getenv, getenv_s
From cppreference.com
C
Concurrency support (C11)
Program support utilities
Program termination
Unreachable control flow
(C23)
Communicating with the environment
getenvgetenv_s
(C11)
Memory alignment query
(C23)
Signals
Signal types
Non-local jumps
Types
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)
rsize_t valuesz, const char *restrict name );
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円' 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_sis 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 or if a runtime constraint 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) { const char *name = "PATH"; const char *env_p = getenv(name); if (env_p) printf ("Your %s is %s\n", name, env_p); }
Possible output:
Your PATH is /home/gamer/.local/bin:/usr/local/bin:/usr/bin:/bin:/usr/share/games
[edit] References
- C23 standard (ISO/IEC 9899:2024):
- 7.22.4.6 The getenv function (p: TBD)
- K.3.6.2.1 The getenv_s function (p: TBD)
- 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):
- 4.10.4.4 The getenv function
[edit] See also
C++ documentation for getenv