munmap() not working when called from function - c

Question

Im writing a library of functions that works with shared memory. The function I am writing to close the shared memory gives me a segfault whenever it gets to the munmap() part, but if i dont use the function and just let the code munmap() and shm_unlink at the end, it works. im very confused as to why. the code is below.
void close_table(table_t *tbl, char* name){
size_t size = sizeof(tbl->table_size);
printf("table_size: %d\n", tbl->table_size);
/* when the first part of if else statement runs its fine */
if(tbl->numP > 1){
printf("first if statement\n");
tbl->numP -= 1;
munmap(tbl, tbl->table_size);
}
/* when second part runs gives seg fault */
else{
tbl->numP -= 1;
munmap(tbl, tbl->table_size);
//close(tbl->shm_fd);
shm_unlink(tbl->name);
}
// Code omission
}
This is the open table function
table_t *open_table(char *name, int record_size, int max_records){
int shm_fd;
int table_size;
void* ptr;
table_t* tbl;
table_t controlBlock;
db_info info;
data_block data;
table_size = max_records * sizeof(controlBlock) * sizeof(info) *
sizeof(data);
if((shm_fd = shm_open(name, O_CREAT | O_RDWR, 0666)) == -1){
exit(EXIT_FAILURE);
}
ftruncate(shm_fd, table_size);
ptr = mmap(NULL, table_size, PROT_WRITE | PROT_READ, MAP_SHARED, shm_fd,
0);
tbl = ptr;
tbl->shm_fd = shm_fd;
tbl->table_size = table_size;
tbl->max_records = max_records;
tbl->record_size = record_size;
if(tbl->numP >= 1){
tbl->numP += 1;
}
else{
tbl->numP = 1;
}
return tbl;
}
these are my structs
typedef struct{
int dbInfoId;
int deleted;
}db_info;
typedef struct{
int shm_fd;
int numP;
int control_id;
char *name;
int table_size;
int record_size;
int max_records;
db_info* dbInfo;
void* db;
}table_t;
typedef struct{
int id;
void* record;
}data_block;
valgrind says its an "invalid read of size 8
enter a command (just the number):
1. Add
2. Delete
3. Process
4. Close
5. # of processes
6. exit
4
table_size: 29360128
==6679== Invalid read of size 8
==6679== at 0x108C42: close_table (in /home/abiodun/Documents/Comp 3713/Assignment3_makeup/main)
==6679== by 0x108F15: main (main.c:40)
==6679== Address 0x5654010 is not stack'd, malloc'd or (recently) free'd
==6679==
==6679==
==6679== Process terminating with default action of signal 11 (SIGSEGV)
==6679== Access not within mapped region at address 0x5654010
==6679== at 0x108C42: close_table (in /home/abiodun/Documents/Comp 3713/Assignment3_makeup/main)
==6679== by 0x108F15: main (main.c:40)
==6679== If you believe this happened as a result of a stack
==6679== overflow in your program's main thread (unlikely but
==6679== possible), you can try to increase the size of the
==6679== main thread stack using the --main-stacksize= flag.
==6679== The main thread stack size used in this run was 8388608.
--6679-- REDIR: 0x50db950 (libc.so.6:free) redirected to 0x4c30cd0 (free)
==6679==
==6679== HEAP SUMMARY:
==6679== in use at exit: 0 bytes in 0 blocks
==6679== total heap usage: 2 allocs, 2 frees, 2,048 bytes allocated
==6679==
==6679== All heap blocks were freed -- no leaks are possible
==6679==
==6679== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
==6679==
==6679== 1 errors in context 1 of 1:
==6679== Invalid read of size 8
==6679== at 0x108C42: close_table (in /home/abiodun/Documents/Comp 3713/Assignment3_makeup/main)
==6679== by 0x108F15: main (main.c:40)
==6679== Address 0x5654010 is not stack'd, malloc'd or (recently) free'd
==6679==
==6679== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
Segmentation fault (core dumped)

Answer 1

I figured it the answer. It turns out I was still trying to access variables in the tbl struct after it was unmapped. so to fix this I just created variabbles in the function and saved what I needed from the tbl struct before unmapping.
void close_table(table_t *tbl){
int size = tbl->table_size;
char* name = tbl->name;
int fd = tbl->shm_fd;
printf("table_size: %d\n", tbl->table_size);
if(tbl->numP > 1){
tbl->numP -= 1;
munmap(tbl, tbl->table_size);
close(fd);
}
else if(tbl->numP == 1){
tbl->numP -= 1;
munmap(tbl, size);
printf("unmap works\n"); fflush(stdout);
shm_unlink(name);
close(fd);
}