setenv(3) — Linux manual page

 setenv(3) Library Functions Manual setenv(3) 

NAME         top

 setenv - change or add an environment variable 

LIBRARY         top

 Standard C library (libc, -lc) 

SYNOPSIS         top

 #include <stdlib.h> int setenv(const char *name, const char *value, int overwrite); int unsetenv(const char *name); Feature Test Macro Requirements for glibc (see feature_test_macros(7)): setenv(), unsetenv(): _POSIX_C_SOURCE >= 200112L || /* glibc <= 2.19: */ _BSD_SOURCE 

DESCRIPTION         top

 The setenv() function adds the variable name to the environment with the value value, if name does not already exist. If name does exist in the environment, then its value is changed to value if overwrite is nonzero; if overwrite is zero, then the value of name is not changed (and setenv() returns a success status). This function makes copies of the strings pointed to by name and value (by contrast with putenv(3)). The unsetenv() function deletes the variable name from the environment. If name does not exist in the environment, then the function succeeds, and the environment is unchanged. 

RETURN VALUE         top

 setenv() and unsetenv() functions return zero on success, or -1 on error, with errno set to indicate the error. 

ERRORS         top

 EINVAL name is NULL, points to a string of length 0, or contains an '=' character. ENOMEM Insufficient memory to add a new variable to the environment. 

ATTRIBUTES         top

 For an explanation of the terms used in this section, see attributes(7). ┌──────────────────────────┬───────────────┬─────────────────────┐ │ Interface │ Attribute │ Value │ ├──────────────────────────┼───────────────┼─────────────────────┤ │ setenv(), unsetenv() │ Thread safety │ MT-Unsafe const:env │ └──────────────────────────┴───────────────┴─────────────────────┘ 

STANDARDS         top

 POSIX.1-2008. 

HISTORY         top

 POSIX.1-2001, 4.3BSD. Prior to glibc 2.2.2, unsetenv() was prototyped as returning void; more recent glibc versions follow the POSIX.1-compliant prototype shown in the SYNOPSIS. 

CAVEATS         top

 POSIX.1 does not require setenv() or unsetenv() to be reentrant. 

BUGS         top

 POSIX.1 specifies that if name contains an '=' character, then setenv() should fail with the error EINVAL; however, versions of glibc before glibc 2.3.4 allowed an '=' sign in name. 

SEE ALSO         top

 clearenv(3), getenv(3), putenv(3), environ(7) 

COLOPHON         top

 This page is part of the man-pages (Linux kernel and C library user-space interface documentation) project. Information about the project can be found at ⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report for this manual page, see ⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩. This page was obtained from the tarball man-pages-6.15.tar.gz fetched from ⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on 2025-08-11. If you discover any rendering problems in this HTML version of the page, or you believe there is a better or more up- to-date source for the page, or you have corrections or improvements to the information in this COLOPHON (which is not part of the original manual page), send a mail to man-pages@man7.org Linux man-pages 6.15 2025-05-17 setenv(3) 

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