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setenvThis article will introduce several methods of using functions to export environment variables in C language .

setenvUse function to export environment variables in C language

Every program running on a Unix-base system has an environment, which is a collection of variable-value pairs used primarily by the shell and other user-space programs. A program can getenvretrieve a single environment variable and its value with the _environment_variable function. But if you want to define a new variable or change an existing one, you should call the _environment_variable setenvfunction. It takes three arguments, the first and second arguments are charpointers to the variable name and its value, respectively. The third argument is of type _environment_variable int, which specifies whether the value of the given variable should be overwritten if it already exists in the environment. A non-zero value for this parameter indicates overwriting behavior, while a zero value indicates the opposite.

But please note that to iterate over all defined environment variables, you need to access environa special global variable named -, which is a NULL-terminated string array. Alternatively, you can declare maina function with a third parameter envpto access the variable. In the following example, we print both the and pointers setenvbefore and after calling the function . Note that after the call, the value of the pointer is the same, while the value of has changed.enironenvpenvpenviron

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

extern char **environ;

int main(int argc, const char *argv[], const char *envp[]) {

    printf("environ:  %p\n", environ);
    printf("envp:     %p\n", envp);

    setenv("NEW_VAR", "new_value", 1);

    puts("----Added NEW_VAR----");
    printf("environ:  %p\n", environ);
    printf("envp:     %p\n", envp);

    exit(EXIT_SUCCESS);
}

Output:

environ:  0x7fffa05a7fe8
envp:     0x7fffa05a7fe8
----Added NEW_VAR----
environ:  0x5646431276b0
envp:     0x7fffa05a7fe8

Use envpvariables to iterate over defined environment variables in C

The previous code example illustrates why you should avoid setenvusing mainthe function parameter envpto retrieve environment variables after calling the function. When setenvthe function is called, the environment is relocated, but envpstill points to the old environment. The following sample code demonstrates the incorrect behavior by setenvdefining a new variable with the call and then iterating over the array of pointers. Note that we also use the statement to jump to the end of the loop envpif NEW_VARa variable named is found.goto

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

int main(int argc, const char *argv[], const char *envp[]) {

    if (setenv("NEW_VAR", "new_value", 1) != 0) {
        perror("setenv");
        exit(EXIT_FAILURE);
    }

    if (envp != NULL) {
        for (size_t i = 0; envp[i] != NULL; ++i) {
            if (strcmp(envp[i], "NEW_VAR=new_value") == 0) {
                puts(envp[i]);
                goto END;
            }
        }
        printf("No such variable found!\n");
    } END:

    exit(EXIT_SUCCESS);
}

Output:

No such variable found!

Use environvariables to iterate over defined environment variables in C

Since the previous solution cannot find the newly defined variables, we should use externally declared environvariables. In this case, we have included multiple preprocessor conditional statements to make the code portable across different systems, but it is only needed in most UNIX-based systems extern char **environ. Once declared, we can use the same loop we implemented in the previous example to iterate over the list of variables.

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

#if defined (_POSIX_) || defined (__USE_POSIX)
extern char **environ;
#elif defined(_WIN32)
_CRTIMP extern char **_environ;
#endif

int main(void) {

    if (setenv("NEW_VAR", "new_value", 1) != 0) {
        perror("setenv");
        exit(EXIT_FAILURE);
    }

    if (environ != NULL) {
        for (size_t i = 0; environ[i] != NULL; ++i) {
            if (strcmp(environ[i], "NEW_VAR=new_value") == 0) {
                puts(environ[i]);
                goto END;
            }
        }
        printf("No such variable found!\n");
    } END:

    exit(EXIT_SUCCESS);
}

Output:

NEW_VAR=new_value