c - Reading a binary file into structure variable giving seg fault -


i have below sample file read binary structure , print length of string stored in structure. segmentation fault core dumped when try print full string. not find reason it.

#include <stdio.h> #include <stdlib.h>  struct sample {         unsigned int m;         unsigned int v;         unsigned int s[128];         int t_length;         char *t; };  int main() {         int i=0;         unsigned int t_len=0;         file *fp;         struct sample sam;         fp =fopen("sample.bin", "rb");         if (fp==null) {                 printf("file not created\n");                 return -1;         }         fread(&sam, sizeof(sam), 1, fp);         printf("t_length %d\n", sam.t_length);         t_len=sam.t_length;         sam.t=(char *)malloc(sizeof(char) * t_len);         fseek(fp,0,seek_set);         fread(&sam, sizeof(sam)+t_len, 1, fp);         printf("t_length %d\n", t_len);         printf("%s\n", sam.t);         fclose(fp);         free(sam.t);         return 0;  }

consider following lines:

    fread(&sam, sizeof(sam), 1, fp);     // ...     sam.t=(char *)malloc(sizeof(char) * t_len);     // ...     fread(&sam, sizeof(sam)+t_len, 1, fp);

you read structure. assign t member. read structure again, overwriting t member.

this lead t member no longer pointing memory allocated, , lead undefined behavior when dereference pointer.

i suggest research flexible array members might solve problem. of course requires program writing file changed use flexible array member.

if can't modify program writing file, don't re-read structure. read structure once, allocate memory, read only data put in memory allocated. skip seek, file should @ correct position anyway.

in code like

fread(&sam, sizeof sam , 1, fp); sam.t = malloc(samr.t_length); fread(sam.t, sam.t_length, 1, fp);

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