The getenv function is a standard library function in C that is used to retrieve the value of an environment variable. An environment variable is a named value that is maintained by the operating system and is used to control various aspects of the system and user environment.

The syntax of the getenv function is as follows:

char *getenv(const char *name);

The getenv function takes a single argument, which is a pointer to a null-terminated string containing the name of the environment variable. If the environment variable exists, the function returns a pointer to a null-terminated string containing its value. If the environment variable does not exist, the function returns a null pointer.

Here is an example that demonstrates how to use the getenv function:

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

int main(void) {
    char *path = getenv("PATH");
    if (path == NULL) {
        printf("PATH environment variable is not set.\n");
    } else {
        printf("PATH=%s\n", path);
    }
    return 0;
}

In this example, the getenv function is used to retrieve the value of the PATH environment variable, which is a list of directories that are searched when a command is entered at the command prompt. The value of the PATH variable is stored in the path variable, and then printed to the console using printf.

Note that the getenv function is not thread-safe, and should not be used in a multithreaded program without proper synchronization.