snprintf function not declared? - c

Question

I'm trying to use the snprintf function which based on the manual I've read is apart of the <stdio.h> header however I'm getting an error that it's been implicitly declared. Here is my code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct users {
char* user_id;
};
typedef struct users users_t;
int save_user_detail(user_t);
int main() {
users_t users;
save_user_detail(users);
return 0;
}
int save_user_detail(users_t users)
{
printf("Type the filename = ");
scanf("%s", users.user_id);
char* extension = ".txt";
char fileSpec[strlen(users.user_id)+strlen(extension)+1];
FILE *file;
snprintf(fileSpec, sizeof(fileSpec), "%s%s", users.user_id, extension);
file = fopen(fileSpec, "w");
if(file==NULL)
{
printf("Error: can't open file.\n");
return 1;
}
else
{
printf("File written successfully.\n");
fprintf(file, "WORKS!\r\n");
}
fclose(file);
return 0;
}

Answer 1

You seem to be using gcc, but this compiler does not necessarily use the glibc, which is compliant with the C Standard and supports snprintf.
On the Windows architecture, you may be using the Microsoft C library, which in older versions did not have snprintf or renamed it as _snprintf.
Here are 2 ways you can try a work around your problem:
try using _snprintf instead of snprintf.
define snprintf manually after including <stdio.h> as
int snprintf(char *buf, size_t size, const char *fmt, ...);
The compiler should stop complaining about the missing prototype and if the runtime library does have a symbol for snprintf with a matching calling convention, it will link to it and the program should behave as expected.