Re: ERRNO is unset until set

[Bart Schaefer <schaefer@brasslantern.com>]

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On Thu, Apr 28, 2022 at 7:17 AM Matthew Martin <phy1729@gmail.com> wrote:
>
> Should have git blamed first. This was changed from 0 to PM_UNSET in
> 32337 for emulation compatibility. Perhaps the docs should be updated
> instead?

We should at least be consistent.

These code changes are pretty minor but perhaps shouldn't go in before
the pending release.

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diff --git a/Doc/Zsh/mod_system.yo b/Doc/Zsh/mod_system.yo
index 884c3e753..e25201faa 100644
--- a/Doc/Zsh/mod_system.yo
+++ b/Doc/Zsh/mod_system.yo
@@ -74,6 +74,11 @@ truncate file to size 0
 )
 enditem()
 
+A return status of 0 indicates the descriptor was successfully opened,
+otherwise an error message is printed, and 1 is returned for an error
+in the parameters to the command, or 2 is returned for a system error.
+The parameter tt(ERRNO) is nonzero for system errors.
+
 To close the file, use one of the following:
 
 example(tt(exec {)var(fd)tt(}<&-)
@@ -123,11 +128,11 @@ error for which a message is printed to standard error.
 )
 item(2)(
 There was an error on the read, or on polling the input file descriptor
-for a timeout.  The parameter tt(ERRNO) gives the error.
+for a timeout.  The parameter tt(ERRNO) identifies the error.
 )
 item(3)(
 Data were successfully read, but there was an error writing them
-to var(outfd).  The parameter tt(ERRNO) gives the error.
+to var(outfd).  The parameter tt(ERRNO) identifies the error.
 )
 item(4)(
 The attempt to read timed out.  Note this does not set tt(ERRNO) as this
@@ -147,6 +152,11 @@ expression. The tt(-u) option allows the file descriptor to be specified. By
 default the offset is specified relative to the start or the file but, with the
 tt(-w) option, it is possible to specify that the offset should be relative to
 the current position or the end of the file.
+
+The return status may be 0 for success, 1 for an error in the parameters
+to the command, or 2 for an error on the seek; no error message is
+printed in the last case, but the parameter tt(ERRNO) reflects
+the error that occurred.
 )
 item(tt(syswrite) [ tt(-c) var(countvar) ] [ tt(-o) var(outfd) ] var(data))(
 The data (a single string of bytes) are written to the file descriptor
@@ -166,7 +176,7 @@ returning early.
 
 The return status may be 0 for success, 1 for an error in the parameters
 to the command, or 2 for an error on the write; no error message is
-printed in the last case, but the parameter tt(ERRNO) will reflect
+printed in the last case, but the parameter tt(ERRNO) reflects
 the error that occurred.
 )
 xitem(tt(zsystem flock) [ tt(-t) var(timeout) ] [ tt(-i) var(interval) ] [ tt(-f) var(var) ] [tt(-er)] var(file))
diff --git a/Doc/Zsh/params.yo b/Doc/Zsh/params.yo
index 6d2d41b7a..b543d1c38 100644
--- a/Doc/Zsh/params.yo
+++ b/Doc/Zsh/params.yo
@@ -750,6 +750,9 @@ as set by the most recently failed system call.
 This value is system dependent and is intended for debugging
 purposes.  It is also useful with the tt(zsh/system) module which
 allows the number to be turned into a name or message.
+
+To use this parameter, it must first be assigned a value (typically
+0 (zero)).  It is initially unset for scripting compatibility.
 )
 vindex(FUNCNEST)
 item(tt(FUNCNEST) <S>)(
diff --git a/Src/Modules/system.c b/Src/Modules/system.c
index ea11ef037..929a8b002 100644
--- a/Src/Modules/system.c
+++ b/Src/Modules/system.c
@@ -74,6 +74,8 @@ bin_sysread(char *nam, char **args, Options ops, UNUSED(int func))
     int infd = 0, outfd = -1, bufsize = SYSREAD_BUFSIZE, count;
     char *outvar = NULL, *countvar = NULL, *inbuf;
 
+    errno = 0;	/* Distinguish non-system errors */
+
     /* -i: input file descriptor if not stdin */
     if (OPT_ISSET(ops, 'i')) {
 	infd = getposint(OPT_ARG(ops, 'i'), nam);
@@ -238,6 +240,8 @@ bin_syswrite(char *nam, char **args, Options ops, UNUSED(int func))
     int outfd = 1, len, count, totcount;
     char *countvar = NULL;
 
+    errno = 0;	/* Distinguish non-system errors */
+
     /* -o: output file descriptor if not stdout */
     if (OPT_ISSET(ops, 'o')) {
 	outfd = getposint(OPT_ARG(ops, 'o'), nam);
@@ -303,6 +307,13 @@ static struct { const char *name; int oflag; } openopts[] = {
     { "trunc", O_TRUNC }
 };
 
+/*
+ * Return values of bin_sysopen:
+ *	0	Success
+ *	1	Error in parameters to command
+ *	2	Error on open, ERRNO set by system
+ */
+
 /**/
 static int
 bin_sysopen(char *nam, char **args, Options ops, UNUSED(int func))
@@ -319,6 +330,8 @@ bin_sysopen(char *nam, char **args, Options ops, UNUSED(int func))
     int fdflags = 0;
 #endif
 
+    errno = 0;	/* Distinguish non-system errors */
+
     if (!OPT_ISSET(ops, 'u')) {
 	zwarnnam(nam, "file descriptor not specified");
 	return 1;
@@ -374,12 +387,12 @@ bin_sysopen(char *nam, char **args, Options ops, UNUSED(int func))
 
     if (fd == -1) {
 	zwarnnam(nam, "can't open file %s: %e", *args, errno);
-	return 1;
+	return 2;
     }
     moved_fd = (explicit > -1) ? redup(fd, explicit) : movefd(fd);
     if (moved_fd == -1) {
 	zwarnnam(nam, "can't open file %s", *args);
-	return 1;
+	return 2;
     }
 
 #ifdef FD_CLOEXEC
@@ -423,6 +436,8 @@ bin_sysseek(char *nam, char **args, Options ops, UNUSED(int func))
     char *whence;
     off_t pos;
 
+    errno = 0;	/* Distinguish non-system errors */
+
     /* -u:  file descriptor if not stdin */
     if (OPT_ISSET(ops, 'u')) {
 	fd = getposint(OPT_ARG(ops, 'u'), nam);