/* https://stackoverflow.com/questions/53220084/proper-input-method-to-take-char-string-input-in-c/53221258#53221258 gcc array_struct_3.c &&./a.out */ #include<stdio.h> #include<stdlib.h> #define ROW 5 /* if you need a constant, #define one (or more) */ #define COL 64 typedef struct student_t { char name[COL]; int score; } student_t; int student_t_cmp_score(const void *x, const void *y) { struct student_t* const *ix = x; struct student_t* const *iy = y; return ((*ix)->score > (*iy)->score) - ((*ix)->score < (*iy)->score); } int main(void) { int n = 0; // student_t student[ROW] = {{.name = ""}}; student_t *student[ROW]; puts("\n[note; press enter alone to end input]\n"); (student[n]) = malloc(sizeof(struct student_t)); for(;;){ char buf[COL]; fputs("Enter student name: ", stdout); if (!fgets(buf, sizeof buf, stdin)) /* read/validate line */ break; if(*buf == '\n') /* check for empty line */ break; /* parse line into name and score - validate! */ if(sscanf(buf,"%63s %d",student[n]->name, &student[n]->score) != 2) { fputs(" error: invalid input, conversion failed.\n",stderr); continue; } n++; student[n] = malloc(sizeof(struct student_t)); if(n== ROW){ fputs("\narray full - input complete.\n", stdout); break; } } printf("sizeof student: %zu\n", sizeof *student); qsort(student, n, sizeof *student,student_t_cmp_score); for(int i = 0; i < n; i++) printf("%-16s %3d\n",student[i]->name, student[i]->score); return 0; } The above version will work. However, I try the following code then it's Segmentation fault. The line to make Segmentation fault is: qsort(&student, n, sizeof *student,student_t_cmp_score); .Since there is only 4 variable, so the only problem will be either &student or student_t_cmp_score for student_t_cmp_score, I just follow/copy other people code. Overall I don't know why it's Segmentation fault.
typedef struct student_t { char name[COL]; int score; } student_t; int student_t_cmp_score(const void *x, const void *y) { struct student_t* const *ix = x; struct student_t* const *iy = y; return ((*ix)->score > (*iy)->score) - ((*ix)->score < (*iy)->score); } int main(void) { int n = 0; student_t student[ROW] = {{.name = ""}}; // student_t *student[ROW]; puts("\n[note; press enter alone to end input]\n"); // (student[n]) = malloc(sizeof(struct student_t)); for(;;){ char buf[COL]; fputs("Enter student name: ", stdout); if (!fgets(buf, sizeof buf, stdin)) /* read/validate line */ break; if(*buf == '\n') /* check for empty line */ break; /* parse line into name and score - validate! */ if(sscanf(buf,"%63s %d",student[n].name, &student[n].score) != 2) { fputs(" error: invalid input, conversion failed.\n",stderr); continue; } n++; // student[n] = malloc(sizeof(struct student_t)); if(n== ROW){ fputs("\narray full - input complete.\n", stdout); break; } } printf("sizeof student: %zu\n", sizeof *student); qsort(&student, n, sizeof *student,student_t_cmp_score); // qsort(student, n, sizeof *student,student_t_cmp_score); for(int i = 0; i < n; i++) printf("%-16s %3d\n",student[i].name, student[i].score); return 0; } I also tried use GDB via following command.
gdb ./a.out (gdb) run <segfault happens here> (gdb) backtrace error show in terminal:
sizeof student: 68 Program received signal SIGSEGV, Segmentation fault. student_t_cmp_score (x=0x7fffffffdbe0, y=0x7fffffffdc24) at array_struct_3.c:22 warning: Source file is more recent than executable. 22 int student_t_cmp_score(const void *x, const void *y) (gdb) backtrace #0 student_t_cmp_score (x=0x7fffffffdbe0, y=0x7fffffffdc24) at array_struct_3.c:22 #1 0x00007ffff7dc6491 in msort_with_tmp (p=p@entry=0x7fffffffdb60, b=b@entry=0x7fffffffdab8, n=n@entry=2) at ./stdlib/msort.c:123 #2 0x00007ffff7dc63a5 in msort_with_tmp (n=2, b=0x7fffffffdab8, p=0x7fffffffdb60) at ./stdlib/msort.c:44 #3 msort_with_tmp (p=p@entry=0x7fffffffdb60, b=0x7fffffffdab8, n=n@entry=5) at ./stdlib/msort.c:52 #4 0x00007ffff7dc6992 in msort_with_tmp (n=5, b=<optimized out>, p=0x7fffffffdb60) at ./stdlib/msort.c:44 #5 __GI___qsort_r (b=<optimized out>, n=<optimized out>, s=68, cmp=<optimized out>, arg=0x0) at ./stdlib/msort.c:253 #6 0x0000555555555417 in main () at array_struct_3.c:60 
