Using string functions like strtoi or strtoumax, linker error

[Gatze]

This should not be very hard (beginners question), I want to convert an array of characters to an integer, I added string.h but it is not working with the function strtoi (linker error)

Should I add a c file or something ? and where to find ?

[Gatze]

Already solved, made a mistake by reading i like an L from the strtoi but should be strtol.....