Error(s):

source_file.c:5:16: warning: ‘struct fraction’ declared inside parameter list
 int add(struct fraction F){ 
                ^
source_file.c:5:16: warning: its scope is only this definition or declaration, which is probably not what you want
source_file.c:5:25: error: parameter 1 (‘F’) has incomplete type
 int add(struct fraction F){ 
                         ^
source_file.c:14:21: warning: ‘struct fraction’ declared inside parameter list
 int subtract(struct fraction F){ 
                     ^
source_file.c:14:30: error: parameter 1 (‘F’) has incomplete type
 int subtract(struct fraction F){ 
                              ^
source_file.c:23:21: warning: ‘struct fraction’ declared inside parameter list
 int multiply(struct fraction F){ 
                     ^
source_file.c:23:30: error: parameter 1 (‘F’) has incomplete type
 int multiply(struct fraction F){ 
                              ^
source_file.c:30:19: warning: ‘struct fraction’ declared inside parameter list
 int divide(struct fraction F){ 
                   ^
source_file.c:30:28: error: parameter 1 (‘F’) has incomplete type
 int divide(struct fraction F){ 
                            ^
source_file.c: In function ‘main’:
source_file.c:48:4: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
    scanf("%d",&c);
    ^
source_file.c:52:13: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d",&F[0].n);
             ^
source_file.c:53:13: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d",&F[0].d);
             ^
source_file.c:54:13: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d",&F[1].n);
             ^
source_file.c:55:13: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d",&F[1].d);
             ^
source_file.c:59:13: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d",&F[0].n);
             ^
source_file.c:60:13: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d",&F[0].d);
             ^
source_file.c:61:13: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d",&F[1].n);
             ^
source_file.c:62:13: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d",&F[1].d);
             ^
source_file.c:67:13: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d",&F[0].n);
             ^
source_file.c:68:13: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d",&F[0].d);
             ^
source_file.c:69:13: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d",&F[1].n);
             ^
source_file.c:70:13: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d",&F[1].d);
             ^
source_file.c:74:13: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d",&F[0].n);
             ^
source_file.c:75:13: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d",&F[0].d);
             ^
source_file.c:76:13: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d",&F[1].n);
             ^
source_file.c:77:13: warning: ignoring return value of ‘scanf’, declared with attribute warn_unused_result [-Wunused-result]
             scanf("%d",&F[1].d);
             ^