C: Passing a pointer down, updated pointer can't be accessed from calling functions! -

i've got problem- assign pointer variable in called function, when trying access updated pointer in calling functions, doesnt show updated, , results in seg fault. help!

"bottommost function":

void comparestrings(char* firstfrag, char* secondfrag, int* longestcommonlength, char* suffix, char* prefix, int* relative_pos) {     suffix = firstfrag;     prefix = secondfrag; } 

it called function (the printf triggers fault)

int findbestoverlap(char* fragmentarray[], int fragmentcount, int* longestcommonlength, char* suffix, char* prefix, int* relative_pos) {     comparestrings(fragmentarray[firstfrag], fragmentarray[secondfrag], longestcommonlength, suffix, prefix, relative_pos);     printf("lcl: %i || prefix: %s || suffix: %s", *longestcommonlength, prefix, suffix); } 

the variables prefix , suffix in turned created in higher calling function, processstrings.

void processstrings(char* fragmentarray[], int fragmentcount) {     int longestcommonlength = 0;     char* suffix;     char* prefix;     int relative_pos;  // first letter of suffix is, vis-a-vis prefix      if ((findbestoverlap(fragmentarray, fragmentcount, &longestcommonlength, suffix, prefix, &relative_pos)) != 0) {     } } 

help!

you're not updating pointer, you're changing local value of argument. need use pointer pointer (e.g. char**) , change pointed-to value instead.

void comparestrings(char* firstfrag, char* secondfrag, int* longestcommonlength, char** suffix, char** prefix, int* relative_pos) {     *suffix = firstfrag;     *prefix = secondfrag; } 

then need dereference appropriately in caller. have fun!