Strtok_r returning NULL

Question

I'm trying to tokenize a string taken from a file. strtok_r works properly on the first sub string and then returns null (and segmentation fault cause I try to strndup into an other var)

char buffer[500];
char * c;
char * c1;
char * c2;
//....
while(fgets(buffer, sizeof(buffer), f) != NULL){
    c2 = buffer;
    printf("%s\n", buffer);
    c = strtok(c2, ":");
    for(int i = 0; i < 4; i++){
        c = strtok(NULL, ":");
        printf("%s\n", c);
    }
    if(strcmp(c, argp->origen) == 0){
        c = strtok(NULL, ":");
        printf("%s\n", c);
        if(strcmp(c, argp->destino) == 0){
            nodo = malloc(sizeof(lista_vuelo));
            c2 = buffer;

            c = strtok_r(c2, ":", &c1); 
            nodo->IdReg = atoi(c);
            printf("\n%d test\n", nodo->IdReg); //Works until here

            c = strtok_r(NULL, ":", &c1);
            printf("\n%s\n", c); //Prints null and then segmentation fault

            nodo->Idvuelo = strndup(c, strlen(c));
            printf("\n%s\n", nodo->Idvuelo);
//....

Input from file :

3:IBE3674:02-04-2019:19-45:Madrid:Berlin:Barajas:Tegel:IBERIA:210:35:6:T4:60:N

Output :

3 test -> Expected output

(null)
Violación de segmento (`core' generado) -> Segmentation fault, (null) should be IBE3674

Answer 1

strtok does not simply modify the pointer it is passed but actually the string itself. It replaces each found delimiter with a null character.

If you had string test:strtok:for:me and call strtok once you had test\0strtok:for:me after that.

So when you iterate ofer the first couple tokens each : is replaced with \0. If you now reset your pointer c2 to the beginning of the string and call strtok again, strtok finds a null charakter before it finds a delimiter and assumes the string ended before a delimiter is found and returns NULL.