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int putenv(char *string);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
putenv(): _SVID_SOURCE || _XOPEN_SOURCE
The putenv() function adds or changes the value of environment vari-
ables. The argument string is of the form name=value. If name does
not already exist in the environment, then string is added to the envi-
ronment. If name does exist, then the value of name in the environment
is changed to value. The string pointed to by string becomes part of
the environment, so altering the string changes the environment.
The putenv() function returns zero on success, or nonzero if an error
occurs. In the event of an error, errno is set to indicate the cause.
ENOMEM Insufficient space to allocate new environment.
SVr4, POSIX.1-2001, 4.3BSD.
The putenv() function is not required to be reentrant, and the one in
libc4, libc5 and glibc 2.0 is not, but the glibc 2.1 version is.
Description for libc4, libc5, glibc: If the argument string is of the
form name, and does not contain an '=' character, then the variable
name is removed from the environment. If putenv() has to allocate a
new array environ, and the previous array was also allocated by
putenv(), then it will be freed. In no case will the old storage asso-
ciated to the environment variable itself be freed.
The libc4 and libc5 and glibc 2.1.2 versions conform to SUSv2: the
pointer string given to putenv() is used. In particular, this string
becomes part of the environment; changing it later will change the
environment. (Thus, it is an error is to call putenv() with an auto-
matic variable as the argument, then return from the calling function
while string is still part of the environment.) However, glibc
2.0-2.1.1 differs: a copy of the string is used. On the one hand this
causes a memory leak, and on the other hand it violates SUSv2. This
has been fixed in glibc 2.1.2.
The 4.4BSD version, like glibc 2.0, uses a copy.
SUSv2 removes the const from the prototype, and so does glibc 2.1.3.