The less well-known but extremely useful (and standard since C89 — meaning ‘forever’) functions in the C library provide the information in a single call. Actually, there are multiple functions — an embarrassment of riches. The relevant ones for this are:
7.21.5.3 The strcspn function
Synopsis
#include <string.h> size_t strcspn(const char *s1, const char *s2);Description
The strcspn function computes the length of the maximum initial segment of the string
pointed to by s1 which consists entirely of characters not from the string pointed to by
s2.Returns
The strcspn function returns the length of the segment.
7.21.5.4 The strpbrk function
Synopsis
#include <string.h> char *strpbrk(const char *s1, const char *s2);Description
The strpbrk function locates the first occurrence in the string pointed to by s1 of any
character from the string pointed to by s2.Returns
The strpbrk function returns a pointer to the character, or a null pointer if no character
from s2 occurs in s1.
The question asks about ‘for each char in string … if it is in list of invalid chars’.
With these functions, you can write:
size_t len = strlen(test);
size_t spn = strcspn(test, "invald");
if (spn != len) { ...there's a problem... }
Or:
if (strpbrk(test, "invald") != 0) { ...there's a problem... }
Which is better depends on what else you want to do. There is also the related strspn() function which is sometimes useful (whitelist instead of blacklist).