An environment variable is a name–value pair of strings supplied through a program’s runtime environment. A C program can inspect this environment list to receive operating-system-dependent settings such as HOME, PATH, LANG, or LC_ALL. section 8.7
void printenv(char const name[static 1]) {
if (getenv(name)) {
char value[256] = { };
if (getenv_s(nullptr, value, sizeof value, name)) {
fprintf(stderr,
"%s: value is longer than %zu\n",
name, sizeof value);
} else {
printf("%s=%s\n", name, value);
}
} else {
fprintf(stderr, "%s not in environment\n", name);
}
}
This helper checks whether a name exists, copies its value into a fixed-size buffer with getenv_s, and reports when the value does not fit. section 8.7
Safe access follows these rules:
- Use getenv to test whether a name is present, then use getenv_s to copy its value into a destination buffer. section 8.7
- Pass nullptr as getenv_s’s first argument when only the copied value is needed; the destination is written only when the result fits. section 8.7
- Treat available names as operating-system-dependent rather than universally portable. section 8.7
Printing a setting
A printenv helper can report whether HOME exists, print HOME=value when its value fits a 256-character buffer, and otherwise report that the value is longer than the buffer. section 8.7
