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getenv, getenv_s

Defined in header `<stdlib.h>`
char getenv( const char name );	(1)
errno_t getenv_s( size_t restrict len, char restrict value, rsize_t valuesz, const char *restrict name );	(2)	(since C11)

1) Searches for an environmental variable with name name in the host-specified environment list and returns a pointer to the string that is associated with the matched environment variable. The set of environmental variables and methods of altering it are implementation-defined.

This function is not required to be thread-safe. Another call to getenv, as well as a call to the POSIX functions setenv(), unsetenv(), and putenv() may invalidate the pointer returned by a previous call or modify the string obtained from a previous call.

Modifying the string returned by getenv invokes undefined behavior.

2) Same as (1), except that the values of the environment variable is written to the user-provided buffer value (unless null) and the number of bytes written is stored in the user-provided location *len (unless null). If the environment variable is not set in the environment, zero is written to *len (unless null) and '\0' is written to value[0] (unless null). In addition, the following errors are detected at runtime and call the currently installed constraint handler function:

name is a null pointer
valuesz is greater than RSIZE_MAX
value is a null pointer and valuesz is not zero

As with all bounds-checked functions, getenv_s is only guaranteed to be available if __STDC_LIB_EXT1__ is defined by the implementation and if the user defines __STDC_WANT_LIB_EXT1__ to the integer constant 1 before including <stdlib.h>.

Parameters

name	-	null-terminated character string identifying the name of the environmental variable to look for
len	-	pointer to a user-provided location where `getenv_s` will store the length of the environment variable
value	-	pointer to a user-provided character array where `getenv_s` will store the contents of the environment variable
valuesz	-	maximum number of characters that `getenv_s` is allowed to write to `dest` (size of the buffer)

Return value

1) character string identifying the value of the environmental variable or null pointer if such variable is not found.

2) zero if the environment variable was found, non-zero if it was not found or if a runtime constraint violation occurred. On any error, writes zero to *len (unless len is a null pointer).

Notes

On POSIX systems, the environment variables are also accessible through the global variable environ, declared as extern char **environ; in <unistd.h>, and through the optional third argument, envp, of the main function.

The call to getenv_s with a null pointer for value and zero for valuesz is used to determine the size of the buffer required to hold the entire result.

Example

#include <stdio.h>
#include <stdlib.h>
 
int main(void)
{
    const char *name = "PATH";
    const char *env_p = getenv(name);
    if (env_p)
        printf("%s = %s\n", name, env_p);
}

Possible output:

PATH = /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

References

C17 standard (ISO/IEC 9899:2018):

7.22.4.6 The getenv function (p: 256-257)
K.3.6.2.1 The getenv_s function (p: 440-441)

C11 standard (ISO/IEC 9899:2011):

7.22.4.6 The getenv function (p: 352-353)
K.3.6.2.1 The getenv_s function (p: 606-607)

C99 standard (ISO/IEC 9899:1999):

7.20.4.5 The getenv function (p: 317)

C89/C90 standard (ISO/IEC 9899:1990):