How to send structure array to function in C?

In your code, you did not create an instance of the PM array or structs. Other than a few other nits, and your sort being in descending order (check the bubble sort logic), the code was not in bad shape. Note: when you are creating an array of some size, it is general practice to #define a maximum size of the array. Below, I just defined MAXS (max structures) to 256 in the line #define MAXS 256. You then check your count as you read from your data file against MAXS-1 (zero based array count). If you exceed MAXS - 1 you will need to increase the size (or if you allocated dynamically, you have the option to reallocate at that point) The example below ran fine.

Remember to initialize all variables! This is one of the easiest things you can do to prevent wandering off into undefined behavior.

Note: I created a tenure to return the number of months served -- make sure that is what you want. Also, I did not recreate a party_time function. That's up to you.

Let me know if you need any additional help.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define MAXS 256

struct Leader
{
    char surname[15];
    char party[15];
    int months_in_power;
};

void bubble (struct Leader arr[],int max);
int tenure (int start_month, int start_year, int end_month, int end_year);

int main () {

    char name[64] = {0};
    char party[64] = {0};
    int start_month=0, start_year=0, end_month=0, end_year=0;
    int months = 0;
    int i = 0;
    int imax = 0;
    FILE *INFILE;

    struct Leader PM[MAXS] = {{ {0}, {0}, 0 }};

    if ((INFILE = (fopen ("PM.txt","r"))) == NULL) 
    { 
        fprintf (stderr, "ERROR: The file could not be opened."); 
        return 1; 
    }

    while(fscanf(INFILE, "%s %d %d %d %d %s", name, 
    &start_month, &start_year, &end_month, &end_year, party)>0)
    {
        strcpy(PM[i].surname, name);
        strcpy(PM[i].party, party);
        months = tenure(start_month, start_year, end_month, end_year);
        PM[i].months_in_power = months;
        // party_time(&PM[i], &Con_months, &Lab_months );
        printf("%-12s: %-12s, %d months\n", PM[i].surname, PM[i].party, PM[i].months_in_power); //This line all works

        i++;

        if (i > MAXS - 1) {
            fprintf (stderr, "Error: lines read from file exceed MAXS, reallocate or increase size.\n");
            return 1;
        }
    }

    imax=i;
    bubble(PM, imax);

    return 0;
}

void bubble(struct Leader arr[],int max)
{
    int j=0, i=0, temp=0;

    for (j=1; j<(max);j++)
    {   
        for (i=0; i<(max-1); i++)
        {
        if (arr[i].months_in_power < arr[i+1].months_in_power)
        { 
            temp = arr[i].months_in_power;
            arr[i].months_in_power=arr[i+1].months_in_power;
            arr[i+1].months_in_power = temp;
        }
            else continue;
        }
    } 

    for( i=0; i<max; i++) 
    {printf("%d\n", arr[i].months_in_power);} 

    return ;        
}

int tenure (int start_month, int start_year, int end_month, int end_year) 
{
    int no_months = (end_year - start_year) * 12 - start_month + end_month;
    return (start_year <= end_year) ? no_months : -(no_months);
}

input:

$ cat PM.txt
Attlee 7 1945 10 1951 Labour
Churchill 11 1951 5 1955 Conservative
Eden 6 1955 12 1956 Conservative

output:

$ ./bin/structleader
Attlee      : Labour      , 75 months
Churchill   : Conservative, 42 months
Eden        : Conservative, 18 months
75
42
18