEND.) At this
point, the only error C exec/spawn functions are designed to report is
that there isn't enough memory available for the child process, and
the R6005 error is issued.

The path searching exec/spawn functions that can be adversely affected
by APPEND are execlp, execlpe, execvp, execvpe, spawnlp, spawnlpe,
spawnvp, and spawnvpe. The spawn functions using P_WAIT are not
affected by APPEND since they do not use sopen().

The Microsoft C run-time library functions do not support the use of
APPEND. There are no plans to do so in the future.

To work around problems caused by APPEND, disabling APPEND with the
DOS command "APPEND;" is recommended. Or, within a program, you may
use the system() library function for this purpose (except for
versions of DOS that support the /E APPEND option to place APPEND in
the environment table). If your program needs to accommodate APPEND,
you could use the _searchenv() function to obtain the path. Then
traverse the path prepending successive drive and directory names to
the child's filename for use with sopen() until an sopen() call
returns a valid file handle indicating the child program has been
found. Once you have the exact path of the .EXE or .COM file, pass
the complete path the the appropriate exec?? or spawn?? function.

Note: DON'T use one of the functions that has a "p" in its name --
these functions search the path for the file, something you've
already done if you've followed these directions.

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