How to check if a string starts with another string in C?

c string comparison startswith
thejh picture thejh · Jan 22, 2011 · Viewed 80.4k times · Source

Is there something like startsWith(str_a, str_b) in the standard C library?

It should take pointers to two strings that end with nullbytes, and tell me whether the first one also appears completely at the beginning of the second one.

Examples:

"abc", "abcdef" -> true
"abcdef", "abc" -> false
"abd", "abdcef" -> true
"abc", "abc"    -> true

Answer

Fred Foo picture Fred Foo · Jan 22, 2011

There's no standard function for this, but you can define

bool prefix(const char *pre, const char *str)
{
    return strncmp(pre, str, strlen(pre)) == 0;
}

We don't have to worry about str being shorter than pre because according to the C standard (7.21.4.4/2):

The strncmp function compares not more than n characters (characters that follow a null character are not compared) from the array pointed to by s1 to the array pointed to by s2."

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