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@(#)getenv.3 8.2 (Berkeley) 12/11/93
$FreeBSD$
.Dd June 20, 2007 .Dt GETENV 3 .Os .Sh NAME .Nm getenv , .Nm putenv , .Nm setenv , .Nm unsetenv .Nd environment variable functions .Sh LIBRARY .Lb libc .Sh SYNOPSIS n stdlib.h .Ft char * .Fn getenv "const char *name" .Ft int .Fn setenv "const char *name" "const char *value" "int overwrite" .Ft int .Fn putenv "char *string" .Ft int .Fn unsetenv "const char *name" .Sh DESCRIPTION These functions set, unset and fetch environment variables from the host .Em environment list .
p The .Fn getenv function obtains the current value of the environment variable, .Fa name . The application should not modify the string pointed to by the .Fn getenv function.
p The .Fn setenv function inserts or resets the environment variable .Fa name in the current environment list. If the variable .Fa name does not exist in the list, it is inserted with the given .Fa value . If the variable does exist, the argument .Fa overwrite is tested; if .Fa overwrite is zero, the variable is not reset, otherwise it is reset to the given .Fa value .
p The .Fn putenv function takes an argument of the form ``name=value'' and puts it directly into the current environment, so altering the argument shall change the environment. If the variable .Fa name does not exist in the list, it is inserted with the given .Fa value . If the variable .Fa name does exist, it is reset to the given .Fa value .
p The .Fn unsetenv function deletes all instances of the variable name pointed to by .Fa name from the list.
p If corruption (e.g., a name without a value) is detected while making a copy of environ for internal usage, then .Fn setenv , .Fn unsetenv and .Fn putenv will output a warning to stderr about the issue, drop the corrupt entry and complete the task without error. .Sh RETURN VALUES The .Fn getenv function returns the value of the environment variable as a .Dv NUL Ns -terminated string. If the variable .Fa name is not in the current environment, .Dv NULL is returned.
p .Rv -std setenv putenv unsetenv .Sh ERRORS l -tag -width Er t Bq Er EINVAL The function .Fn getenv , .Fn setenv or .Fn unsetenv failed because the .Fa name is a .Dv NULL pointer, points to an empty string, or points to a string containing an .Dq Li = character.
p The function .Fn putenv failed because .Fa string is a .Dv NULL pointer, .Fa string is without an .Dq Li = character or .Dq Li = is the first character in .Fa string . This does not follow the .Tn POSIX specification. t Bq Er ENOMEM The function .Fn setenv , .Fn unsetenv or .Fn putenv failed because they were unable to allocate memory for the environment. .El .Sh SEE ALSO .Xr csh 1 , .Xr sh 1 , .Xr execve 2 , .Xr environ 7 .Sh STANDARDS The .Fn getenv function conforms to .St -isoC . The .Fn setenv , .Fn putenv and .Fn unsetenv functions conforms to .St -p1003.1-2001 . .Sh HISTORY The functions .Fn setenv and .Fn unsetenv appeared in .At v7 . The .Fn putenv function appeared in x 4.3 Reno .
p Until .Fx 7.0 , .Fn putenv would make a copy of .Fa string and insert it into the environment using .Fn setenv . This was changed to use .Fa string as the memory location of the ``name=value'' pair to follow the .Tn POSIX specification. .Sh BUGS Successive calls to .Fn setenv that assign a larger-sized .Fa value than any previous value to the same .Fa name will result in a memory leak. The .Fx semantics for this function (namely, that the contents of .Fa value are copied and that old values remain accessible indefinitely) make this bug unavoidable. Future versions may eliminate one or both of these semantic guarantees in order to fix the bug.