stdio.h
typedef FILE ...; FILE *stdin, *stdout, *stderr; FILE *fopen(char *fileName, char *mode); int fgetc(FILE *inf); int fputc(int ch, FILE *outf); char *fgets(char *strbuf, int max, FILE *inf); int fputs(char *str, FILE *outf); ...
Many more examples available in .h files under /usr/include
.h
/usr/include