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gutils: Add functions for working with environment arrays

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When spawning a child process, it is not safe to call setenv() before
the fork() (because setenv() isn't thread-safe), but it's also not
safe to call it after the fork() (because it's not async-signal-safe).
So the only safe way to alter the environment for a child process from
a threaded program is to pass a fully-formed envp array to
exec*/g_spawn*/etc.

So, add g_environ_getenv(), g_environ_setenv(), and
g_environ_unsetenv(), which act like their namesakes, but work on
arbitrary arrays rather than working directly on the environment.

http://bugzilla.gnome.org/show_bug.cgi?id=659326
This commit is contained in:
Dan Winship 2011-10-15 15:52:28 -04:00 committed by Matthias Clasen
parent 5ff803d91f
commit 409d93148f
5 changed files with 225 additions and 48 deletions
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3
docs/reference/glib/glib-sections.txt
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@ -1665,6 +1665,9 @@ g_set_application_name
g_get_prgname
g_set_prgname
g_get_environ
g_environ_getenv
g_environ_setenv
g_environ_unsetenv
g_getenv
g_setenv
g_unsetenv

3
glib/glib.symbols
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@ -1310,6 +1310,9 @@ g_setenv PRIVATE
g_get_host_name
g_listenv
g_get_environ
g_environ_getenv
g_environ_setenv
g_environ_unsetenv
#ifdef G_OS_WIN32
g_find_program_in_path_utf8
g_get_current_dir_utf8

36
glib/gspawn.h
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@ -97,24 +97,36 @@ typedef enum
* @user_data: user data to pass to the function.
*
* Specifies the type of the setup function passed to g_spawn_async(),
* g_spawn_sync() and g_spawn_async_with_pipes(). On POSIX platforms it
* is called in the child after GLib has performed all the setup it plans
* to perform but before calling exec(). On POSIX actions taken in this
* function will thus only affect the child, not the parent.
* g_spawn_sync() and g_spawn_async_with_pipes(), which can, in very
* limited ways, be used to affect the child's execution.
*
* Note that POSIX allows only async-signal-safe functions (see signal(7))
* to be called in the child between fork() and exec(), which drastically
* limits the usefulness of child setup functions.
* On POSIX platforms, the function is called in the child after GLib
* has performed all the setup it plans to perform, but before calling
* exec(). Actions taken in this function will only affect the child,
* not the parent.
*
* Also note that modifying the environment from the child setup function
* may not have the intended effect, since it will get overridden by
* a non-%NULL @env argument to the <literal>g_spawn...</literal> functions.
*
* On Windows the function is called in the parent. Its usefulness on
* On Windows, the function is called in the parent. Its usefulness on
* Windows is thus questionable. In many cases executing the child setup
* function in the parent can have ill effects, and you should be very
* careful when porting software to Windows that uses child setup
* functions.
*
* However, even on POSIX, you are extremely limited in what you can
* safely do from a #GSpawnChildSetupFunc, because any mutexes that
* were held by other threads in the parent process at the time of the
* fork() will still be locked in the child process, and they will
* never be unlocked (since the threads that held them don't exist in
* the child). POSIX allows only async-signal-safe functions (see
* <citerefentry><refentrytitle>signal</refentrytitle><manvolnum>7</manvolnum></citerefentry>)
* to be called in the child between fork() and exec(), which
* drastically limits the usefulness of child setup functions.
*
* In particular, it is not safe to call any function which may
* call malloc(), which includes POSIX functions such as setenv().
* If you need to set up the child environment differently from
* the parent, you should use g_get_environ(), g_environ_setenv(),
* and g_environ_unsetev(), and then pass the complete environment
* list to the <literal>g_spawn...</literal> function.
*/
typedef void (* GSpawnChildSetupFunc) (gpointer user_data);

204
glib/gutils.c
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@ -1215,13 +1215,13 @@ _g_getenv_nomalloc (const gchar *variable,
*
* Sets an environment variable. Both the variable's name and value
* should be in the GLib file name encoding. On UNIX, this means that
* they can be any sequence of bytes. On Windows, they should be in
* they can be arbitrary byte strings. On Windows, they should be in
* UTF-8.
*
* Note that on some systems, when variables are overwritten, the memory
* Note that on some systems, when variables are overwritten, the memory
* used for the previous variables and its value isn't reclaimed.
*
* <warning><para>
* <warning>
* Environment variable handling in UNIX is not thread-safe, and your
* program may crash if one thread calls g_setenv() while another
* thread is calling getenv(). (And note that many functions, such as
@ -1229,7 +1229,12 @@ _g_getenv_nomalloc (const gchar *variable,
* use at the very start of your program, before creating any other
* threads (or creating objects that create worker threads of their
* own).
* </para></warning>
*
* If you need to set up the environment for a child process, you can
* use g_get_environ() to get an environment array, modify that with
* g_environ_setenv() and g_environ_unsetenv(), and then pass that
* array directly to execvpe(), g_spawn_async(), or the like.
* </warning>
*
* Returns: %FALSE if the environment variable couldn't be set.
*
@ -1331,7 +1336,7 @@ extern char **environ;
* Note that on some systems, when variables are overwritten, the memory
* used for the previous variables and its value isn't reclaimed.
*
* <warning><para>
* <warning>
* Environment variable handling in UNIX is not thread-safe, and your
* program may crash if one thread calls g_unsetenv() while another
* thread is calling getenv(). (And note that many functions, such as
@ -1339,7 +1344,12 @@ extern char **environ;
* use at the very start of your program, before creating any other
* threads (or creating objects that create worker threads of their
* own).
* </para></warning>
*
* If you need to set up the environment for a child process, you can
* use g_get_environ() to get an environment array, modify that with
* g_environ_setenv() and g_environ_unsetenv(), and then pass that
* array directly to execvpe(), g_spawn_async(), or the like.
* </warning>
*
* Since: 2.4
**/
@ -1354,31 +1364,13 @@ g_unsetenv (const gchar *variable)
unsetenv (variable);
#else /* !HAVE_UNSETENV */
int len;
gchar **e, **f;
g_return_if_fail (variable != NULL);
g_return_if_fail (strchr (variable, '=') == NULL);
len = strlen (variable);
/* Mess directly with the environ array.
* This seems to be the only portable way to do this.
*
* Note that we remove *all* environment entries for
* the variable name, not just the first.
*/
e = f = environ;
while (*e != NULL)
{
if (strncmp (*e, variable, len) != 0 || (*e)[len] != '=')
{
*f = *e;
f++;
}
e++;
}
*f = NULL;
g_environ_unsetenv (environ, variable);
#endif /* !HAVE_UNSETENV */
#else /* G_OS_WIN32 */
@ -1487,7 +1479,7 @@ g_listenv (void)
/**
* g_get_environ:
*
*
* Gets the list of environment variables for the current process. The
* list is %NULL terminated and each item in the list is of the form
* 'NAME=VALUE'.
@ -1498,7 +1490,8 @@ g_listenv (void)
* The return value is freshly allocated and it should be freed with
* g_strfreev() when it is no longer needed.
*
* Returns: (array zero-terminated=1) (transfer full): the list of environment variables
* Returns: (array zero-terminated=1) (transfer full): the list of
* environment variables
*
* Since: 2.28
*/
@ -1524,6 +1517,163 @@ g_get_environ (void)
#endif
}
static gint
g_environ_find (gchar **envp,
const gchar *variable)
{
gint len, i;
len = strlen (variable);
for (i = 0; envp[i]; i++)
{
if (strncmp (envp[i], variable, len) == 0 &&
envp[i][len] == '=')
return i;
}
return -1;
}
/**
* g_environ_getenv:
* @envp: (array zero-terminated=1) (transfer none): an environment
* list (eg, as returned from g_get_environ())
* @variable: the environment variable to get, in the GLib file name
* encoding
*
* Returns the value of the environment variable @variable in the
* provided list @envp.
*
* The name and value are in the GLib file name encoding.
* On UNIX, this means the actual bytes which might or might not
* be in some consistent character set and encoding. On Windows,
* it is in UTF-8. On Windows, in case the environment variable's
* value contains references to other environment variables, they
* are expanded.
*
* Return value: the value of the environment variable, or %NULL if
* the environment variable is not set in @envp. The returned
* string is owned by @envp, and will be freed if @variable is
* set or unset again.
*
* Since: 2.32
*/
const gchar *
g_environ_getenv (gchar **envp,
const gchar *variable)
{
gint index;
g_return_val_if_fail (envp != NULL, NULL);
g_return_val_if_fail (variable != NULL, NULL);
index = g_environ_find (envp, variable);
if (index != -1)
return envp[index] + strlen (variable) + 1;
else
return NULL;
}
/**
* g_environ_setenv:
* @envp: (array zero-terminated=1) (transfer full): an environment
* list (eg, as returned from g_get_environ())
* @variable: the environment variable to set, must not contain '='
* @value: the value for to set the variable to
* @overwrite: whether to change the variable if it already exists
*
* Sets the environment variable @variable in the provided list
* @envp to @value.
*
* Both the variable's name and value should be in the GLib
* file name encoding. On UNIX, this means that they can be
* arbitrary byte strings. On Windows, they should be in UTF-8.
*
* Return value: (array zero-terminated=1) (transfer full): the
* updated environment
*
* Since: 2.32
*/
gchar **
g_environ_setenv (gchar **envp,
const gchar *variable,
const gchar *value,
gboolean overwrite)
{
gint index;
g_return_val_if_fail (envp != NULL, NULL);
g_return_val_if_fail (variable != NULL, NULL);
g_return_val_if_fail (strchr (variable, '=') == NULL, NULL);
index = g_environ_find (envp, variable);
if (index != -1)
{
if (overwrite)
{
g_free (envp[index]);
envp[index] = g_strdup_printf ("%s=%s", variable, value);
}
}
else
{
gint length;
length = g_strv_length (envp);
envp = g_renew (gchar *, envp, length + 2);
envp[length] = g_strdup_printf ("%s=%s", variable, value);
envp[length + 1] = NULL;
}
return envp;
}
/**
* g_environ_unsetenv:
* @envp: (array zero-terminated=1) (transfer full): an environment
* list (eg, as returned from g_get_environ())
* @variable: the environment variable to remove, must not contain '='
*
* Removes the environment variable @variable from the provided
* environment @envp.
*
* Return value: (array zero-terminated=1) (transfer full): the
* updated environment
*
* Since: 2.32
*/
gchar **
g_environ_unsetenv (gchar **envp,
const gchar *variable)
{
gint len;
gchar **e, **f;
g_return_val_if_fail (envp != NULL, NULL);
g_return_val_if_fail (variable != NULL, NULL);
g_return_val_if_fail (strchr (variable, '=') == NULL, NULL);
len = strlen (variable);
/* Note that we remove *all* environment entries for
* the variable name, not just the first.
*/
e = f = envp;
while (*e != NULL)
{
if (strncmp (*e, variable, len) != 0 || (*e)[len] != '=')
{
*f = *e;
f++;
}
e++;
}
*f = NULL;
return envp;
}
G_LOCK_DEFINE_STATIC (g_utils_global);
static gchar *g_tmp_dir = NULL;

27
glib/gutils.h
View File

@ -265,17 +265,26 @@ void g_nullify_pointer (gpointer *nullify_location);
#endif
#endif
const gchar * g_getenv (const gchar *variable);
gboolean g_setenv (const gchar *variable,
const gchar *value,
gboolean overwrite);
void g_unsetenv (const gchar *variable);
gchar** g_listenv (void);
gchar** g_get_environ (void);
const gchar * g_getenv (const gchar *variable);
gboolean g_setenv (const gchar *variable,
const gchar *value,
gboolean overwrite);
void g_unsetenv (const gchar *variable);
gchar ** g_listenv (void);
gchar ** g_get_environ (void);
const gchar * g_environ_getenv (gchar **envp,
const gchar *variable);
gchar ** g_environ_setenv (gchar **envp,
const gchar *variable,
const gchar *value,
gboolean overwrite) G_GNUC_WARN_UNUSED_RESULT;
gchar ** g_environ_unsetenv (gchar **envp,
const gchar *variable) G_GNUC_WARN_UNUSED_RESULT;
/* private */
const gchar* _g_getenv_nomalloc (const gchar *variable,
gchar buffer[1024]);
const gchar* _g_getenv_nomalloc (const gchar *variable,
gchar buffer[1024]);
/**
* GVoidFunc: