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The value of an environment variable can be accessed with the
getenv function. This is declared in the header file
stdlib.h.
Libraries should use secure_getenv instead of getenv, so
that they do not accidentally use untrusted environment variables.
Modifications of environment variables are not allowed in
multi-threaded programs. The getenv and secure_getenv
functions can be safely used in multi-threaded programs.
Preliminary: | MT-Safe env | AS-Safe | AC-Safe | See POSIX Safety Concepts.
This function returns a string that is the value of the environment
variable name. You must not modify this string. In some non-Unix
systems not using the GNU C Library, it might be overwritten by subsequent
calls to getenv (but not by any other library function). If the
environment variable name is not defined, the value is a null
pointer.
Preliminary: | MT-Safe env | AS-Safe | AC-Safe | See POSIX Safety Concepts.
This function is similar to getenv, but it returns a null
pointer if the environment is untrusted. This happens when the
program file has SUID or SGID bits set. General-purpose libraries
should always prefer this function over getenv to avoid
vulnerabilities if the library is referenced from a SUID/SGID program.
This function is a GNU extension.
Preliminary: | MT-Unsafe const:env | AS-Unsafe heap lock | AC-Unsafe corrupt lock mem | See POSIX Safety Concepts.
The putenv function adds or removes definitions from the environment.
If the string is of the form ‘name=value’, the
definition is added to the environment. Otherwise, the string is
interpreted as the name of an environment variable, and any definition
for this variable in the environment is removed.
If the function is successful it returns 0. Otherwise the return
value is nonzero and errno is set to indicate the error.
The difference to the setenv function is that the exact string
given as the parameter string is put into the environment. If the
user should change the string after the putenv call this will
reflect automatically in the environment. This also requires that
string not be an automatic variable whose scope is left before the
variable is removed from the environment. The same applies of course to
dynamically allocated variables which are freed later.
This function is part of the extended Unix interface. You should define _XOPEN_SOURCE before including any header.
Preliminary: | MT-Unsafe const:env | AS-Unsafe heap lock | AC-Unsafe corrupt lock mem | See POSIX Safety Concepts.
The setenv function can be used to add a new definition to the
environment. The entry with the name name is replaced by the
value ‘name=value’. Please note that this is also true
if value is the empty string. To do this a new string is created
and the strings name and value are copied. A null pointer
for the value parameter is illegal. If the environment already
contains an entry with key name the replace parameter
controls the action. If replace is zero, nothing happens. Otherwise
the old entry is replaced by the new one.
Please note that you cannot remove an entry completely using this function.
If the function is successful it returns 0. Otherwise the
environment is unchanged and the return value is -1 and
errno is set.
This function was originally part of the BSD library but is now part of the Unix standard.
Preliminary: | MT-Unsafe const:env | AS-Unsafe lock | AC-Unsafe lock | See POSIX Safety Concepts.
Using this function one can remove an entry completely from the
environment. If the environment contains an entry with the key
name this whole entry is removed. A call to this function is
equivalent to a call to putenv when the value part of the
string is empty.
The function return -1 if name is a null pointer, points to
an empty string, or points to a string containing a = character.
It returns 0 if the call succeeded.
This function was originally part of the BSD library but is now part of the Unix standard. The BSD version had no return value, though.
There is one more function to modify the whole environment. This function is said to be used in the POSIX.9 (POSIX bindings for Fortran 77) and so one should expect it did made it into POSIX.1. But this never happened. But we still provide this function as a GNU extension to enable writing standard compliant Fortran environments.
Preliminary: | MT-Unsafe const:env | AS-Unsafe heap lock | AC-Unsafe lock mem | See POSIX Safety Concepts.
The clearenv function removes all entries from the environment.
Using putenv and setenv new entries can be added again
later.
If the function is successful it returns 0. Otherwise the return
value is nonzero.
You can deal directly with the underlying representation of environment objects to add more variables to the environment (for example, to communicate with another program you are about to execute; see Executing a File).
The environment is represented as an array of strings. Each string is of the format ‘name=value’. The order in which strings appear in the environment is not significant, but the same name must not appear more than once. The last element of the array is a null pointer.
This variable is declared in the header file unistd.h.
If you just want to get the value of an environment variable, use
getenv.
Unix systems, and GNU systems, pass the initial value of
environ as the third argument to main.
See Program Arguments.
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