释放内存时出现分段错误

Question

Here is a program I made this program has segmentation fault I checked it in gdb in the second last of code free(somepath); .I do not have any reason for why is this segmentation fault coming? Some one please suggest some thing.

#include<dirent.h>
#include<unistd.h>
#include<string.h>
#include<sys/stat.h>
#include<stdlib.h>
#include<stdio.h>
char *directs[20], *files[20];
int i = 0;
int j = 0;
int count = 0;

void printdir(char *);
int count_dirs(char *);
int count_files(char *);
void new_printdir(int ,int ,char *);
int main()
{
    char startdir[20];
    printf("Scanning user directories\n");
    scanf("%s", startdir);
    printdir(startdir);
}

void printdir(char *dir)
{


    DIR *dp = opendir(dir);
    int nDirs, nFiles, nD, nF;

    nDirs = 0;
    nFiles = 0;
    nD = 0;
    nF = 0;
    if (dp) {
        struct dirent *entry = 0;
        struct stat statBuf;

        nDirs = count_dirs(dir);
        nFiles = count_files(dir);



  new_printdir(nDirs,nFiles,dir);


        while ((entry = readdir(dp)) != 0) {
            if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0) {
                continue;
            }

    char *     filepath = malloc(strlen(dir) + strlen(entry->d_name) + 2);
            if (filepath) {
                sprintf(filepath, "%s/%s", dir, entry->d_name);
                if (lstat(filepath, &statBuf) == 0) {
                    if (S_ISDIR(statBuf.st_mode)) {
                    printdir(filepath);
                        }
                    else {
                    }
                }

            }    

            free(filepath);
        }        //2nd while

        closedir(dp);
    }

    else {
        fprintf(stderr, "Error, cannot open directory %s\n", dir);
    }

}                //printdir

int count_dirs(char *dir)
{
    DIR *dp = opendir(dir);
    int nD;
    nD = 0;
    if (dp) {
        struct dirent *entry = 0;
        struct stat statBuf;

        while ((entry = readdir(dp)) != 0) {
            if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0) {
                continue;
            }

            char *filepath =  malloc(strlen(dir) + strlen(entry->d_name) + 2);

            if (filepath) {
                sprintf(filepath, "%s/%s", dir, entry->d_name);

                if (lstat(filepath, &statBuf) != 0) {
                    fprintf(stderr, "File Not found? %s\n",filepath);
                }

                if (S_ISDIR(statBuf.st_mode)) {
                    nD++;

                } else {
                    continue;
                }

                free(filepath);
            }
        }

        closedir(dp);
    } else {
        fprintf(stderr, "Error, cannot open directory %s\n", dir);
    }
    return nD;
}

int count_files(char *dir)
{
    DIR *dp = opendir(dir);
    int nF;
    nF = 0;
    if (dp) {
        struct dirent *entry = 0;
        struct stat statBuf;

        while ((entry = readdir(dp)) != 0) {
            if (strcmp(entry->d_name, ".") == 0 || strcmp(entry->d_name, "..") == 0) {
                continue;
            }

            char *filepath =
                malloc(strlen(dir) + strlen(entry->d_name) + 2);

            if (filepath) {
                sprintf(filepath, "%s/%s", dir, entry->d_name);

                if (lstat(filepath, &statBuf) != 0) {
                    fprintf(stderr, "File Not found? %s\n",    filepath);
                }

                if (S_ISDIR(statBuf.st_mode)) {

                    continue;
                } else {
                    nF++;

                }

                free(filepath);
            }
        }

        closedir(dp);
    } else {
        fprintf(stderr, "Error, cannot open file %s\n", dir);
    }
    return nF;
}

void new_printdir(int nDirs,int nFiles,char *dir)
{

      struct dirent **namelist;
        DIR *dop;
        int i, j,t,re,nD,nF;

        char userd[20],*somepath;

       i = scandir(dir, &namelist, 0, alphasort);
        t=0;

        if (i < 0)
                perror ("Scandir failed to open directory I hope you understand \n");
        else {
                for (j = 0; j < i; j++) {
                if(strcmp(".",namelist[j]->d_name)==0||strcmp("..",namelist[j]->d_name)==0)
                continue;
              somepath = malloc(strlen(dir)+strlen(namelist[j]->d_name)+2);
                  sprintf(somepath,"%s/%s",dir,namelist[j]->d_name);
                        dop=opendir(somepath);

                        if(dop)
                         {

                nD++;
                            if ((nDirs-nD)<3)    
                        {printf("%s ",namelist[j]->d_name);}
                          }
                     else {
                nF++;
                if ((nFiles-nF)<3)    
                printf("%s ",namelist[j]->d_name);
              }
            closedir(dop);
                        free(namelist[j]);
                }
        }
        free(namelist);
 free(somepath);

}

Why is this segmentation fault happening this is not clear to me. What can I do to get rid of it?

Answer 1

In your code, you are not guaranteed to allocate memory and assign it to somepath (you also fail to free somepath except for the last iteration of the loop). You should put your free(somepath) statement at the end of the for-loop, and you should initialize somepath to NULL both at the very beginning of the function and at the start of each for loop to avoid similar careless errors.

Answer 2

You're not initializing the somepath variable - it has an undefined value unless the malloc() executes (ie for empty folders). You should initialize it to NULL at the place of definition.

Answer 3

You do not initialize somepath to 0 (NULL), but you could under some circumstances release the value. This means you release a random value - which is not a good idea.