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# getenv, getenv_s

From cppreference.com
< c‎ | program

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 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`.

### [edit] 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)

### [edit] 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 of if a runtime constrant violation occurred. On any error, writes zero to *len (unless `len` is a null pointer).

### [edit] 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.

### [edit] Example

Run this code

    #include <stdio.h>
    #include <stdlib.h>

    int main(void)
    {
        char *env_p = getenv("PATH");
        if (env_p)
            printf("PATH = %s\n", env_p);
    }

Possible output:

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

### [edit] References

- 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):

  - 4.10.4.4 The getenv function

### [edit] See also

C++ documentation for getenv

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