The puts function in the stdio.h library of the C programming language is used to write a string to the standard output stream (stdout). The syntax of the puts function is as follows:

int puts(const char *str);

Here, the argument str is a pointer to the null-terminated string to be written to the standard output stream.

The puts function writes the specified string to the standard output stream, and returns a non-negative value on success. In case of an error, it returns EOF. The EOF macro is a constant defined in the stdio.h library that represents the end-of-file condition or an error condition.

For example, the following code writes the string "Hello, world!" to the standard output stream using the puts function:

#include <stdio.h>

int main() {
   int result;
   result = puts("Hello, world!");
   if (result == EOF) {
      printf("Error writing to stream\n");
   }
   return 0;
}

In this example, the puts function writes the string "Hello, world!" to the standard output stream (stdout). If the write operation is successful, the function returns a non-negative value (strlen(str) + 1), which is the number of characters written plus one for the newline character added by the function. If an error occurs during the write operation, the function returns EOF, and the code in the if statement is executed.